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"content": "<p><strong>FP Complete Publishes Results of Independent 3rd Party Audits of Hedera Platform and New Hedera Token Service</strong></p>\n<p><em>FP Complete Corporation development specialists conducted a comprehensive review of Hedera's code and technical documentation</em></p>\n<p><strong>Zug, Switzerland – February 9, 2021 –</strong> As part of its goal to deliver\ntransparency to the development community,\n<a href=\"http://www.hedera.com/\">Hedera Hashgraph</a>, the enterprise-grade public\ndistributed ledger, engaged FP Complete, an IT engineering specialist,\nto perform an independent audit of the engineering work by Hedera's\ndevelopment team on the Hedera platform, including the new Hedera Token\nService. The full completed audit reports can be found at:</p>\n<ul>\n<li><a href=\"https://hedera.com/fp-complete-hedera\">Hedera Platform</a></li>\n<li><a href=\"https://hedera.com/fp-complete-hts\">Hedera Token Service</a></li>\n</ul>\n<p>Founded by the former head of Microsoft's own in-house engineering\ntools, Aaron Contorer, FP Complete Corporation is the world's leading\nsupplier of commercial-grade tools and engineering for advanced\nprogramming languages, distributed systems, blockchain, and DevOps\ntechnologies. FP Complete performed an in-depth code review to examine\nthe Hedera software quality, focusing on robustness, security, and\naudibility.</p>\n<p>FP Complete also completed a review of Hedera's code and technical\ndocumentation, enabling the development team to use this ongoing work to\noptimize the engineering methods, tools, and coding standards used to\nimplement the Hedera network. The publication of these results\ndemonstrates the Company's commitment to technical rigor and\ntransparency.</p>\n<p>Dr. Leemon Baird, co-founder and Chief Scientist of Hedera Hashgraph,\ncomments: "These third-party audits by FP Complete illustrate our\ncommitment to high-quality engineering, project transparency, and a\nrigorous and independent auditing process. We are pleased to be able to\npublish these audit results today and look forward to sharing additional\naudit findings as they are completed in the future."</p>\n<p>Wesley Crook, CEO of FP Complete, comments: "We have worked with the\nHedera team to conduct a third-party audit of their codebase to assess\nsecurity, stability, and correctness. Our team of software, blockchain,\nand network architecture experts has provided feedback throughout the\ndevelopment process."</p>\n<hr />\n<h2 id=\"about-hedera\">About Hedera</h2>\n<p>Hedera is a decentralized enterprise-grade public network on which\nanyone can build secure, fair applications with near real-time finality.\nThe platform is owned and governed by a council of the world's leading\norganizations including Avery Dennison, Boeing, Dentons, Deutsche\nTelekom, DLA Piper, eftpos, FIS (WorldPay), Google, IBM, LG Electronics,\nMagalu, Nomura, Swirlds, Tata Communications, University College London\n(UCL), Wipro, and Zain Group.</p>\n<p>For more information, visit\nhttps://www.hedera.com, or follow us on Twitter\nat <a href=\"https://twitter.com/hedera\">@hedera</a>, Telegram at\n<a href=\"https://t.me/hederahashgraph\">t.me/hederahashgraph</a>, or Discord at\n<a href=\"https://www.hedera.com/discord\">www.hedera.com/discord</a>. The Hedera\nwhitepaper can be found at\n<a href=\"https://hedera.com/papers\">www.hedera.com/papers</a>.</p>\n<h2 id=\"about-fp-complete\">About FP Complete</h2>\n<p>FP Complete is an advanced server-side software development and DevOps\nconsulting Company. We specialize in helping FinTech companies solve\ntheir unique set of problems related to data and information integrity,\ndata security, architectural design, systems integration, and regulatory\ncompliance. We are recognized worldwide for our contributions to the\nfunctional programming community using the Haskell programming language.\nOur people and processes have helped countless companies increase the\nvelocity and quality of their delivered software products. From fortune\n500 biotech companies to small blockchain FinTech software companies we\nhave solved unique and complicated problems with expert results.</p>\n<p><a href=\"https://www.fpcomplete.com/\">https://www.fpcomplete.com/</a></p>\n",
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"content": "<p>Earlier this week, Amazon removed Parler from its platform. As a company hosting a network service on a cloud provider today, should you worry about such actions from cloud vendors? And what steps should you be taking now?</p>\n<p>In this post, we'll explore some of the risks associated with being tied to a single vendor, and the costs involved in breaking the dependency. I'll also give some recommendations on low hanging fruit.</p>\n<p>Ultimately, how far down the vendor neutrality path you want to go is a company specific risk mitigation strategy. In this post, we'll explore the raw information, but deeper analysis would be based on your company's specific situation. As usual, if you would like more direct help from the team at FP Complete in understanding these topics, please <a href=\"https://www.fpcomplete.com/contact-us/\">contact us for a consultation</a>.</p>\n<h2 id=\"what-is-vendor-neutrality\">What is vendor neutrality?</h2>\n<p>Vendor neutrality is not a binary. There are various levels on a spectrum from an application that leverages many vendor-specific services to an application which runs on any Linux machine in the world. Achieving complete vendor neutrality is almost never the goal. Instead, most companies interested in this topic are looking to reduce their dependencies where reasonable.</p>\n<p>To be more concrete, let's say you're on Amazon, and you're looking into what database options to use in your application. Your team comes up with three options:</p>\n<ol>\n<li>Build it using DynamoDB, an Amazon-specific proprietary offering</li>\n<li>Build it using PostgreSQL hosted on Amazon's RDS service</li>\n<li>Build it using PostgreSQL which your team manages themselves</li>\n</ol>\n<p>Option (1) provides no vendor neutrality. If you, for any reason, decide to leave Amazon, you'll need to rewrite large parts of your application to move from DynamoDB. This may be a significant undertaking, introducing a major barrier to exit from Amazon.</p>\n<p>Option (2), while still leveraging an Amazon service, does not fall into that same trap. Your application will speak to PostgreSQL, an open source database that can be hosted anywhere in the world. If you're dissatisfied with RDS, you can migrate to another offering fairly easy. PostgreSQL hosted offerings are available on other cloud providers. And by using RDS, you'll get some features more easily, such as backups and replication.</p>\n<p>Option (3) is the most vendor neutral. You'll be forced to implement all features of PostgreSQL you want yourself. Maybe this will entail creating a Docker image with a fully configured PostgreSQL instance. Moving this to Azure or on-prem is even easier than option (2). But we may be at the point of diminishing returns, as we'll discuss below.</p>\n<p>To summarize: vendor neutrality is a spectrum measuring how tied you are to a specific vendor, and how difficult it would be to move to a different one.</p>\n<h2 id=\"advantages-of-vendor-neutrality\">Advantages of vendor neutrality</h2>\n<p>The current situation with Parler is an extreme example of the advantages of vendor neutrality. I would imagine most companies doing business with Amazon don't have a reasonable expectation that Amazon would decide to remove them from their platform. Again, this is a risk assessment scenario, and you need to analyze the risk for your own business. A company hosting uncensored political discourse is in a different risk category from a someone running a personal blog.</p>\n<p>But this is far from the only advantage of vendor neutrality. Let's analyze some of the most common concerns I've seen for companies to remain vendor neutral.</p>\n<ul>\n<li><strong>Price sensitivity</strong> Cloud costs can be a major part of a company's budget, and costs can vary radically between providers. Various providers are also willing to give large incentives for companies to switch platforms. But if you've designed your application deeply around one provider, the cost of switching may not exceed the long term cost savings, leaving you at your current provider's mercy.</li>\n<li><strong>Regulatory obligations</strong> Some governments may have requirements that your software run on specific vendor hardware, or specific on-prem environments. Building up your software around one provider may prevent you from offering your services in those cases.</li>\n<li><strong>Client preference</strong> Similarly, if you provide managed software to companies, they may have a built-in cloud provider preference. If you've built your software on Google Cloud, but they have a corporate policy that all new projects live on Azure, you may lose the sale.</li>\n<li><strong>Geographic distribution</strong> For lowest latency, you'll want to put your services as close to the clients as possible. And it may turn out that the provider you've chosen simply doesn't have a presence there. Or a competitor may be closer. Or a service you want to peer with is on different provider, and the data costs will be much lower if you switch providers.</li>\n</ul>\n<p>There are many more examples, this isn't an exhaustive list. What I want to motivate here is that vendor neutrality isn't just a fringe ideal for companies afraid of platform eviction. There are many reasons a normal company in its normal course of business may wish to be vendor neutral. You should analyze these cases, as well as others that may apply to your company, and assess the value of neutrality.</p>\n<h2 id=\"costs-of-vendor-neutrality\">Costs of vendor neutrality</h2>\n<p>Vendor neutrality does not come for free. A primary value proposition of most cloud providers is quick time to market. By leveraging existing services, your team can offload creation and maintenance of complex systems. Eschewing such services and building from scratch will impact your time to market, and potentially have other impacts (like increase bug rate, reduced reliability, etc).</p>\n<p>I often see engineers decrying the evils of vendor lock-in without taking these costs into account. As a business, you'll need to find a way to adequately and accurately measure these costs as you make decisions, instead of turning it into a quasi-religious crusade against all forms of lock-in.</p>\n<p>With these trade-offs in mind, I'll finish off this post by explaining some of the most bang-for-the-buck moves you can make, which:</p>\n<ul>\n<li>Move you much farther along the vendor neutral spectrum</li>\n<li>Do not cost significant engineering work, if undertaken early on and designed correctly</li>\n<li>Provide additional benefits whenever possible</li>\n</ul>\n<h2 id=\"leverage-open-source-tools\">Leverage open source tools</h2>\n<p>The hardest lock-in to overcome is dedication to a proprietary tool. Without naming names, some large 6-letter database companies have made a great reputation of leveraging lock-in with major increases in licensing fees. Once you're tied into that model, it's difficult to disengage.</p>\n<p>Open source tools provide a major protection against this. Assuming the licenses are correct—and you should be sure to check that—no one can ever take your open source tools away from you. Sure, a provider may decide to stop maintaining the software. Or perhaps future releases may be closed source instead. Or perhaps they won't address your bug reports without paying for a support contract. But ultimately, you retain lots of freedom to take the software, modify it as necessary, and deploy it everywhere.</p>\n<p>There has long been a debate between the features and maturity of proprietary versus open source tooling. As always, we cannot make our decisions in a vacuum, and the flexibility of open source is not the be-all and end-all for a business. However, in the past decade in particular, open source has come to dominate large parts of the deployment space.</p>\n<p>To pick on the example above: while DynamoDB is a powerful and flexible database option on AWS, it's far from unique. Cassandra, Redis, PostgreSQL, and dozens of other open source databases are readily available, with companies offering support, commercial hosting, and paid consulting services.</p>\n<p>We've seen a major shift occur as well in the software development language space. Many of the biggest tech companies in the world not only <em>use</em> open source languages, but provide their own complete language ecosystems, free of charge. Google's Go, Microsoft's .NET Core, Mozilla's <a href=\"https://www.fpcomplete.com/rust/\">Rust</a>, and Apple's Swift are some prime examples.</p>\n<p>Far from being the scrappy underdog, we've seen a shift where open source is the de facto standard, and proprietary options are viewed as niche. You're no longer trading quality for flexibility. You can often have your cake and eat it too.</p>\n<h3 id=\"kubernetes\">Kubernetes</h3>\n<p>I decided to give one open source player its own subsection in this context. Kubernetes is an orchestration management tool, managing various cloud resources for hosting containerized applications in both Linux and Windows. The first notable thing in this context is that Kubernetes has effectively supplanted other proprietary and cloud-specific offerings. Those offerings still exist, but from a market share standpoint, Kubernetes is clearly in a dominant position.</p>\n<p>The second thing to note is that Kubernetes is a tool supported by many of the largest cloud providers. Google created Kubernetes, Microsoft provides significant support, and all three top cloud providers (Google, Azure, and AWS) offer native Kubernetes services.</p>\n<p>The final thing to note is that Kubernetes really goes beyond a single service. In many ways, it functions as a cloud abstraction layer. When you use Kubernetes, you often times write your applications to target Kubernetes <em>instead of</em> targeting the underlying vendor. Instead of using a cloud Load Balancer, you'll use an ingress and service in Kubernetes. This drastically reduces the cost of remaining vendor neutral.</p>\n<p>As a plug, in <a href=\"https://www.fpcomplete.com/products/kube360/\">our own Kubernetes offering</a>, we've focused on combining commonly used open source components to provide a batteries-included experience with minimized vendor lock-in. We've already used it internally and for customers to easily migrate services between different cloud providers, and from the cloud to on-prem.</p>\n<div class=\"text-center\"><a href=\"/products/kube360\" class=\"button-coral\">Learn more about Kube360</a></div>\n<h2 id=\"high-value-cloud-services\">High value cloud services</h2>\n<p>Some cloud services provide an interesting combination of delivering high value with minimal lock-in costs. The greatest example of that is blob storage services, such as S3. The durability and availability guarantees cloud providers offer around your data is far greater than most teams would be able to provide on their own. The cost of usage is significantly far lower than rolling your own solution using block storage in the cloud. And finally: the lock-in risks tend to be small. There are tools available to abstract the different vendor APIs for blob storage (and we include such a tool in Kube360). And even without such tools, generally the impact on a codebase from blob storage selection is minimal.</p>\n<p>Another example is services which host open source offerings. The RDS example above fits in nicely here. We generally recommend using hosted database offerings from cloud providers, since the cost is close to what you would pay to set it up yourself, you get lots of features quickly, and migration to a different option is trivial.</p>\n<p>And one final example is services like load balancers and auto-scaling groups. These are services that are impossible to implement fully yourself, would be far more expensive to implement to any extent using cloud virtual machines, and introduce virtually no lock-in. If you're moving from AWS to Azure, you'll need to change your infrastructure code to use Azure equivalents to those services. But generally, these can be seen at the same level of commodity as the virtual machines themselves. You're paying for a fairly standard service, you're rarely locking yourself in to a vendor-specific feature.</p>\n<h2 id=\"multicloud-vs-hybrid-cloud\">Multicloud vs hybrid cloud</h2>\n<p>In previous discussions, the topic of vendor neutrality typically introduces the two confusing terms "multicloud" and "hybrid cloud." There is some disagreement in the tech space around what the former term means, but I'm going to define these two terms as:</p>\n<ul>\n<li><strong>Multicloud</strong> means that your service is capable of running on multiple different cloud providers and/or on-prem environments, but each environment will be autonomous from others</li>\n<li><strong>Hybrid cloud</strong> means that you can simultaneously run your service on multiple cloud providers, and they will replicate data, load balance, and perform other intelligent operations between the different providers</li>\n</ul>\n<p>Multicloud is a much easier thing to attain than hybrid cloud. Hybrid cloud introduces many new kinds of distributed systems failure models, as well as risks around major data transfer costs and latencies. There are certainly some potential advantages for hybrid cloud setups, but in our experience the much lower hanging fruit is in targeting multicloud.</p>\n<h2 id=\"conclusion\">Conclusion</h2>\n<p>Summing up, there are many reasons a company may decide to keep their applications vendor neutral. Each of these reasons can be seen as a risk mitigation strategy, and a proper risk assessment and cost analysis should be performed. While current events has people's attention on vendor eviction, plenty of other reasons exist.</p>\n<p>On the other hand, vendor neutrality is not free, and should not be pursued to the detriment of the business. Finding high value, low cost moves to increase your neutrality is your best bet. Such moves may include:</p>\n<ul>\n<li>Opting for open source where possible</li>\n<li>Using a platform like <a href=\"https://www.fpcomplete.com/products/kube360/\">Kubernetes</a> that encourages more neutrality</li>\n<li>Opt for cloud services that are more easily swappable, such as load balancers</li>\n</ul>\n<p>If you would like more information or help with a vendor neutrality risk assessment, we would love to chat.</p>\n<div class=\"text-center\"><a href=\"/contact-us/\" class=\"button-coral\">Contact us for more information</a></div>\n<p>If you liked this post, you may also like:</p>\n<ul>\n<li><a href=\"https://www.fpcomplete.com/blog/why-we-built-kube360/\">Why we built Kube360</a></li>\n<li><a href=\"https://www.fpcomplete.com/blog/understanding-cloud-deployments/\">Understanding Cloud Software Deployments</a></li>\n<li><a href=\"https://www.fpcomplete.com/blog/rust-for-devops-tooling/\">Using Rust for DevOps tooling</a></li>\n</ul>\n",
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"description": "Amazon recently removed Parler from its platform, causing some people to ask if and how they should protect themselves from cloud providers. In this post, we'll explore costs and benefits of keeping yourself cloud vendor neutral, and how to approach it expediently.",
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"content": "<p>A few years back, we <a href=\"https://www.fpcomplete.com/blog/2018/07/deploying-rust-with-docker-and-kubernetes/\">published a blog post</a> about deploying a Rust application using Docker and Kubernetes. That application was a Telegram bot. We're going to do something similar today, but with a few meaningful differences:</p>\n<ol>\n<li>We're going to be deploying a web app. Don't get too excited: this will be an incredibly simply piece of code, basically copy-pasted from the <a href=\"https://actix.rs/docs/application/\">actix-web documentation</a>.</li>\n<li>We're going to build the deployment image on Github Actions</li>\n<li>And we're going to be building this using Windows Containers instead of Linux. (Sorry for burying the lead.)</li>\n</ol>\n<p>We put this together for testing purposes when rolling out Windows support in our <a href=\"https://www.fpcomplete.com/products/kube360/\">managed Kubernetes product, Kube360®</a> here at FP Complete. I wanted to put this post together to demonstrate a few things:</p>\n<ul>\n<li>How pleasant and familiar Windows Containers workflows were versus the more familiar Linux approaches</li>\n<li>Github Actions work seamlessly for building Windows Containers</li>\n<li>With the correct configuration, Kubernetes is a great platform for deploying Windows Containers</li>\n<li>And, of course, how wonderful the Rust toolchain is on Windows</li>\n</ul>\n<p>Alright, let's dive in! And if any of those topics sound interesting, and you'd like to learn more about FP Complete offerings, please <a href=\"https://www.fpcomplete.com/contact-us/\">contact us for more information on our offerings</a>.</p>\n<h2 id=\"prereqs\">Prereqs</h2>\n<p>Quick sidenote before we dive in. Windows Containers only run on Windows machines. Not even all Windows machines will support Windows Containers. You'll need Windows 10 Pro or a similar license, and have Docker installed on that machine. You'll also need to ensure that Docker is set to use Windows instead of Linux containers.</p>\n<p>If you have all of that set up, you'll be able to follow along with most of the steps below. If not, you won't be able to build or run the Docker images on your local machine.</p>\n<p>Also, for running the application on Kubernetes, you'll need a Kubernetes cluster with Windows nodes. I'll be using the FP Complete Kube360 test cluster on Azure in this blog post, though we've previously tested in on both AWS and on-prem clusters too.</p>\n<h2 id=\"the-rust-application\">The Rust application</h2>\n<p>The source code for this application will be, by far, the most uninteresting part of this post. As mentioned, it's basically a copy-paste of an example straight from the actix-web documentation featuring mutable state. It turns out this was a great way to test out basic Kubernetes functionality like health checks, replicas, and autohealing.</p>\n<p>We're going to build this using the latest stable Rust version as of writing this post, so create a <code>rust-toolchain</code> file with the contents:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#657b83;\">1.47.0\n</span></code></pre>\n<p>Our <code>Cargo.toml</code> file will be pretty vanilla, just adding in the dependency on <code>actix-web</code>:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#657b83;\">[</span><span style=\"color:#b58900;\">package</span><span style=\"color:#657b83;\">]\n</span><span style=\"color:#268bd2;\">name </span><span style=\"color:#657b83;\">= </span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">windows-docker-web</span><span style=\"color:#839496;\">"\n</span><span style=\"color:#268bd2;\">version </span><span style=\"color:#657b83;\">= </span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">0.1.0</span><span style=\"color:#839496;\">"\n</span><span style=\"color:#268bd2;\">authors </span><span style=\"color:#657b83;\">= [</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">Michael Snoyman <msnoyman@fpcomplete.com></span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">]\n</span><span style=\"color:#268bd2;\">edition </span><span style=\"color:#657b83;\">= </span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">2018</span><span style=\"color:#839496;\">"\n\n</span><span style=\"color:#657b83;\">[</span><span style=\"color:#b58900;\">dependencies</span><span style=\"color:#657b83;\">]\n</span><span style=\"color:#268bd2;\">actix-web </span><span style=\"color:#657b83;\">= </span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">3.1</span><span style=\"color:#839496;\">"\n</span></code></pre>\n<p>If you want to see the <code>Cargo.lock</code> file I compiled with, it's <a href=\"https://github.com/fpco/windows-docker-web/blob/f8a3192e63f2e699cc67716488a633f5e0893446/Cargo.lock\">available in the source repo</a>.</p>\n<p>And finally, the actual code in <code>src/main.rs</code>:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#859900;\">use </span><span style=\"color:#657b83;\">actix_web::{get, web, App, HttpServer};\n</span><span style=\"color:#859900;\">use </span><span style=\"color:#657b83;\">std::sync::Mutex;\n\n</span><span style=\"color:#268bd2;\">struct </span><span style=\"color:#b58900;\">AppState </span><span style=\"color:#657b83;\">{\n </span><span style=\"color:#268bd2;\">counter</span><span style=\"color:#657b83;\">: Mutex<</span><span style=\"color:#268bd2;\">i32</span><span style=\"color:#657b83;\">>,\n}\n\n#[</span><span style=\"color:#268bd2;\">get</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">/</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">)]\nasync </span><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">index</span><span style=\"color:#657b83;\">(</span><span style=\"color:#268bd2;\">data</span><span style=\"color:#657b83;\">: web::Data<AppState>) -> String {\n </span><span style=\"color:#268bd2;\">let </span><span style=\"color:#586e75;\">mut</span><span style=\"color:#657b83;\"> counter = data.counter.</span><span style=\"color:#859900;\">lock</span><span style=\"color:#657b83;\">().</span><span style=\"color:#859900;\">unwrap</span><span style=\"color:#657b83;\">();\n *counter += </span><span style=\"color:#6c71c4;\">1</span><span style=\"color:#657b83;\">;\n </span><span style=\"color:#859900;\">format!</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">Counter is at </span><span style=\"color:#cb4b16;\">{}</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, counter)\n}\n\n#[</span><span style=\"color:#268bd2;\">actix_web</span><span style=\"color:#657b83;\">::</span><span style=\"color:#268bd2;\">main</span><span style=\"color:#657b83;\">]\nasync </span><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">main</span><span style=\"color:#657b83;\">() -> std::io::</span><span style=\"color:#859900;\">Result</span><span style=\"color:#657b83;\"><()> {\n </span><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> host = </span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">0.0.0.0:8080</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">;\n </span><span style=\"color:#859900;\">println!</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">Trying to listen on </span><span style=\"color:#cb4b16;\">{}</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, host);\n </span><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> app_state = web::Data::new(AppState {\n counter: Mutex::new(</span><span style=\"color:#6c71c4;\">0</span><span style=\"color:#657b83;\">),\n });\n HttpServer::new(</span><span style=\"color:#586e75;\">move </span><span style=\"color:#859900;\">|| </span><span style=\"color:#657b83;\">App::new().</span><span style=\"color:#859900;\">app_data</span><span style=\"color:#657b83;\">(app_state.</span><span style=\"color:#859900;\">clone</span><span style=\"color:#657b83;\">()).</span><span style=\"color:#859900;\">service</span><span style=\"color:#657b83;\">(index))\n .</span><span style=\"color:#859900;\">bind</span><span style=\"color:#657b83;\">(host)</span><span style=\"color:#859900;\">?\n </span><span style=\"color:#657b83;\">.</span><span style=\"color:#859900;\">run</span><span style=\"color:#657b83;\">()\n .await\n}\n</span></code></pre>\n<p>This code creates an application state (a mutex of an <code>i32</code>), defines a single <code>GET</code> handler that increments that variable and prints the current value, and then hosts this on <code>0.0.0.0:8080</code>. Not too shabby.</p>\n<p>If you're following along with the code, now would be a good time to <code>cargo run</code> and make sure you're able to load up the site on your <code>localhost:8080</code>.</p>\n<h2 id=\"dockerfile\">Dockerfile</h2>\n<p>If this is your first foray into Windows Containers, you may be surprised to hear me say "Dockerfile." Windows Container images can be built with the same kind of Dockerfiles you're used to from the Linux world. This even supports more advanced features, such as multistage Dockerfiles, which we're going to take advantage of here.</p>\n<p>There are a number of different base images provided by Microsoft for Windows Containers. We're going to be using Windows Server Core. It provides enough capabilities for installing Rust dependencies (which we'll see shortly), without including too much unneeded extras. Nanoserver is a much lighterweight image, but it doesn't play nicely with the Microsoft Visual C++ runtime we're using for the <code>-msvc</code> Rust target.</p>\n<p><strong>NOTE</strong> I've elected to use the <code>-msvc</code> target here instead of <code>-gnu</code> for two reasons. Firstly, it's closer to the actual use cases we need to support in Kube360, and therefore made a better test case. Also, as the default target for Rust on Windows, it seemed appropriate. It should be possible to set up a more minimal nanoserver-based image based on the <code>-gnu</code> target, if someone's interested in a "fun" side project.</p>\n<p>The <a href=\"https://github.com/fpco/windows-docker-web/blob/f8a3192e63f2e699cc67716488a633f5e0893446/Dockerfile\">complete Dockerfile is available on Github</a>, but let's step through it more carefully. As mentioned, we'll be performing a multistage build. We'll start with the build image, which will install the Rust build toolchain and compile our application. We start off by using the Windows Server Core base image and switching the shell back to the standard <code>cmd.exe</code>:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#657b83;\">FROM mcr.microsoft.com/windows/servercore:1809 as build\n\n# Restore the default Windows shell for correct batch processing.\nSHELL ["cmd", "/S", "/C"]\n</span></code></pre>\n<p>Next we're going to install the Visual Studio buildtools necessary for building Rust code:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#657b83;\"># Download the Build Tools bootstrapper.\nADD https://aka.ms/vs/16/release/vs_buildtools.exe /vs_buildtools.exe\n\n# Install Build Tools with the Microsoft.VisualStudio.Workload.AzureBuildTools workload,\n# excluding workloads and components with known issues.\nRUN vs_buildtools.exe --quiet --wait --norestart --nocache \\\n --installPath C:\\BuildTools \\\n --add Microsoft.Component.MSBuild \\\n --add Microsoft.VisualStudio.Component.Windows10SDK.18362 \\\n --add Microsoft.VisualStudio.Component.VC.Tools.x86.x64\t\\\n || IF "%ERRORLEVEL%"=="3010" EXIT 0\n</span></code></pre>\n<p>And then we'll modify the entrypoint to include the environment modifications necessary to use those buildtools:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#657b83;\"># Define the entry point for the docker container.\n# This entry point starts the developer command prompt and launches the PowerShell shell.\nENTRYPOINT ["C:\\\\BuildTools\\\\Common7\\\\Tools\\\\VsDevCmd.bat", "&&", "powershell.exe", "-NoLogo", "-ExecutionPolicy", "Bypass"]\n</span></code></pre>\n<p>Next up is installing <code>rustup</code>, which is fortunately pretty easy:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#657b83;\">RUN curl -fSLo rustup-init.exe https://win.rustup.rs/x86_64\nRUN start /w rustup-init.exe -y -v && echo "Error level is %ERRORLEVEL%"\nRUN del rustup-init.exe\n\nRUN setx /M PATH "C:\\Users\\ContainerAdministrator\\.cargo\\bin;%PATH%"\n</span></code></pre>\n<p>Then we copy over the relevant source files and kick off a build, storing the generated executable in <code>c:\\output</code>:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#657b83;\">COPY Cargo.toml /project/Cargo.toml\nCOPY Cargo.lock /project/Cargo.lock\nCOPY rust-toolchain /project/rust-toolchain\nCOPY src/ /project/src\nRUN cargo install --path /project --root /output\n</span></code></pre>\n<p>And with that, we're done with our build! Time to jump over to our runtime image. We don't need the Visual Studio buildtools in this image, but we do need the Visual C++ runtime:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#657b83;\">FROM mcr.microsoft.com/windows/servercore:1809\n\nADD https://download.microsoft.com/download/6/A/A/6AA4EDFF-645B-48C5-81CC-ED5963AEAD48/vc_redist.x64.exe /vc_redist.x64.exe\nRUN c:\\vc_redist.x64.exe /install /quiet /norestart\n</span></code></pre>\n<p>With that in place, we can copy over our executable from the build image and set it as the default <code>CMD</code> in the image:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#657b83;\">COPY --from=build c:/output/bin/windows-docker-web.exe /\n\nCMD ["/windows-docker-web.exe"]\n</span></code></pre>\n<p>And just like that, we've got a real life Windows Container. If you'd like to, you can test it out yourself by running:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#657b83;\">> docker run --rm -p 8080:8080 fpco/windows-docker-web:f8a3192e63f2e699cc67716488a633f5e0893446\n</span></code></pre>\n<p>If you connect to port 8080, you should see our painfully simple app. Hurrah!</p>\n<h2 id=\"building-with-github-actions\">Building with Github Actions</h2>\n<p>One of the nice things about using a multistage Dockerfile for performing the build is that our CI scripts become very simple. Instead of needing to set up an environment with correct build tools or any other configuration, our script:</p>\n<ul>\n<li>Logs into the Docker Hub registry</li>\n<li>Performs a <code>docker build</code></li>\n<li>Pushes to the Docker Hub registry</li>\n</ul>\n<p>The downside is that there is no build caching at play with this setup. There are multiple methods to mitigate this problem, such as creating helper build images that pre-bake the dependencies. Or you can perform the builds on the host on CI and only use the Dockerfile for generating the runtime image. Those are interesting tweaks to try out another time. </p>\n<p>Taking on the simple multistage approach though, we have the following in our <code>.github/workflows/container.yml</code> file:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">name</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">Build a Windows container\n\n</span><span style=\"color:#b58900;\">on</span><span style=\"color:#657b83;\">:\n </span><span style=\"color:#268bd2;\">push</span><span style=\"color:#657b83;\">:\n </span><span style=\"color:#268bd2;\">branches</span><span style=\"color:#657b83;\">: [</span><span style=\"color:#2aa198;\">master</span><span style=\"color:#657b83;\">]\n\n</span><span style=\"color:#268bd2;\">jobs</span><span style=\"color:#657b83;\">:\n </span><span style=\"color:#268bd2;\">build</span><span style=\"color:#657b83;\">:\n </span><span style=\"color:#268bd2;\">runs-on</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">windows-latest\n\n steps</span><span style=\"color:#657b83;\">:\n - </span><span style=\"color:#268bd2;\">uses</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">actions/checkout@v1\n\n </span><span style=\"color:#657b83;\">- </span><span style=\"color:#268bd2;\">name</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">Build and push\n shell</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">bash\n run</span><span style=\"color:#657b83;\">: </span><span style=\"color:#859900;\">|\n</span><span style=\"color:#2aa198;\"> echo "${{ secrets.DOCKER_HUB_TOKEN }}" | docker login --username fpcojenkins --password-stdin\n IMAGE_ID=fpco/windows-docker-web:$GITHUB_SHA\n docker build -t $IMAGE_ID .\n docker push $IMAGE_ID\n</span></code></pre>\n<p>I like following the convention of tagging my images with the Git SHA of the commit. Other people prefer different tagging schemes, it's all up to you.</p>\n<h2 id=\"manifest-files\">Manifest files</h2>\n<p>Now that we have a working Windows Container image, the next step is to deploy it to our Kube360 cluster. Generally, we use ArgoCD and Kustomize for managing app deployments within Kube360, which lets us keep a very nice Gitops workflow. Instead, for this blog post, I'll show you the raw manifest files. It will also let us play with the <code>k3</code> command line tool, which also happens to be written in Rust.</p>\n<p>First we'll have a Deployment manifest to manage the pods running the application itself. Since this is a simple Rust application, we can put very low resource limits on this. We're going to disable the Istio sidebar, since it's not compatible with Windows. We're going to ask Kubernetes to use the Windows machines to host these pods. And we're going to set up some basic health checks. All told, this is what our manifest file looks like:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">apiVersion</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">apps/v1\nkind</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">Deployment\nmetadata</span><span style=\"color:#657b83;\">:\n </span><span style=\"color:#268bd2;\">name</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">windows-docker-web\n labels</span><span style=\"color:#657b83;\">:\n </span><span style=\"color:#268bd2;\">app.kubernetes.io/component</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">webserver\nspec</span><span style=\"color:#657b83;\">:\n </span><span style=\"color:#268bd2;\">replicas</span><span style=\"color:#657b83;\">: </span><span style=\"color:#6c71c4;\">1\n </span><span style=\"color:#268bd2;\">minReadySeconds</span><span style=\"color:#657b83;\">: </span><span style=\"color:#6c71c4;\">5\n </span><span style=\"color:#268bd2;\">selector</span><span style=\"color:#657b83;\">:\n </span><span style=\"color:#268bd2;\">matchLabels</span><span style=\"color:#657b83;\">:\n </span><span style=\"color:#268bd2;\">app.kubernetes.io/component</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">webserver\n template</span><span style=\"color:#657b83;\">:\n </span><span style=\"color:#268bd2;\">metadata</span><span style=\"color:#657b83;\">:\n </span><span style=\"color:#268bd2;\">labels</span><span style=\"color:#657b83;\">:\n </span><span style=\"color:#268bd2;\">app.kubernetes.io/component</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">webserver\n annotations</span><span style=\"color:#657b83;\">:\n </span><span style=\"color:#268bd2;\">sidecar.istio.io/inject</span><span style=\"color:#657b83;\">: </span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">false</span><span style=\"color:#839496;\">"\n </span><span style=\"color:#268bd2;\">spec</span><span style=\"color:#657b83;\">:\n </span><span style=\"color:#268bd2;\">runtimeClassName</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">windows-2019\n containers</span><span style=\"color:#657b83;\">:\n - </span><span style=\"color:#268bd2;\">name</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">windows-docker-web\n image</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">fpco/windows-docker-web:f8a3192e63f2e699cc67716488a633f5e0893446\n ports</span><span style=\"color:#657b83;\">:\n - </span><span style=\"color:#268bd2;\">name</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">http\n containerPort</span><span style=\"color:#657b83;\">: </span><span style=\"color:#6c71c4;\">8080\n </span><span style=\"color:#268bd2;\">readinessProbe</span><span style=\"color:#657b83;\">:\n </span><span style=\"color:#268bd2;\">httpGet</span><span style=\"color:#657b83;\">:\n </span><span style=\"color:#268bd2;\">path</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">/\n port</span><span style=\"color:#657b83;\">: </span><span style=\"color:#6c71c4;\">8080\n </span><span style=\"color:#268bd2;\">initialDelaySeconds</span><span style=\"color:#657b83;\">: </span><span style=\"color:#6c71c4;\">10\n </span><span style=\"color:#268bd2;\">periodSeconds</span><span style=\"color:#657b83;\">: </span><span style=\"color:#6c71c4;\">10\n </span><span style=\"color:#268bd2;\">livenessProbe</span><span style=\"color:#657b83;\">:\n </span><span style=\"color:#268bd2;\">httpGet</span><span style=\"color:#657b83;\">:\n </span><span style=\"color:#268bd2;\">path</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">/\n port</span><span style=\"color:#657b83;\">: </span><span style=\"color:#6c71c4;\">8080\n </span><span style=\"color:#268bd2;\">initialDelaySeconds</span><span style=\"color:#657b83;\">: </span><span style=\"color:#6c71c4;\">10\n </span><span style=\"color:#268bd2;\">periodSeconds</span><span style=\"color:#657b83;\">: </span><span style=\"color:#6c71c4;\">10\n </span><span style=\"color:#268bd2;\">resources</span><span style=\"color:#657b83;\">:\n </span><span style=\"color:#268bd2;\">requests</span><span style=\"color:#657b83;\">:\n </span><span style=\"color:#268bd2;\">memory</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">128Mi\n cpu</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">100m\n limits</span><span style=\"color:#657b83;\">:\n </span><span style=\"color:#268bd2;\">memory</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">128Mi\n cpu</span><span style=\"color:#657b83;\">: </span><span style=\"color:#2aa198;\">100m\n</span></code></pre>\n<p>Awesome, that's the most complicated by far of the three manifests. Next we'll put a fairly stock-standard Service in front of that deployment:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">apiVersion</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">v1\nkind</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">Service\nmetadata</span><span style=\"color:#657b83;\">:\n </span><span style=\"color:#268bd2;\">name</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">windows-docker-web\n labels</span><span style=\"color:#657b83;\">:\n </span><span style=\"color:#268bd2;\">app.kubernetes.io/component</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">webserver\nspec</span><span style=\"color:#657b83;\">:\n </span><span style=\"color:#268bd2;\">ports</span><span style=\"color:#657b83;\">:\n - </span><span style=\"color:#268bd2;\">name</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">http\n port</span><span style=\"color:#657b83;\">: </span><span style=\"color:#6c71c4;\">80\n </span><span style=\"color:#268bd2;\">targetPort</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">http\n type</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">ClusterIP\n selector</span><span style=\"color:#657b83;\">:\n </span><span style=\"color:#268bd2;\">app.kubernetes.io/component</span><span style=\"color:#657b83;\">: </span><span style=\"color:#2aa198;\">webserver\n</span></code></pre>\n<p>This exposes a services on port 80, and targets the <code>http</code> port (port 8080) inside the deployment. Finally, we have our Ingress. Kube360 uses external DNS to automatically set DNS records, and cert-manager to automatically grab TLS certificates. Our manifest looks like this:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">apiVersion</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">networking.k8s.io/v1beta1\nkind</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">Ingress\nmetadata</span><span style=\"color:#657b83;\">:\n </span><span style=\"color:#268bd2;\">annotations</span><span style=\"color:#657b83;\">:\n </span><span style=\"color:#268bd2;\">cert-manager.io/cluster-issuer</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">letsencrypt-ingress-prod\n kubernetes.io/ingress.class</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">nginx\n nginx.ingress.kubernetes.io/force-ssl-redirect</span><span style=\"color:#657b83;\">: </span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">true</span><span style=\"color:#839496;\">"\n </span><span style=\"color:#268bd2;\">name</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">windows-docker-web\nspec</span><span style=\"color:#657b83;\">:\n </span><span style=\"color:#268bd2;\">rules</span><span style=\"color:#657b83;\">:\n - </span><span style=\"color:#268bd2;\">host</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">windows-docker-web.az.fpcomplete.com\n http</span><span style=\"color:#657b83;\">:\n </span><span style=\"color:#268bd2;\">paths</span><span style=\"color:#657b83;\">:\n - </span><span style=\"color:#268bd2;\">backend</span><span style=\"color:#657b83;\">:\n </span><span style=\"color:#268bd2;\">serviceName</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">windows-docker-web\n servicePort</span><span style=\"color:#657b83;\">: </span><span style=\"color:#6c71c4;\">80\n </span><span style=\"color:#268bd2;\">tls</span><span style=\"color:#657b83;\">:\n - </span><span style=\"color:#268bd2;\">hosts</span><span style=\"color:#657b83;\">:\n - </span><span style=\"color:#268bd2;\">windows-docker-web.az.fpcomplete.com\n secretName</span><span style=\"color:#657b83;\">: </span><span style=\"color:#2aa198;\">windows-docker-web-tls\n</span></code></pre>\n<p>Now that we have our application inside a Docker image, and we have our manifest files to instruct Kubernetes on how to run it, we just need to deploy these manifests and we'll be done.</p>\n<h2 id=\"launch\">Launch</h2>\n<p>With our manifests in place, we can finally deploy them. You can use <code>kubectl</code> directly to do this. Since I'm deploying to Kube360, I'm going to use the <code>k3</code> command line tool, which automates the process of logging in, getting temporary Kubernetes credentials, and providing those to the <code>kubectl</code> command via an environment variable. These steps could be run on Windows, Mac, or Linux. But since we've done the rest of this post on Windows, I'll use my Windows machine for this too.</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#657b83;\">> k3 init test.az.fpcomplete.com\n> k3 kubectl apply -f deployment.yaml\nWeb browser opened to https://test.az.fpcomplete.com/k3-confirm?nonce=c1f764d8852f4ff2a2738fb0a2078e68\nPlease follow the login steps there (if needed).\nThen return to this terminal.\nPolling the server. Please standby.\nChecking ...\nThanks, got the token response. Verifying token is valid\nRetrieving a kubeconfig for use with k3 kubectl\nKubeconfig retrieved. You are now ready to run kubectl commands with `k3 kubectl ...`\ndeployment.apps/windows-docker-web created\n> k3 kubectl apply -f ingress.yaml\ningress.networking.k8s.io/windows-docker-web created\n> k3 kubectl apply -f service.yaml\nservice/windows-docker-web created\n</span></code></pre>\n<p>I told <code>k3</code> to use the <code>test.az.fpcomplete.com</code> cluster. On the first <code>k3 kubectl</code> call, it detected that I did not have valid credentials for the cluster, and opened up my browser to a page that allowed me to log in. One of the design goals in Kube360 is to strongly leverage existing identity providers, such as Azure AD, Google Directory, Okta, Microsoft 365, and others. This is not only more secure than copy-pasting <code>kubeconfig</code> files with permanent credentials around, but more user friendly. As you can see, the process above was pretty automated.</p>\n<p>It's easy enough to check that the pods are actually running and healthy:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#657b83;\">> k3 kubectl get pods\nNAME READY STATUS RESTARTS AGE\nwindows-docker-web-5687668cdf-8tmn2 1/1 Running 0 3m2s\n</span></code></pre>\n<p>Initially, the ingress controller looked like this while it was getting TLS certificates:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#657b83;\">> k3 kubectl get ingress\nNAME CLASS HOSTS ADDRESS PORTS AGE\ncm-acme-http-solver-zlq6j <none> windows-docker-web.az.fpcomplete.com 80 0s\nwindows-docker-web <none> windows-docker-web.az.fpcomplete.com 80, 443 3s\n</span></code></pre>\n<p>And after cert-manager gets the TLS certificate, it will switch over to:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#657b83;\">> k3 kubectl get ingress\nNAME CLASS HOSTS ADDRESS PORTS AGE\nwindows-docker-web <none> windows-docker-web.az.fpcomplete.com 52.151.225.139 80, 443 90s\n</span></code></pre>\n<p>And finally, our site is live! Hurrah, a Rust web application compiled for Windows and running on Kubernetes inside Azure.</p>\n<p><strong>NOTE</strong> Depending on when you read this post, the web app may or may not still be live, so don't be surprised if you don't get a response if you try to connect to that host.</p>\n<h2 id=\"conclusion\">Conclusion</h2>\n<p>This post was a bit light on actual Rust code, but heavy on a lot of Windows scripting. As I think many Rustaceans already know, the dev experience for Rust on Windows is top notch. What may not have been obvious is how pleasant the Docker experience is on Windows. There are definitely some pain points, like the large images involved and needing to install the VC runtime. But overall, with a bit of cargo-culting, it's not too bad. And finally, having a cluster with Windows support ready via Kube360 makes deployment a breeze.</p>\n<p>If anyone has follow up questions about anything here, please <a href=\"https://twitter.com/snoyberg\">reach out to me on Twitter</a> or <a href=\"https://www.fpcomplete.com/contact-us/\">contact our team at FP Complete</a>. In addition to our <a href=\"https://www.fpcomplete.com/products/kube360/\">Kube360 product offering</a>, FP Complete provides many related services, including:</p>\n<ul>\n<li><a href=\"https://www.fpcomplete.com/devops/\">DevOps consulting</a></li>\n<li><a href=\"https://www.fpcomplete.com/rust/\">Rust consulting and training</a></li>\n<li><a href=\"https://www.fpcomplete.com/services/\">General training and consulting services</a></li>\n<li><a href=\"https://www.fpcomplete.com/haskell/\">Haskell consulting and training</a></li>\n</ul>\n<p>If you liked this post, please check out some related posts:</p>\n<ul>\n<li><a href=\"https://www.fpcomplete.com/blog/2018/07/deploying-rust-with-docker-and-kubernetes/\">Deploying Rust with Docker and Kubernetes</a></li>\n<li><a href=\"https://www.fpcomplete.com/blog/rust-for-devops-tooling/\">Using Rust for DevOps tooling</a></li>\n<li><a href=\"https://www.fpcomplete.com/rust/crash-course/\">The Rust Crash Course eBook</a></li>\n<li><a href=\"https://www.fpcomplete.com/blog/devops-for-developers/\">DevOps for (Skeptical) Developers</a></li>\n<li><a href=\"https://www.fpcomplete.com/blog/understanding-cloud-auth/\">Understanding cloud auth</a></li>\n</ul>\n",
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"content": "<p>It used to be that being technically mature was thought to be a good thing; now, that view is not so cut and dried. As you look at topics like containerization, cloud migration, and DevOps, it is easy to see why young companies get to claim the term “Cloud Native.” At the same time, those who have been in business for decades are frequently relegated to the legions of those needing ‘transforming.’ While this is, of course, an overgeneralization, it feels right more often than not. So, what are the ‘mature’ to do? </p>\n<p>Talking to several older small and medium sized businesses, a few strategic changes help propel those who are thinking about tech ‘transformation’ into becoming better, faster, more cost-effective, and more secure. These strategies include focusing on containerizing business logic, cloud-enabling their enterprise, and taking a fresh look at open source offerings for their infrastructure. If we look at these topics from an executive seat rather than an engineering one, a path and a plan emerges. </p>\n<a href=\"/devops/why-what-how/\">\n<p style=\"text-align:center;font-size:2em;border-width: 3px 0;border-color:#ff8d6e;border-style: dashed;margin:1em 0;padding:0.25em 0;font-weight: bold\">\nCheck Out The Why, What, and How of DevSecOps\n</p>\n</a>\n<p>Containerization is not a new topic; it has just evolved. We have all gone from monolithic solutions to distributed computing. From there, we bought small Linux servers, and they felt like containers; then, virtualization came to market, and the VM became the new container. Now, we have Docker and Kubernetes. Docker containers represent a considerable paradigm shift in that they do not require a lot of hardware or yet another OS license…., and when managed by Kubernetes, they create an entire ecosystem with little overhead. Kubernetes take Docker containers and handle horizontal scaling, fault tolerance, automated monitoring, etc. within a DevOps toolset and frame. What makes this setup even more impressive is Open Source; yet, supported by ‘the most prominent’ tech infrastructure firms. </p>\n<p>Once we start embracing modern container architectures, the conversation gets fascinating. All cloud and virtualization providers are now battling each other to get customers to deploy these standardized workloads onto their proprietary platforms. While there are always a few complications, Docker and Kubernetes run on AWS, Azure, VMWare, GCP, etc., with little (or no) alterations if you follow the Open Source path. </p>\n<p>So imagine....once we were trying to figure out how to build in fault tolerance, scalability, continuous develop/deploy, and automate testing.....now all we need to do is follow a DevOps approach using Open Source frameworks like Docker and Kubernetes....and voila....you are there (well it isn’t that easy....but a darn sight easier than it used to be). Oh....and by the way, all of this is far easier to deploy in the cloud than on-premise, but that is a topic for another day. </p>\n<p><a href=\"https://www.fpcomplete.com/devops/why-what-how/\"><img src=\"/images/cta/why-what-how.png\" alt=\"A Quick Guide to DevOps\" /></a></p>\n",
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"content": "<p>Is it Enterprise DevOps or DevOps in the enterprise? I guess it all depends on where you sit. DevOps has been a significant change to how many modern technology organizations approach systems development and support. While many have found it to be a major productivity boost, it represents a threat in "BTTWWHADI" evangelists in some organizations. Let's start with two definitions: </p>\n<ul>\n<li>\n<p>DevOps: DevOps is a set of practices that combines software development (Dev) and IT operations (Ops). It aims to shorten the systems development life cycle and provide continuous delivery with high software quality. DevOps is complementary with Agile software development; several DevOps aspects came from Agile methodology. Credit: https://en.wikipedia.org/wiki/DevOps </p>\n</li>\n<li>\n<p>BTTWWHADI : This is shorthand for "But That's The Way We Have Always Done It." Credit: Unknown </p>\n</li>\n</ul>\n<h2 id=\"where-we-come-from\">Where we come from...</h2>\n<p>If we look at some successful Enterprise technology areas, they have had long term success by sticking with what works. Cleanly partitioned technical responsibilities (analysts, developers, DBAs, network admins, sysadmins, etc.), a waterfall approach to development, a "stay in your lane" accountability matrix (e.g., you write the app, I'll get it platformed), rack 'em and stack 'em approach to hardware, etc.</p>\n<p>While no one can deny this type of discipline has served many well, Enterprise technology's current generation offers us a much more flexible approach. Today, virtually all hardware is virtualized (on and off-premise), and cloud vendors offer things like platforms as a service, databases as a service, security as a service...etc. These innovations have allowed my companies to completely re-think how they want to be spending their technology resource (budget, people, mindshare)….with the most enlightened organizations quickly concluding that they should spend their human capital in spaces where they can create competitive advantages while purchasing those parts of their technology ecosystem what more commoditized.</p>\n<p>An example of this would be in a retail company to think more about creating business intelligence than setting up new hardware for a database server. A database can be scaled in the cloud, leaving the retail enterprise more human capital to figure out how to drive revenue. Those who are not embracing the change DevOps affords are most often using a BTTWWHADI argument. </p>\n<h2 id=\"not-everyone-is-ready-for-a-revolution\">Not everyone is ready for a revolution...</h2>\n<p>So, if DevOps is such a revolution, why do you have so many corporations having such an issue trying to get DevOps strategies to work for them? The answer lies in culture. For DevOps to be effective, an organization needs to be willing to take out a blank sheet of paper and draw a picture of what could be if they tore down yesterday's constraints and looked toward today's innovations. They need to match that picture up against their current staff, recognize that many jobs (and many skills) need to be re-learned or acquired. No longer is so much specialization required in many specific fixed assets (like data centers, computers, network devices, security devices, etc.) In a modern DevOps world, much of the infrastructure is virtualized (giving rise to infrastructure as code). </p>\n<p>To some extent, this means that your infrastructure staff will start to look more and more like developers. Instead of a team plugging in servers, routers, and load balancers into a network backbone, they will be using scripting to configure equivalent services on virtualized hardware. On the development and operational side, CI/CD pipelines and process automation drive out many manual processes involved in yesterday's software development lifecycle. For development, the beginnings of this revolution date back to test-driven development. Today's modern pipelines go from development through testing, integration, and deployment. While everything is automatable, many have stopping points in their pipeline where human interactions are required to review test results or require confirmation about final deployments to production. Whether you are in infrastructure or development, BTTWWHADI just won't do and more. To compete, everyone will need to skill up and focus on architecture, automation, XaaS, and scripting/coding to decrease time to market while improving quality and resilience. </p>\n<a href=\"/devops/creating-ecosystem/\">\n<p style=\"text-align:center;font-size:2em;border-width: 3px 0;border-color:#ff8d6e;border-style: dashed;margin:1em 0;padding:0.25em 0;font-weight: bold\">\nLearn How to Create Your DevOps Ecosystem\n</p>\n</a>\n<h2 id=\"so-what-s-the-big-deal\">So, what's the big deal…</h2>\n<p>DevOps can be a threat to those who aren't ready for it (the BTTWWHADI crowd). If your job is configuring hardware or running manual software tests, you might see these functions being automated into 'coding' jobs. This function change could pose a severe career problem for those team members who don't see this evolution coming and fail to get prepared through education and training. Unprepared staff becomes resistive to change (understandably), yet, those who are prepared end up in a better position (read: more career security, mobility, and better paid) as automation experts are now far more sought after than traditional hardware configuration engineers (as a gross generalization). Please do not misunderstand; traditional system engineers are still valuable members of most enterprise teams, but as DevOps and virtualization take hold, those jobs will change. Get prepared, train your staff, and address the culture change head-on. </p>\n<p>If you need help with your journey, <a href=\"https://www.fpcomplete.com/contact-us/\">contact FP Complete</a>. This is who we are and what we do. </p>\n<p><a href=\"https://www.fpcomplete.com/devops/creating-ecosystem/\"><img src=\"/images/cta/creating-devops-ecosystem.png\" alt=\"A Quick Guide to DevOps\" /></a></p>\n",
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"content": "<p>Does this mean if you weren't born in the cloud, you'll never be as good as those who are? </p>\n<p>When thinking about building from scratch or modernizing an existing technology environment, we tend to see one of a few different things happening: </p>\n<ul>\n<li>Staff will read up, and you will try it on your own. </li>\n<li>Managers will hire someone who says they have done it before. </li>\n<li>Leaders will engage a large software vendor or consulting firm to help get them to the promised land. </li>\n</ul>\n<p>While all of these strategies can work, we often find one of the following happens: </p>\n<ul>\n<li>Trial and error result in very expensive under delivery. </li>\n<li>Existing teams become disaffected and resistive because they perceive being left behind. </li>\n<li>Something gets delivered, but costs go up, and reliability goes down. </li>\n<li>New hires come in, make the magic happen, and then move on without leaving enough knowhow to continue without them. </li>\n<li>Vendors use proprietary software, and a new age of vendor lock-in ensues. </li>\n</ul>\n<p>There is a better way of approaching modernizing a business-focused, legacy world. Our core approach at FP complete is: </p>\n<ul>\n<li>Be vendor agnostic </li>\n<li>Build a road map based on business outcomes </li>\n<li>Deeply understand and implement DevOps concepts </li>\n<li>Be ruthlessly focused on architecture from the start </li>\n<li>Containerize everything* </li>\n<li>Virtualize everything*</li>\n</ul>\n<p>While this approach is straightforward, staying focused on outcomes is the key: </p>\n<ul>\n<li>The business logic is the key to build your ecosystem once and properly so you can focus on what matters. </li>\n<li>Integrate security by design as security is a non-non-negotiable. </li>\n<li>All alerts and logs centrally as managing and operating via complete transparency is key. </li>\n<li>Ensure Containers are made to scale horizontally and be fault-tolerant from the start. </li>\n<li>Ensure you are on-prem and cloud-agnostic. </li>\n<li>Be open-source but get enterprise support. </li>\n</ul>\n<a href=\"/devops/quick-guide/\">\n<p style=\"text-align:center;font-size:2em;border-width: 3px 0;border-color:#ff8d6e;border-style: dashed;margin:1em 0;padding:0.25em 0;font-weight: bold\">\nCheck out our Quick Guide to DevOps\n</p>\n</a>\n<p>How do you get help without breaking the bank, compromising your values, or getting locked in? </p>\n<p>At FP Complete, we believe the way to get started is to: </p>\n<ul>\n<li>Build DevOps expertise, acquire DevOps Tooling. </li>\n<li>Get help constructing your roadmap to ensure technical focus aligns with business results. </li>\n<li>Get help designing how your applications will get containerized to be cloud-ready. </li>\n<li>Acquire Enterprise support for your newly open-sourced world. </li>\n</ul>\n<p>FP Complete has a unique track record in these activities. We are not built on recurring revenue from long term consulting. We are built on helping our customers build better software, run better technology operations, and achieve better business outcomes. We come from diverse backgrounds and have serviced a myriad of industries. We often find that others have already solved many of our client's problems, and our expertise lies in matching existing solutions to places where they are needed most. </p>\n<p>So, what is the best way to get started: </p>\n<ol>\n<li>Please send us a mail or call us up. </li>\n<li>We will walk through your aspirations and provide a high-level road map for achieving your goals at no cost. </li>\n<li>If you like what you see, invite us in for a POC based on a 100% ROI. </li>\n<li>Scale from there. </li>\n</ol>\n<p>If you are unsure about the claims in this post, shoot me an email...you won't get a bot response… you'll get me. </p>\n<p>*Note: the exceptions to these rules are usually around ultra-low latency requirements. </p>\n<p><a href=\"https://www.fpcomplete.com/devops/quick-guide/\"><img src=\"/images/cta/quick-guide-devops.png\" alt=\"A Quick Guide to DevOps\" /></a></p>\n",
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"content": "<p>A beginner's guide to writing your DevOps tools in Rust.</p>\n<h2 id=\"introduction\">Introduction</h2>\n<p>In this blog post we'll cover some basic DevOps use cases for Rust and why \nyou would want to use it.\nAs part of this, we'll also cover a few common libraries you will likely use\nin a Rust-based DevOps tool for AWS.</p>\n<p>If you're already familiar with writing DevOps tools in other languages,\nthis post will explain why you should try Rust.</p>\n<p>We'll cover why Rust is a particularly good choice of language to write your DevOps\ntooling and critical cloud infrastructure software in.\nAnd we'll also walk through a small demo DevOps tool written in Rust. \nThis project will be geared towards helping someone new to the language ecosystem \nget familiar with the Rust project structure.</p>\n<p>If you're brand new to Rust, and are interested in learning the language, you may want to start off with our <a href=\"https://www.fpcomplete.com/rust/crash-course/\">Rust Crash Course eBook</a>.</p>\n<h2 id=\"what-makes-the-rust-language-unique\">What Makes the Rust Language Unique</h2>\n<blockquote>\n<p>Rust is a systems programming language focused on three goals: safety, speed, \nand concurrency. It maintains these goals without having a garbage collector, \nmaking it a useful language for a number of use cases other languages aren’t \ngood at: embedding in other languages, programs with specific space and time \nrequirements, and writing low-level code, like device drivers and operating systems. </p>\n</blockquote>\n<p><em>The Rust Book (first edition)</em></p>\n<p>Rust was initially created by Mozilla and has since gained widespread adoption and\nsupport. As the quote from the Rust book alludes to, it was designed to fill the \nsame space that C++ or C would (in that it doesn’t have a garbage collector or a runtime).\nBut Rust also incorporates zero-cost abstractions and many concepts that you would\nexpect in a higher level language (like Go or Haskell).\nFor that, and many other reasons, Rust's uses have expanded well beyond that\noriginal space as low level safe systems language.</p>\n<p>Rust's ownership system is extremely useful in efforts to write correct and \nresource efficient code. Ownership is one of the killer features of the Rust \nlanguage and helps programmers catch classes of resource errors at compile time \nthat other languages miss or ignore.</p>\n<p>Rust is an extremely performant and efficient language, comparable to the speeds \nyou see with idiomatic everyday C or C++.\nAnd since there isn’t a garbage collector in Rust, it’s a lot easier to get \npredictable deterministic performance.</p>\n<h2 id=\"rust-and-devops\">Rust and DevOps</h2>\n<p>What makes Rust unique also makes it very useful for areas stemming from robots \nto rocketry, but are those qualities relevant for DevOps?\nDo we care if we have efficient executables or fine grained control over \nresources, or is Rust a bit overkill for what we typically need in DevOps?</p>\n<p><em>Yes and no</em></p>\n<p>Rust is clearly useful for situations where performance is crucial and actions \nneed to occur in a deterministic and consistent way. That obviously translates to \nlow-level places where previously C and C++ were the only game in town. \nIn those situations, before Rust, people simply had to accept the inherent risk and \nadditional development costs of working on a large code base in those languages.\nRust now allows us to operate in those areas but without the risk that C and C++\ncan add.</p>\n<p>But with DevOps and infrastructure programming we aren't constrained by those \nrequirements. For DevOps we've been able to choose from languages like Go, Python, \nor Haskell because we're not strictly limited by the use case to languages without \ngarbage collectors. Since we can reach for other languages you might argue \nthat using Rust is a bit overkill, but let's go over a few points to counter this.</p>\n<h3 id=\"why-you-would-want-to-write-your-devops-tools-in-rust\">Why you would want to write your DevOps tools in Rust</h3>\n<ul>\n<li>Small executables relative to other options like Go or Java</li>\n<li>Easy to port across different OS targets</li>\n<li>Efficient with resources (which helps cut down on your AWS bill) </li>\n<li>One of the fastest languages (even when compared to C)</li>\n<li>Zero cost abstractions - Rust is a low level performant language which also\ngives the us benefits of a high level language with its generics and abstractions.</li>\n</ul>\n<p>To elaborate on some of these points a bit further:</p>\n<h4 id=\"os-targets-and-cross-compiling-rust-for-different-architectures\">OS targets and Cross Compiling Rust for different architectures</h4>\n<p>For DevOps it's also worth mentioning the (relative) ease with which you can \nport your Rust code across different architectures and different OS's. </p>\n<p>Using the official Rust toolchain installer <code>rustup</code>, it's easy to get the \nstandard library for your target platform.\nRust <a href=\"https://doc.rust-lang.org/nightly/rustc/platform-support.html\">supports a great number of platforms</a>\nwith different tiers of support.\nThe docs for the <code>rustup</code> tool has <a href=\"https://rust-lang.github.io/rustup/cross-compilation.html\">a section</a>\ncovering how you can access pre-compiled artifacts for various architectures.\nTo install the target platform for an architecture (other than the host platform which is installed by default)\nyou simply need to run <code>rustup target add</code>:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#657b83;\">$ rustup target add x86_64-pc-windows-msvc \ninfo: downloading component 'rust-std' for 'x86_64-pc-windows-msvc'\ninfo: installing component 'rust-std' for 'x86_64-pc-windows-msvc'\n</span></code></pre>\n<p>Cross compilation is already built into the Rust compiler by default. \nOnce the <code>x86_64-pc-windows-msvc</code> target is installed you can build for Windows \nwith the <code>cargo</code> build tool using the <code>--target</code> flag:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#657b83;\">cargo build --target=x86_64-pc-windows-msvc\n</span></code></pre>\n<p>(the default target is always the host architecture)</p>\n<p>If one of your dependencies links to a native (i.e. non-Rust) library, you will\nneed to make sure that those cross compile as well. Doing <code>rustup target add</code>\nonly installs the Rust standard library for that target. However for the other \ntools that are often needed when cross-compiling, there is the handy\n<a href=\"https://github.com/rust-embedded/cross\">github.com/rust-embedded/cross</a> tool.\nThis is essentially a wrapper around cargo which does all cross compilation in \ndocker images that have all the necessary bits (linkers) and pieces installed.</p>\n<h4 id=\"small-executables\">Small Executables</h4>\n<p>A key unique feature of Rust is that it doesn't need a runtime or a garbage collector.\nCompare this to languages like Python or Haskell: with Rust the lack of any runtime\ndependencies (Python), or system libraries (as with Haskell) is a huge advantage \nfor portability.</p>\n<p>For practical purposes, as far as DevOps is concerned, this portability means \nthat Rust executables are much easier to deploy than scripts.\nWith Rust, compared to Python or Bash, we don't need to set up the environment for \nour code ahead of time. This frees us up from having to worry if the runtime \ndependencies for the language are set up.</p>\n<p>In addition to that, with Rust you're able to produce 100% static executables for \nLinux using the MUSL libc (and by default Rust will statically link all Rust code). \nThis means that you can deploy your Rust DevOps tool's binaries across your Linux \nservers without having to worry if the correct <code>libc</code> or other libraries were \ninstalled beforehand.</p>\n<p>Creating static executables for Rust is simple. As we discussed before, when discussing\ndifferent OS targets, it's easy with Rust to switch the target you're building against.\nTo compile static executables for the Linux MUSL target all you need to do is add \nthe <code>musl</code> target with:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#657b83;\">$ rustup target add x86_64-unknown-linux-musl\n</span></code></pre>\n<p>Then you can using this new target to build your Rust project as a fully static \nexecutable with:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#657b83;\">$ cargo build --target x86_64-unknown-linux-musl\n</span></code></pre>\n<p>As a result of not having a runtime or a garbage collector, Rust executables \ncan be extremely small. For example, there is a common DevOps tool called \nCredStash that was originally written in Python but has since been \nported to Go (GCredStash) and now Rust (RuCredStash).</p>\n<p>Comparing the executable sizes of the Rust versus Go implementations of CredStash,\nthe Rust executable is nearly a quarter of the size of the Go variant. </p>\n<table><thead><tr><th>Implementation</th><th>Executable Size</th></tr></thead><tbody>\n<tr><td>Rust CredStash: (RuCredStash Linux amd64)</td><td>3.3 MB</td></tr>\n<tr><td>Go CredStash: (GCredStash Linux amd64 v0.3.5)</td><td>11.7 MB</td></tr>\n</tbody></table>\n<p>Project links:</p>\n<ul>\n<li><a href=\"https://github.com/psibi/rucredstash\">github.com/psibi/rucredstash</a></li>\n<li><a href=\"https://github.com/winebarrel/gcredstash\">github.com/winebarrel/gcredstash</a></li>\n</ul>\n<p>This is by no means a perfect comparison, and 8 MB may not seem like a lot, but\nconsider the advantage automatically of having executables that are a quarter of the \nsize you would typically expect. </p>\n<p>This cuts down on the size your Docker images, AWS AMI's, or Azure VM images need\nto be - and that helps speed up the time it takes to spin up new deployments.</p>\n<p>With a tool of this size, having an executable that is 75% smaller than it \nwould be otherwise is not immediately apparent. On this scale the difference, 8 MB,\nis still quite cheap.\nBut with larger tools (or collections of tools and Rust based software) the benefits\nadd up and the difference begins to be a practical and worthwhile consideration.</p>\n<p>The Rust implementation was also not strictly written with the resulting size of \nthe executable in mind. So if executable size was even more important of a \nfactor other changes could be made - but that's beyond the scope of this post.</p>\n<h4 id=\"rust-is-fast\">Rust is fast</h4>\n<p>Rust is very fast even for common idiomatic everyday Rust code. And not only that\nit's arguably easier to work with than with C and C++ and catch errors in your \ncode.</p>\n<p>For the Fortunes benchmark (which exercises the ORM, \ndatabase connectivity, dynamic-size collections, sorting, server-side templates, \nXSS countermeasures, and character encoding) Rust is second and third, only lagging \nbehind the first place C++ based framework by 4 percent. </p>\n<img src=\"/images/blog/techempower-benchmarks-round-19-fortunes.png\" style=\"max-width:95%\">\n<p>In the benchmark for database access for a single query Rust is first and second:</p>\n<img src=\"/images/blog/techempower-benchmarks-round-19-single-query.png\" style=\"max-width:95%\">\n<p>And in a composite of all the benchmarks Rust based frameworks are second and third place.</p>\n<img src=\"/images/blog/techempower-benchmarks-round-19-composite.png\" style=\"max-width:95%\">\n<p>Of course language and framework benchmarks are not real life, however this is \nstill a fair comparison of the languages as they relate to others (within the context \nand the focus of the benchmark).</p>\n<p>Source: <a href=\"https://www.techempower.com/benchmarks/\">https://www.techempower.com/benchmarks</a></p>\n<h3 id=\"why-would-you-not-want-to-write-your-devops-tools-in-rust\">Why would you not want to write your DevOps tools in Rust?</h3>\n<p>For medium to large projects, it’s important to have a type system and compile \ntime checks like those in Rust versus what you would find in something like Python\nor Bash.\nThe latter languages let you get away with things far more readily. This makes \ndevelopment much "faster" in one sense.</p>\n<p>Certain situations, especially those with involving small project codebases, would \nbenefit more from using an interpreted language. In these cases, being able to quickly \nchange pieces of the code without needing to re-compile and re-deploy the project\noutweighs the benefits (in terms of safety, execution speed, and portability)\nthat languages like Rust bring. </p>\n<p>Working with and iterating on a Rust codebase in those circumstances, with frequent\nbut small codebases changes, would be needlessly time-consuming\nIf you have a small codebase with few or no runtime dependencies, then it wouldn't\nbe worth it to use Rust.</p>\n<h2 id=\"demo-devops-project-for-aws\">Demo DevOps Project for AWS</h2>\n<p>We'll briefly cover some of the libraries typically used for an AWS focused \nDevOps tool in a walk-through of a small demo Rust project here. \nThis aims to provide a small example that uses some of the libraries you'll likely\nwant if you’re writing a CLI based DevOps tool in Rust. Specifically for this \nexample we'll show a tool that does some basic operations against AWS S3 \n(creating new buckets, adding files to buckets, listing the contents of buckets).</p>\n<h3 id=\"project-structure\">Project structure</h3>\n<p>For AWS integration we're going to utilize the <a href=\"https://www.rusoto.org/\">Rusoto</a> library.\nSpecifically for our modest demo Rust DevOps tools we're going to pull in the \n<a href=\"https://docs.rs/rusoto_core/0.45.0/rusoto_core/\">rusoto_core</a> and the \n<a href=\"https://docs.rs/rusoto_s3/0.45.0/rusoto_s3/\">rusoto_s3</a> crates (in Rust a <em>crate</em>\nis akin to a library or package).</p>\n<p>We're also going to use the <a href=\"https://docs.rs/structopt/0.3.16/structopt/\">structopt</a> crate\nfor our CLI options. This is a handy, batteries included CLI library that makes \nit easy to create a CLI interface around a Rust struct. </p>\n<p>The tool operates by matching the CLI option and arguments the user passes in \nwith a <a href=\"https://github.com/fpco/rust-aws-devops/blob/54d6cfa4bb7a9a15c2db52976f2b7057431e0c5e/src/main.rs#L211\"><code>match</code> expression</a>.</p>\n<p>We can then use this to match on that part of the CLI option struct we've defined \nand call the appropriate functions for that option.</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#859900;\">match</span><span style=\"color:#657b83;\"> opt {\n Opt::Create { bucket: bucket_name } </span><span style=\"color:#859900;\">=> </span><span style=\"color:#657b83;\">{\n </span><span style=\"color:#859900;\">println!</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">Attempting to create a bucket called: </span><span style=\"color:#cb4b16;\">{}</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, bucket_name);\n </span><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> demo = S3Demo::new(bucket_name);\n </span><span style=\"color:#859900;\">create_demo_bucket</span><span style=\"color:#657b83;\">(</span><span style=\"color:#859900;\">&</span><span style=\"color:#657b83;\">demo);\n },\n</span></code></pre>\n<p>This matches on the <a href=\"https://github.com/fpco/rust-aws-devops/blob/54d6cfa4bb7a9a15c2db52976f2b7057431e0c5e/src/main.rs#L182\"><code>Create</code></a>\nvariant of the <code>Opt</code> enum. </p>\n<p>We then use <code>S3Demo::new(bucket_name)</code> to create a new <code>S3Client</code> which we can\nuse in the standalone <code>create_demo_bucket</code> function that we've defined \nwhich will create a new S3 bucket.</p>\n<p>The tool is fairly simple with most of the code located in \n<a href=\"https://github.com/fpco/rust-aws-devops/blob/54d6cfa4bb7a9a15c2db52976f2b7057431e0c5e/src/main.rs\">src/main.rs</a></p>\n<h3 id=\"building-the-rust-project\">Building the Rust project</h3>\n<p>Before you build the code in this project, you will need to install Rust. \nPlease follow <a href=\"https://www.rust-lang.org/tools/install\">the official install instructions here</a>.</p>\n<p>The default build tool for Rust is called Cargo. It's worth getting familiar \nwith <a href=\"https://doc.rust-lang.org/cargo/guide/\">the docs for Cargo</a>\nbut here's a quick overview for building the project.</p>\n<p>To build the project run the following from the root of the \n<a href=\"https://github.com/fpco/rust-aws-devops\">git repo</a>:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#657b83;\">cargo build\n</span></code></pre>\n<p>You can then use <code>cargo run</code> to run the code or execute the code directly\nwith <code>./target/debug/rust-aws-devops</code>:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#657b83;\">$ ./target/debug/rust-aws-devops \n\nRunning tool\nRustAWSDevops 0.1.0\nMike McGirr <mike@fpcomplete.com>\n\nUSAGE:\n rust-aws-devops <SUBCOMMAND>\n\nFLAGS:\n -h, --help Prints help information\n -V, --version Prints version information\n\nSUBCOMMANDS:\n add-object Add the specified file to the bucket\n create Create a new bucket with the given name\n delete Try to delete the bucket with the given name\n delete-object Remove the specified object from the bucket\n help Prints this message or the help of the given subcommand(s)\n list Try to find the bucket with the given name and list its objects``\n</span></code></pre>\n<p>Which will output the nice CLI help output automatically created for us \nby <code>structopt</code>.</p>\n<p>If you're ready to build a release version (with optimizations turn on which \nwill make compilation take slightly longer) run the following:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#657b83;\">cargo build --release\n</span></code></pre><h2 id=\"conclusion\">Conclusion</h2>\n<p>As this small demo showed, it's not difficult to get started using Rust to write\nDevOps tools. And even then we didn't need to make a trade-off between ease of\ndevelopment and performant fast code. </p>\n<p>Hopefully the next time you're writing a new piece of DevOps software, \nanything from a simple CLI tool for a specific DevOps operation or you're writing \nthe next Kubernetes, you'll consider reaching for Rust.\nAnd if you have further questions about Rust, or need help implementing your Rust \nproject, please feel free to reach out to FP Complete for Rust engineering \nand training!</p>\n<p>Want to learn more Rust? Check out our <a href=\"https://www.fpcomplete.com/rust/crash-course/\">Rust Crash Course eBook</a>. And for more information, check out our <a href=\"https://www.fpcomplete.com/rust/\">Rust homepage</a>.</p>\n",
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"content": "<p>The term DevOps has been around for many years. Small and big companies adopt DevOps concepts for different purposes, e.g. to increase the quality of software. In this blog post, we define DevOps, present its pros and cons, highlight a few concepts and see how these can impact the entire organization.</p>\n<h2 id=\"what-is-devops\">What is DevOps?</h2>\n<p>At a high level, DevOps is understood as a technical, organizational and cultural shift in a company to run software more efficiently, reliably, and securely. From this first definition, we can see that DevOps is much more than "use tool X" or "move to the cloud". DevOps starts with the understanding that development (Dev), operations (Ops) and quality assurance (QA) are not treated as siloed disciplines anymore. Instead, they all come together in shared processes and responsibilities across collaborating teams. DevOps achieves this through various techniques. In the section "Implementation", we present a few of these concepts.</p>\n<h2 id=\"benefits\">Benefits</h2>\n<p>Benefits of applying DevOps include:</p>\n<ul>\n<li>Cost savings through higher efficiency.</li>\n<li>Faster software iteration cycles, where updates take less time from development to running in production.</li>\n<li>More security, reliability, and fault tolerance when running software.</li>\n<li>Stronger bonds between different stakeholders in the organization including non-technical staff.</li>\n<li>Enable more data-driven decisions.</li>\n</ul>\n<p>Let's have a look <em>how</em> these benefits can be achieved by applying DevOps ideas:</p>\n<h2 id=\"how-to-implement-devops\">How to implement DevOps</h2>\n<h3 id=\"automation-and-continuous-integration-ci-continuous-delivery-cd\">Automation and Continuous Integration (CI) / Continuous Delivery (CD)</h3>\n<p>Automation refers to a key aspect of the engineering-driven part of DevOps. With automation, we aim to reduce the need for human action, and thus the possibility of human error, as far as possible by sending your software through an automated and well-understood pipeline of actions. These automated actions can build your software, run unit tests, integrate it with existing systems, run system tests, deploy it, and provide feedback on each step. What we are\ndescribing here is usually referred to as <strong>Continuous Integration (CI)</strong> and <strong>Continuous Delivery (CD)</strong>. Adopting CI/CD invests in a low-risk and low-cost way of crossing the chasm between "software that is working on an engineer's laptop" and "software that running securely and reliably on production servers".</p>\n<p>CI/CD is usually tied to a platform on top of which the automated actions are run, e.g., Gitlab. The platform accepts software that should be passed through the pipeline, executes the automated actions on servers which are usually abstracted away, and provides feedback to the engineering team. These actions can be highly customized and tied together in different ways. For example, one action only compiles the source code and provides the build artifacts to subsequent actions. Another action can be responsible for running a test-suite, another one can deploy software. Such actions can be defined for different types of software: A website can be automatically deployed to a server, or a Desktop application can be made available to your customers without human interaction.</p>\n<p>Besides the fact that CI/CD can be used for all kinds of software, there are other advantages to consider:</p>\n<ol>\n<li><strong>The CI/CD pipeline is well-understood and maintained by the teams</strong>: the actions that are run in a pipeline can be flexibly updated, extended, etc. <a href=\"https://www.fpcomplete.com/blog/devops-for-developers/#infra-as-code\">Infrastructure as Code</a> can be a powerful concept here.</li>\n<li><strong>Run in standardized environments</strong>: Version conflicts between tools and configuration or dependency mismatches only have to be fixed once when the pipeline is built. Once a pipeline is working, it will continue to work as the underlying servers and their software versions don't change. No more conflicts between operating systems, tools, versions of tools across different engineers. Pipelines are highly reproducible. Containerization can be a game-changer here.</li>\n<li><strong>Feedback</strong>: Actions sometimes fail, e.g. because a unit test does not pass. The CI/CD platform usually allows different reporting mechanisms: E-mail someone, update the project status on your repository overview page, block subsequent actions or cancel other pipelines.</li>\n</ol>\n<p>The next sections cover more DevOps concepts that benefit from automation.</p>\n<h3 id=\"multiple-environments\">Multiple Environments</h3>\n<p>The CI/CD can be extended by deploying software to different environments. These deployments can happen in individual actions defined in your pipeline. Besides the production environment, which runs user-facing software, staging and testing environments can be defined where software is deployed to. For example, a testing environment can be used by the engineering team for peer-reviewing and validating software changes. Once the team agreed on new software, it can be deployed to a staging environment. A usual purpose of the staging environment is to mimic the production environment as closely as possible. Further tests can be run in a staging environment to make sure the software is ready to be used by real users. Finally, the software reaches production-readiness and is deployed to a production environment. Such a production deployment can be designed using a gradual rollout, i.e. canary deployments.</p>\n<p>Different environments not only realize different semantics and confidence levels of running software, e.g. as described in the previous paragraph, but also serve as an agreed-upon view on software in the entire organization. Multi-environment deployments make your software and quality thereof easier to understand. This is because of the gained insights when running software, in particular on infrastructure that is close to a production setting. Generally, running software gives much more insights into the performance, reliability, security, production-readiness and overall quality. Different teams, e.g. security experts or a dedicated QA-team (if your organization follows this practice) can be consulted at different software quality stages, i.e. different environments in which software runs. Additionally, non-technical staff can use environments, e.g. specialized ones for demo purposes.</p>\n<p>Ultimately, integrating multiple environments structures QA and smoothens the interactions between different teams.</p>\n<h3 id=\"fail-early\">Fail early</h3>\n<p>No matter how well things are working in an organization that builds software, bugs happen and bugs are expensive. The cost of bugs can be projected to the manpower invested into fixing the bug, the loss of reputation due to angry customers, and generally negative business impact. Since we can't fully avoid bugs, there exist concepts to reduce both the frequency and impact of bugs. "Fail early" is one of these concepts.</p>\n<p>The basic idea is to catch bugs and other flaws in your software as early in the development process as possible. When software is developed, unit tests, compiler errors and peer reviews count towards the early and cheap mechanisms to detect and fix flaws. Ideally, a unit test tells the developer that the software is not correct, or, a second pair of eyes reveals a potential performance issue during a code review. In both cases, not much time and effort is lost and the flaw can be easily fixed. However, other bugs might make it through these initial checks and land in testing or staging environments. Other types of tests and QA should be in place to check the software quality. Worst case, the bug outlives all checks and is in production. There, bugs have much higher impact and require more effort by many stakeholders, e.g. the bug fix by the engineering team and the apology to the customers.</p>\n<p>To save costs, cheap checks such as running a test suite in an automated pipeline should be executed early. This will save costs as flaws discovered later in the process result in higher costs. Thus, failing early increases cost efficiency.</p>\n<h3 id=\"rollbacks\">Rollbacks</h3>\n<p>DevOps can also help to react quickly to changes. One example of a sudden change is a bug, as described in the last section, which is discovered in the production environment. Rollbacks, for example as manually triggered pipelines, can recover the well-functioning of a production service in a timely manner. This can be useful when the bug is a hard one and needs hours to be identified and fixed. These hours of degraded customer experience or even downtime makes paying customers unhappy. A faster mechanism is desired, which minimizes the gap between a faulty system and a recovered system. A rollback can be a fast and effective way to recover system state without exposing customers to company failure much.</p>\n<h3 id=\"policies\">Policies</h3>\n<p>DevOps concepts impose a challenge to security and permission management as these span the entire organization. Policies can help to formulate authorizations and rules during operations. For example, implementing the following security requirements may be required:</p>\n<ul>\n<li>A deployment or rollback in production should not be triggered by anyone but a well-defined set of people in authority.</li>\n<li>Some actions in a CI/CD pipeline should always be run while other actions are intended to be triggered manually or only run under certain conditions.</li>\n<li>The developers might require slightly different permissions than a dedicated QA team to perform their day-to-day work.</li>\n<li>Humans and machine users can have different capabilities but should always have the least privileges assigned to them.</li>\n</ul>\n<p>The authentication and authorization tools provided by CI/CD providers or cloud vendors can help to design such policies according to your organizational needs.</p>\n<h3 id=\"observability\">Observability</h3>\n<p>As software is running and users are interacting with your applications, insights such as error rates, performance statistics, resource usages, etc. can help to identify bottlenecks, mitigate future issues, and drive business decisions through data. There exist two major ways to establish different forms of observability:</p>\n<ul>\n<li><strong>Logging</strong>: Events in text form that software outputs to inform about the application's status and health. Different types of logging messages, e.g. indicating the severity of an error event, can help to aggregate and display log messages in a central place, where it can be used by engineering teams for debugging purposes.</li>\n<li><strong>Metrics</strong>: Information about the running software that is not generated by the application itself. For example, the CPU or memory usage of the underlying machine that runs the software, network statistics, HTTP error rates, etc. As with logging, metrics can help to spot bottlenecks and mitigate them before they have a business impact. Visualizing aggregated metrics data facilitates communication across technical and non-technical teams and leverages data-driven decisions. Metrics dashboards can strengthen the shared ownership of software across teams.</li>\n</ul>\n<p>Logging and metrics can help to define goals and to align a development team with a QA team for example.</p>\n<h2 id=\"disadvantages\">Disadvantages</h2>\n<p>So far, we only looked at the benefits and characteristics of DevOps. Let's have a brief look at the other side of the coin by commenting on the possible negative side effects and disadvantages of adopting DevOps concepts.</p>\n<ul>\n<li>\n<p>The investment into DevOps can be huge as it is a company-wide, multi-discipline, and multi-team transformation that not only requires technical implementation effort but also training for people, re-structuring and aligning teams.</p>\n</li>\n<li>\n<p>This goes along with the first point but it's worth emphasizing it: The cultural impact on your organization can be challenging due to human factors. While a new automation mechanism can be estimated and implemented reasonably well, tracking the progress of changing people's way of communication, feeling of ownership, aligning to new processes can be hard and might lead to no gained efficiencies, which DevOps promises, short-term. Due to the high impact of DevOps, it is a long-term investment.</p>\n</li>\n<li>\n<p>The technical backbone of DevOps, e.g. CI/CD pipelines, cloud vendors, integration of authorization and authentication, likely results in increased expenses through new contracts and licenses with new players. However, through the dominance of open source in modern DevOps tooling, e.g. through using Kubernetes, vendor lock-in can be avoided.</p>\n</li>\n</ul>\n<h2 id=\"conclusion\">Conclusion</h2>\n<p>In this blog post, we explored the definition of DevOps and presented several DevOps concepts and use-cases. Furthermore, we evaluated benefits and disadvantages. Adopting DevOps is an investment into a low-friction and automated way of developing, testing, and running software. Technical improvements, e.g. automation, as well as increased collaboration between teams of different disciplines ultimately improve the efficiency in your organization long-term.</p>\n<p>However, DevOps represents not only technical effort but also impacts the entire company, e.g. how teams communicate with each other, how issues are resolved, and what teams feel responsible for. Finding the right balance and choosing the best concepts and tools for your teams represents a challenge. We can help you with identifying and solving the DevOps transformation in your organization.</p>\n",
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"description": "The term DevOps has been around for many years. Small and big companies adopt DevOps concepts for different purposes, e.g. to increase the quality of software. In this blog post, we define DevOps, present its pros and cons, highlight a few concepts and see how these can impact the entire organization.",
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"content": "<p>In this post, I describe my personal journey as a developer skeptical\nof the seemingly ever-growing, ever more complex, array of "ops"\ntools. I move towards adopting some of these practices, ideas and\ntools. I write about how this journey helps me to write software\nbetter and understand discussions with the ops team at work.</p>\n<div style=\"border:1px solid black;background-color:#f8f8f8;margin-bottom:1em;padding: 0.5em 0.5em 0 0.5em;\">\n<p><strong>Table of Contents</strong></p>\n<ul>\n<li><a href=\"https://www.fpcomplete.com/blog/devops-for-developers/#on-being-skeptical\">On being skeptical</a></li>\n<li><a href=\"https://www.fpcomplete.com/blog/devops-for-developers/#the-humble-app\">The humble app</a></li>\n<li><a href=\"https://www.fpcomplete.com/blog/devops-for-developers/#disk-failures-arent-that-common\">Disk failures aren't that common</a></li>\n<li><a href=\"https://www.fpcomplete.com/blog/devops-for-developers/#auto-deployment-is-better-than-manual\">Auto-deployment is better than manual</a></li>\n<li><a href=\"https://www.fpcomplete.com/blog/devops-for-developers/#backups-become-worth-it\">Backups become worth it</a></li>\n<li><a href=\"https://www.fpcomplete.com/blog/devops-for-developers/#deployment-staging\">Deployment staging</a></li>\n<li><a href=\"https://www.fpcomplete.com/blog/devops-for-developers/#packaging-with-docker-is-good\">Packaging with Docker is good</a></li>\n<li><a href=\"https://www.fpcomplete.com/blog/devops-for-developers/#custodiansmultiple-processes-are-useful\">Custodians/multiple processes are useful</a></li>\n<li><a href=\"https://www.fpcomplete.com/blog/devops-for-developers/#kubernetes-provides-exactly-that\">Kubernetes provides exactly that</a></li>\n<li><a href=\"https://www.fpcomplete.com/blog/devops-for-developers/#declarative-is-good-vendor-lock-in-is-bad\">Declarative is good, vendor lock-in is bad</a></li>\n<li><a href=\"https://www.fpcomplete.com/blog/devops-for-developers/#more-advanced-rollout\">More advanced rollout</a></li>\n<li><a href=\"https://www.fpcomplete.com/blog/devops-for-developers/#relationship-between-code-and-deployed-state\">Relationship between code and deployed state</a></li>\n<li><a href=\"https://www.fpcomplete.com/blog/devops-for-developers/#argocd\">ArgoCD</a></li>\n<li><a href=\"https://www.fpcomplete.com/blog/devops-for-developers/#infra-as-code\">Infra-as-code</a></li>\n<li><a href=\"https://www.fpcomplete.com/blog/devops-for-developers/#where-the-dev-meets-the-ops\">Where the dev meets the ops</a></li>\n<li><a href=\"https://www.fpcomplete.com/blog/devops-for-developers/#what-we-do\">What we do</a></li>\n</ul>\n</div>\n<h2 id=\"on-being-skeptical\">On being skeptical</h2>\n<p>I would characterise my attitudes to adopting technology in two\nstages:</p>\n<ul>\n<li>Firstly, I am conservative and dismissive, in that I will usually\ndisregard any popular new technology as a bandwagon or trend. I'm a\nslow adopter.</li>\n<li>Secondly, when I actually encounter a situation where I've suffered,\nI'll then circle back to that technology and give it a try, and if I\ncan really find the nugget of technical truth in there, then I'll\nadopt it.</li>\n</ul>\n<p>Here are some things that I disregarded for a year or more before\ntrying: Emacs, Haskell, Git, Docker, Kubernetes, Kafka. The whole\nNoSQL trend came, wrecked havoc, and went, while I had my back turned,\nbut I am considering using Redis for a cache at the moment.</p>\n<h2 id=\"the-humble-app\">The humble app</h2>\n<p>If you’re a developer like me, you’re probably used to writing your\nsoftware, spending most of your time developing, and then finally\ndeploying your software by simply creating a machine, either a\ndedicated machine or a virtual machine, and then uploading a binary of\nyour software (or source code if it’s interpreted), and then running\nit with the copy pasted config of systemd or simply running the\nsoftware inside GNU screen. It's a secret shame that I've done this,\nbut it's the reality.</p>\n<p>You might use nginx to reverse-proxy to the service. Maybe you set up\na PostgreSQL database or MySQL database on that machine. And then you\nwalk away and test out the system, and later you realise you need some\nslight changes to the system configuration. So you SSH into the system\nand makes the small tweaks necessary, such as port settings, encoding\nsettings, or an additional package you forgot to add. Sound familiar?</p>\n<p>But on the whole, your work here is done and for most services this is\npretty much fine. There are plenty of services running that you have\nseen in the past 30 years that have been running like this.</p>\n<h2 id=\"disk-failures-aren-t-that-common\">Disk failures aren't that common</h2>\n<p>Rhetoric about processes going down due to a hardware failure are\nprobably overblown. Hard drives don’t crash very often. They don’t\nreally wear out as quickly as they used to, and you can be running a\nsystem for years before anything even remotely concerning happens.</p>\n<h2 id=\"auto-deployment-is-better-than-manual\">Auto-deployment is better than manual</h2>\n<p>When you start to iterate a little bit quicker, you get bored of\nmanually building and copying and restarting the binary on the\nsystem. This is especially noticeable if you forget the steps later\non.</p>\n<!-- Implementing Auto-Deployment -->\n<p>If you’re a little bit more advanced you might have some special\nscripts or post-merge git hooks, so that when you push to your repo it\nwould apply to the same machine and you have some associated token on\nyour CI machine that is capable of uploading a binary and running a\ncommand like copy and restart (e.g. SSH key or API\nkey). Alternatively, you might implement a polling system on the\nactual production system which will check if any updates have occurred\nin get and if so pull down a new binary. This is how we were doing\nthings in e.g. 2013.</p>\n<h2 id=\"backups-become-worth-it\">Backups become worth it</h2>\n<p>Eventually, if you're lucky, your service starts to become slightly\nmore important; maybe it’s used in business and people actually are\nusing it and storing valuable things in the database. You start to\nthink that back-ups are a good idea and worth the investment.</p>\n<!-- Redundancy of DB -->\n<p>You probably also have a script to back up the database, or replicate\nit on a separate machine, for redundancy.</p>\n<h2 id=\"deployment-staging\">Deployment staging</h2>\n<p>Eventually, you might have a staged deployment strategy. So you might\nhave a developer testing machine, you might have a QA machine, a\nstaging machine, and finally a production machine. All of these are\nconfigured in pretty much the same way, but they are deployed at\ndifferent times and probably the system administrator is the only one\nwith access to deploy to production.</p>\n<!-- Continuum -->\n<p>It’s clear by this point that I’m describing a continuum from "hobby\nproject" to "enterprise serious business synergy solutions".</p>\n<h2 id=\"packaging-with-docker-is-good\">Packaging with Docker is good</h2>\n<p>Docker effectively leads to collapsing all of your system dependencies\nfor your binary to run into one contained package. This is good,\nbecause dependency management is hell. It's also highly wasteful,\nbecause its level of granularity is very wide. But this is a trade-off\nwe accept for the benefits.</p>\n<h2 id=\"custodians-multiple-processes-are-useful\">Custodians/multiple processes are useful</h2>\n<p>Docker doesn’t have much to say about starting and restarting\nservices. I’ve explored using CoreOS with the hosting provider Digital\nOcean, and simply running a fresh virtual machine, with the given\nDocker image.</p>\n<p>However, you quickly run into the problem of starting up and tearing\ndown:</p>\n<ul>\n<li>When you start the service, you need certain liveness checks\nand health checks, so if the service fails to start then you should\nnot stop the existing service from running, for example. You should\nkeep the existing ones running.</li>\n<li>If the process fails at any time during running then you should also\nrestart the process. I thought about this point a lot, and came to the\nconclusion that it’s better to have your process be restarted than to\nassume that the reason it failed was so dangerous that the process\nshouldn’t start again. Probably it’s more likely that there is an\nexception or memory issue that happened in a pathological case which\nyou can investigate in your logging system. But it doesn’t mean that\nyour users should suffer by having downtime.</li>\n<li>The natural progression of this functionality is to support\ndifferent rollout strategies. Do you want to switch everything to the\nnew system in one go, do you want it to be deployed piece-by-piece?</li>\n</ul>\n<!-- Summary: You Realise Worth Of Ops Tools -->\n<p>It’s hard to fully appreciate the added value of ops systems like\nKubernetes, Istio/Linkerd, Argo CD, Prometheus, Terraform, etc. until\nyou decide to design a complete architecture yourself, from scratch,\nthe way you want it to work in the long term.</p>\n<h2 id=\"kubernetes-provides-exactly-that\">Kubernetes provides exactly that</h2>\n<p>What system happens to accept Docker images, provide custodianship,\nroll out strategies, and trivial redeploy? Kubernetes.</p>\n<p>It provides this classical monitoring and custodian responsibilities\nthat plenty of other systems have done in the past. However, unlike\nsimply running a process and testing if it’s fine and then turning off\nanother process, Kubernetes buys into Docker all the way. Processes\nare isolated from each other, in both the network on the file\nsystem. Therefore, you can very reliably start and stop the services\non the same machine. Nothing about a process's machine state is\npersistent, therefore you are forced to design your programs in a way\nthat state is explicitly stored either ephemerally, or elsewhere.</p>\n<!-- Cloud Managed Databases Make This Practical -->\n<p>In the past it might be a little bit scarier to have your database\nrunning in such system, what if it automatically wipes out the\ndatabase process? With today’s cloud base deployments, it's more\ncommon to use a managed database such as that provided by Amazon,\nDigital Ocean, Google or Azure. The whole problem of updating and\nbacking up your database can pretty much be put to one\nside. Therefore, you are free to mess with the configuration or\ntopology of your cluster as much as you like without affecting your\ndatabase.</p>\n<h2 id=\"declarative-is-good-vendor-lock-in-is-bad\">Declarative is good, vendor lock-in is bad</h2>\n<p>A very appealing feature of a deployment system like Kubernetes is\nthat everything is automatic and declarative. You stick all of your\nconfiguration in simple YAML files (which is also a curse because YAML\nhas its own warts and it's not common to find formal schemas for it).\nThis is also known as "infrastructure as code".</p>\n<p>Ideally, you should have as much as possible about your infrastructure\nin code checked in to a repo so that you can reproduce it and track\nit.</p>\n<p>There is also a much more straight-forward path to migrate from one\nservice provider to another service provider. Kubernetes is supported\non all the major service providers (Google, Amazon, Azure), therefore\nyou are less vulnerable to vendor lock-in. They also all provide\nmanaged databases that are standard (PostgreSQL, for example) with\ntheir normal wire protocols. If you were using the vendor-specific\nAPIs to achieve some of this, you'd be stuck on one vendor. I, for\nexample, am not sure whether to go with Amazon or Azure on a big\npersonal project right now. If I use Kubernetes, I am mitigating risk.</p>\n<p>With something like Terraform you can go one step further, in which\nyou write code that can create your cluster completely from\nscratch. This is also more vendor independent/mitigated.</p>\n<h2 id=\"more-advanced-rollout\">More advanced rollout</h2>\n<p>Your load balancer and your DNS can also be in code. Typically a load\nbalancer that does the job is nginx. However, for more advanced\ndeployments such as A/B or green/blue deployments, you may need\nsomething more advanced like Istio or Linkerd.</p>\n<p>Do I really want to deploy a new feature to all of my users? Maybe,\nthat might be easier. Do I want to deploy a different way of marketing\nmy product on the website to all users at once? If I do that, then I\ndon’t exactly know how effective it is. So, I could perhaps do a\ndeployment in which half of my users see one page and half of the\nusers see another page. These kinds of deployments are\nstraight-forwardly achieved with Istio/Linkerd-type service meshes,\nwithout having to change any code in your app.</p>\n<h2 id=\"relationship-between-code-and-deployed-state\">Relationship between code and deployed state</h2>\n<p>Let's think further than this.</p>\n<p>You've set up your cluster with your provider, or Terraform. You've\nset up your Kubernetes deployments and services. You've set up your CI\nto build your project, produce a Docker image, and upload the images\nto your registry. So far so good.</p>\n<p>Suddenly, you’re wondering, how do I actually deploy this? How do I\ncall Kubernetes, with the correct credentials, to apply this new\nDoctor image to the appropriate deployment?</p>\n<p>Actually, this is still an ongoing area of innovation. An obvious way\nto do it is: you put some details on your CI system that has access to\nrun kubectl, then set the image with the image name and that will try\nto do a deployment. Maybe the deployment fails, you can look at that\nresult in your CI dashboard.</p>\n<p>However, the question comes up as what is currently actually deployed\non production? Do we really have infrastructure as code here?</p>\n<p>It’s not that I edited the file and that update suddenly got\nreflected. There’s no file anywhere in Git that contains what the\ncurrent image is. Head scratcher.</p>\n<p>Ideally, you would have a repository somewhere which states exactly\nwhich image should be deployed right now. And if you change it in a\ncommit, and then later revert that commit, you should expect the\nproduction is also reverted to reflect the code, right?</p>\n<h2 id=\"argocd\">ArgoCD</h2>\n<p>One system which attempts to address this is ArgoCD. They implement\nwhat they call "GitOps". All state of the system is reflected in a Git\nrepo somewhere. In Argo CD, after your GitHub/Gitlab/Jenkins/Travis CI\nsystem has pushed your Docker image to the Docker repository, it makes\na gRPC call to Argo, which becomes aware of the new image. As an\nadmin, you can now trivially look in the UI and click "Refresh" to\nredeploy the new version.</p>\n<h2 id=\"infra-as-code\">Infra-as-code</h2>\n<p>The common running theme in all of this is\ninfrastructure-as-code. It’s immutability. It’s declarative. It’s\nremoving the number of steps that the human has to do or care\nabout. It’s about being able to rewind. It’s about redundancy. And\nit’s about scaling easily.</p>\n<!-- Circling Back -->\n<p>When you really try to architect your own system, and your business\nwill lose money in the case of ops mistakes, then you start to think\nthat all of these advantages of infrastructure as code start looking\nreally attractive.</p>\n<p>But before you really sit down and think about this stuff, however, it\nis pretty hard to empathise or sympathise with the kind of concerns\nthat people using these systems have.</p>\n<!-- Downsides/Tax -->\n<p>There are some downsides to these tools, as with any:</p>\n<ul>\n<li>Docker is quite wasteful of time and space</li>\n<li>Kubernetes is undoubtedly complex, and leans heavily on YAML</li>\n<li><a href=\"https://www.joelonsoftware.com/2002/11/11/the-law-of-leaky-abstractions/\">All abstractions are leaky</a>,\ntherefore tools like this all leak</li>\n</ul>\n<h2 id=\"where-the-dev-meets-the-ops\">Where the dev meets the ops</h2>\n<p>Now that I’ve started looking into these things and appreciating their\nuse, I interact a lot more with the ops side of our DevOps team at work,\nand I can also be way more helpful in assisting them with the\ninformation that they need, and also writing apps which anticipate the\nkind of deployment that is going to happen. The most difficult\nchallenge typically is metrics and logging, for run-of-the-mill apps,\nI’m not talking about high-performance apps.</p>\n<!-- An Exercise -->\n<p>One way way to bridge the gap between your ops team and dev team,\ntherefore, might be an exercise meeting in which you do have a dev\nperson literally sit down and design an app architecture and\ninfrastructure, from the ground up using the existing tools that we\nhave that they are aware of and then your ops team can point out the\nadvantages and disadvantages of their proposed solution. Certainly,\nI think I would have benefited from such a mentorship, even for an\nhour or two.</p>\n<!-- Head-In-The-Sand Also Works -->\n<p>It may be that your dev team and your ops team are completely separate\nand everybody’s happy. The devs write code, they push it, and then it\nmagically works in production and nobody has any issues. That’s\ncompletely fine. If anything it would show that you have a very good\nprocess. In fact, that’s pretty much how I’ve worked for the past\neight years at this company.</p>\n<p>However, you could derive some benefit if your teams are having\ndifficulty communicating.</p>\n<p>Finally, the tools in the ops world aren't perfect, and they're made\nby us devs. If you have a hunch that you can do better than these\ntools, you should learn more about them, and you might be right.</p>\n<h2 id=\"what-we-do\">What we do</h2>\n<p>FP Complete are using a great number of these tools, and we're writing\nour own, too. If you'd like to know more, email use at\n<a href=\"mailto:sales@fpcomplete.com\">sales@fpcomplete.com</a>.</p>\n",
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"content": "<p>FP Complete has been working with containerization (or OS-level virtualization) since before it was popularized by Docker. What follows is a brief history of how and why we got started using containers, and how our use of containerization has evolved as new technology has emerged.</p>\n<h2 id=\"brief-history\">Brief history</h2>\n<p>Our first foray into containerization started at the beginning of the company, when we were building a web-based integrated development environment for Haskell. We needed a secure and cost-effective way to be able to compile and run Haskell code on the server side. While giving each active user their own virtual machine with dedicated CPU and memory would have satisfied the first requirement (security), it would have been far from cost effective. GHC, the de-facto standard Haskell compiler, is notoriously resource hungry, so the VM would have to be quite large (it's not uncommon to need 4 GB or more of RAM to compile a fairly straightforward piece of software). We needed a way to share CPU and memory resources between multiple users securely and be able to shift load around a cluster of virtual machines to keep usage balanced and avoid one heavy user from impacting the experience of others users on the same VM. This sounds like a job for container orchestration! Unfortunately, Docker didn't exist yet, let alone Kubernetes. The state of the art for Linux containers at the time was LXC, which was mostly a collection of shell scripts that helped with using the Linux kernel features that underly all Linux container solutions, but at a much lower level than Docker. On top of this we built everything we needed to distribute "images" of a base filesystem plus overlay for local changes, isolated container networks, and ability to shift load based on VM and container utilization -- that is, many of the things Docker and Kubernetes do now, but tailored specifically for our application's needs.</p>\n<p>When Docker came on the scene, we embraced it despite some early growing pains, since it was much easier to use and more general purpose than our "bespoke" system and we thought it likely that it would soon become a de-facto standard, which is exactly what happened. For internal and customer solutions, Docker allowed us to create much more nimble and efficient deployment solutions that satisfied the requirement for <a href=\"https://www.fpcomplete.com/devops/immutable-infrastructure/\">immutable infrastructure</a>. Prior to Docker, we achieved immutability by building VM images and spinning up virtual machines; a much slower and heavier process than building a Docker image and running it on an already-provisioned VM. This also allowed us to run multiple applications isolated from one another on a single VM without worry of interference with each other.</p>\n<p>Finally Kubernetes arrived. While it was not the first orchestration platform, it was the first that wholeheartedly standardized on Docker containers. Once again we embraced it, despite some early growing pains, due to its ease of use, multi-cloud support, fast pace of improvement, and backing of a major company (Google). We once again bet that Kubernetes would become the de-facto standard, which is again exactly what happened. With Kubernetes, instead of having to think about which VM a container would run on, we can have a cluster of general-purpose nodes and let the orchestrator worry about what runs on which node. This lets us squeeze yet more efficiency out of our resources. Due to its ease of use and built-in support for common rollout strategies, we can give developers the ability to deploy their apps directly, and since it is so easy to tie into CI/CD pipelines we can drastically simplify automated deployment processes.</p>\n<p>Going forward, we continue to keep up with the latest developments in containerization and are constantly evaluating new and alternative technologies, to stay on the forefront of DevOps.</p>\n<h2 id=\"why-we-really-like-it\">Why we really like it</h2>\n<ul>\n<li>\n<p>Supports <a href=\"https://www.fpcomplete.com/devops/immutable-infrastructure/\">immutable infrastructure</a>.</p>\n</li>\n<li>\n<p>Fast build and deployment processes.</p>\n</li>\n<li>\n<p>Low overhead and efficient use of compute resources.</p>\n</li>\n<li>\n<p>Easy integration with CI/CD pipelines.</p>\n</li>\n<li>\n<p>Isolation of applications from others running on the same machine.</p>\n</li>\n<li>\n<p>Bundles dependencies with the application, so they can be tested together and there's no risk of deploying to an incorrect environment.</p>\n</li>\n<li>\n<p>Developers on various platforms can build and test the application in a consistent environment.</p>\n</li>\n</ul>\n<h2 id=\"limitations-of-the-technology\">Limitations of the technology</h2>\n<ul>\n<li>\n<p>Containers and container orchestration are most mature on Linux, although Docker and Kubernetes do now support running Windows containers on machines running Windows, and most modern server operating system have support for some kind of containerization (but not necessarily Docker or Kubernetes).</p>\n</li>\n<li>\n<p>Containers and container orchestration add additional layers of abstraction and complexity. This can, at times, make diagnosing problems more difficult.</p>\n</li>\n<li>\n<p>Legacy applications can be tricky to containerize since they assume they are running on a persistent machine rather than an ephemeral one. While this can be mitigated using persistent volumes, it makes the containerization strategy less straightforward.</p>\n</li>\n<li>\n<p>While properly configured containers are relatively secure, all containers running on a host share a single operating system kernel which means there is greater risk that a process can use a security vulnerability to "break out" of its container than when using VMs.</p>\n</li>\n</ul>\n<h2 id=\"resources\">Resources</h2>\n<p>From FP Complete:</p>\n<ul>\n<li><a href=\"https://www.fpcomplete.com/devops/containerization/\">Introduction to Containerization concepts</a></li>\n<li><a href=\"https://www.fpcomplete.com/devops/immutable-infrastructure/\">Introduction to Immutable Infrastructure concepts</a></li>\n<li><a href=\"https://www.fpcomplete.com/blog/deploying_haskell_apps_with_kubernetes/\">Webinar: Deploying Haskell apps with Kubernetes</a></li>\n<li><a href=\"https://www.fpcomplete.com/blog/2018/07/deploying-rust-with-docker-and-kubernetes/\">Blog post: Deploying rust with Docker and Kubernetes</a></li>\n<li><a href=\"https://www.fpcomplete.com/blog/2017/02/immutability-docker-haskells-st-type/\">Blog post: Immutability, Docker, and Haskell's ST type</a></li>\n<li><a href=\"https://www.fpcomplete.com/blog/2017/01/containerize-legacy-app/\">Blog post: Containerizing a legacy application: an overview</a></li>\n</ul>\n<p>From the web:</p>\n<ul>\n<li><a href=\"https://www.docker.com/resources/what-container\">What is a container?</a></li>\n<li><a href=\"https://www.docker.com/get-started\">Get started with Docker</a></li>\n<li><a href=\"https://kubernetes.io/docs/concepts/\">Kubernetes concepts</a></li>\n<li><a href=\"https://kubernetes.io/docs/setup/\">Getting started with Kubernetes</a></li>\n</ul>\n",
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"content": "<p>In this post we show a couple of options when it comes to a cloud\ndeployment model. Depending on the needs of your organization some\noptions may suit you better than others.</p>\n<h1 id=\"private-cloud\">Private Cloud</h1>\n<p>A private cloud is cloud infrastructure that only members of your organization\ncan utilize. It is typically owned and managed by the organization itself and\nis hosted on premises but it could also be managed by a third party in a secure\ndatacenter. This deployment model is best suited for organizations that deal\nwith sensitive data and/or are required to uphold certain security standards by\nvarious regulations.</p>\n<p>Advantages:</p>\n<ul>\n<li>Organization specific</li>\n<li>High degree of security and level of control</li>\n<li>Ability to choose your resources (ie. specialized hardware)</li>\n</ul>\n<p>Disadvantages:</p>\n<ul>\n<li>Lack of elasticity and capacity to scale (bursts)</li>\n<li>Higher cost</li>\n<li>Requires a significant amount of engineering effort</li>\n</ul>\n<h1 id=\"public-cloud\">Public Cloud</h1>\n<p>Public cloud refers to cloud infrastructure that is located and\naccessed over the public network. It provides a convenient way to\nburst and scale your project depending on the use and is typically\npay-per-use. Popular examples include <a href=\"https://aws.amazon.com\">Amazon AWS</a>,\n<a href=\"https://cloud.google.com/\">Google Cloud Platform</a> and <a href=\"https://azure.microsoft.com/\">Microsoft\nAzure</a>.</p>\n<p>Advantages:</p>\n<ul>\n<li>Scalability/Flexibility/Bursting</li>\n<li>Cost effective</li>\n<li>Ease of use</li>\n</ul>\n<p>Disadvantages:</p>\n<ul>\n<li>Shared resources</li>\n<li>Operated by third party</li>\n<li>Unreliability</li>\n<li>Less secure</li>\n</ul>\n<h1 id=\"hybrid-cloud\">Hybrid Cloud</h1>\n<p>This type of cloud infrastructure assumes that you are hosting your system both\non private and public cloud . One use case might be regulation requiring data\nto be stored in a locked down private data center but have the application\nprocessing parts available on the public cloud and talking to the private\ncomponents over a secure tunnel.</p>\n<p>Another example is hosting most of the system inside a private cloud and having\na clone of the system on the public cloud to allow for rapid scaling and\naccommodating bursts of new usage that would otherwise not be possible on the\nprivate cloud.</p>\n<p>Advantages:</p>\n<ul>\n<li>Cost effective</li>\n<li>Scalability/Flexibility</li>\n<li>Balance of convenience and security</li>\n</ul>\n<p>Disadvantages:</p>\n<ul>\n<li>Same disadvantages as the public cloud</li>\n</ul>\n<h1 id=\"multi-cloud\">Multi-Cloud</h1>\n<p>This option is a variant of the hybrid cloud but we refer to it when we mean\n"using multiple public cloud providers". It is mostly used for mission critical\nsystems that want to minimize the amount of down time if a specific service on\na particular cloud goes down (e.g., the S3 outage of 2017 that took down a lot\nof web services with it). This option is arguably the most advanced option and\nsacrifices convenience for security and reliability. It requires significant\nexpertise and engineering effort to get right since most platforms vary widely\nbetween the type of resources and services that they provide in subtle ways.</p>\n<p>When chosing a cloud deployment model weigh the advantages and disadvantages of\neach option as it relates to your business objectives. </p>\n<p>If you liked this post you may also like: <a href=\"https://www.fpcomplete.com/blog/intro-to-devops-on-govcloud/\">Introduction to DevOps on AWS Gov Cloud</a></p>\n",
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"content": "<p>The topics of authentication and authorization usually appear simple but turn out to hide significant complexity. That's because, at its core, auth is all about answering two questions:</p>\n<ul>\n<li>Who are you</li>\n<li>What are you allowed to do</li>\n</ul>\n<p>However, the devil is in the details. Seasoned IT professionals, software developers, and even typical end users are fairly accustomed at this point to many of the most common requirements and pain points around auth.</p>\n<p>Cloud authentication and authorization is not drastically different from non-cloud systems, at least in principle. However, there are a few things about the cloud and its common use cases that introduce some curve balls:</p>\n<ul>\n<li>As with most auth systems, cloud providers each have their own idiosyncracies</li>\n<li>Cloud auth systems have almost always been designed from the outset to work API first, and interact with popular web technologies</li>\n<li>Security is usually taken very seriously in cloud, leading to workflows arguably more complex than other systems</li>\n<li>Cloud services themselves typically need some method to authenticate to the cloud, e.g. a virtual machine gaining access to private blob storage</li>\n<li>Many modern DevOps tools are commonly deployed to cloud systems, and introduce extra layers of complexity and indirection</li>\n</ul>\n<p>This blog post series is going to focus on the full picture of authentication and authorization, focusing on a cloud mindset. There is significant overlap with non-cloud systems in this, but we'll be covering those details as well to give a complete picture. Once we have those concepts and terms in place, we'll be ready to tackle the quirks of individual cloud providers and commonly used tooling.</p>\n<h2 id=\"goals-of-authentication\">Goals of authentication</h2>\n<p>We're going to define authentication as proving your identity to a service provider. A service provider can be anything from a cloud provider offering virtual machines, to your webmail system, to a bouncer at a bartender who has your name on a list. The identity is an equally flexible concept, and could be "my email address" or "my user ID in a database" or "my full name."</p>\n<p>To help motivate the concepts we'll be introducing, let's understand what goals we're trying to achieve with typical authentication systems.</p>\n<ul>\n<li>Allow a user to prove who he/she is</li>\n<li>Minimize the number of passwords a user has to memorize</li>\n<li>Minimize the amount of work IT administrator have to do to create new user accounts, maintain them, and ultimately shut them down\n<ul>\n<li>That last point is especially important; no one wants the engineer who was just fired to still be able to authenticate to one of the systems</li>\n</ul>\n</li>\n<li>Provide security against common attack vectors, like compromised passwords or lost devices</li>\n<li>Provide a relatively easy-to-use method for user authentication</li>\n<li>Allow a computer program/application/service (lets call these all apps) to prove what it is</li>\n<li>Provide a simple way to allocate, securely transmit, and store credentials necessary for those proofs</li>\n<li>Ensure that credentials can be revoked when someone leaves a company or an app is no longer desired (or is compromised)</li>\n</ul>\n<h2 id=\"goals-of-authorization\">Goals of authorization</h2>\n<p>Once we know the identity of something or someone, the next question is: what are they allowed to do? That's where authorization comes into play. A good authorization provides these kinds of features:</p>\n<ul>\n<li>Fine grained control, when necessary, of who can do what</li>\n<li>Ability to grant common sets of permissions as a bundle, avoiding tedium and mistakes</li>\n<li>A centralized collection of authorization rules</li>\n<li>Ability to revoke a permission, and see that change propagated quickly to multiple systems</li>\n<li>Ability to delegate permissions from one identity to another\n<ul>\n<li>For example: if I'm allowed to read a file on some cloud storage server, it would be nice if I could let my mail client do that too, without the mail program pretending it's me</li>\n</ul>\n</li>\n<li>To avoid mistakes, it would be nice to assume a smaller set of permissions when performing some operations\n<ul>\n<li>For example: as a super user/global admin/root user, I'd like to be able to say "I don't want to accidentally delete systems files right now"</li>\n</ul>\n</li>\n</ul>\n<p>In simple systems, the two concepts of authentication and authorization is straightforward. For example, on a single-user computer system, my username would be my identity, I would authenticate using my password, and as that user I would be authorized to do anything on the computer system.</p>\n<p>However, most modern systems end up with many additional layers of complexity. Let's step through what some of these concepts are.</p>\n<h2 id=\"users-and-policies\">Users and policies</h2>\n<p>A basic concept of authentication would be a <em>user</em>. This typically would refer to a real human being accessing some service. Depending on the system, they may use identifiers like usernames or email addresses. User accounts are often times given to non-users, like automated processes or Continuous Integration (CI) jobs. However, most modern systems would recommend using a service account (discussed below) or similar instead.</p>\n<p>Sometimes, the user is the end of the story. When I log into my personal Gmail account, I'm allowed to read and write emails in that account. However, when dealing with multiuser shared systems, some form of permissions management comes along as well. Most cloud providers have a robust and sophisticated set of policies, where you can specify fine-grained individual permissions within a policy.</p>\n<p>As an example, with AWS, the S3 file storage service provides an array of individual actions from the obvious (read, write, and delete an object) to the more obscure (like setting retention policies on an object). You can also specify which files can be affected by these permissions, allowing a user to, for example, have read and write access in one directory, but read-only access in another.</p>\n<p>Managing all of these individual permissions each time for each user is tedious and error prone. It makes it difficult to understand what a user can actually do. Common practice is to create a few policies across your organization, and assign them appropriately to each user, trying to minimize the amount of permissions granted out.</p>\n<h2 id=\"groups\">Groups</h2>\n<p>Within the world of authorization, groups are a natural extensions of users and policies. Odds are you'll have multiple users and multiple policies. And odds are that you're likely to have groups of users who need to have similar sets of policy documents. You <em>could</em> create a large master policy that encompasses the smaller policies, but that could be difficult to maintain. You could also apply each individual policy document to each user, but that's difficult to keep track of.</p>\n<p>Instead, with groups, you can assign multiple policies to a group, and multiple groups to a user. If you have a billing team that needs access to the billing dashboard, plus the list of all users in the system, you may have a <code>BillingDashboard</code> policy as well as a <code>ListUsers</code> policy, and assign both policies to a <code>BillingTeam</code> group. You may then also assign the <code>ListUsers</code> policy to the <code>Operators</code> group.</p>\n<h2 id=\"roles\">Roles</h2>\n<p>There's a downside with this policies and groups setup described above. Even if I'm a superadmin on my cloud account, I may not want to have the responsibility of all those powers at all times. It's far too easy to accidentally destroy vital resources like a database server. Often, we would like to artificially limit our permissions while operating with a service.</p>\n<p>Roles allow us to do this. With roles, we create a named role for some set of operations, assign a set of policies to it, and provide some way for users to <em>assume</em> that role. When you assume that role, you can perform actions using that set of permissions, but audit trails will still be able to trace back to the original user who performed the actions.</p>\n<p>Arguably a cloud best practice is to grant users only enough permissions to assume various roles, and otherwise unable to perform any meaningful actions. This forces a higher level of stated intent when interacting with cloud APIs.</p>\n<h2 id=\"service-accounts\">Service accounts</h2>\n<p>Some cloud providers and tools support the concept of a service account. While users <em>can</em> be used for both real human beings and services, there is often a mismatch. For example, we typically want to enable multi-factor authentication on real user accounts, but alternative authentication schemes on services.</p>\n<p>One approach to this is service accounts. Service accounts vary among different providers, but typically allow defining some kind of service, receiving some secure token or password, and assigning either roles or policies to that service account.</p>\n<p>In some cases, such as Amazon's EC2, you can assign roles directly to cloud machines, allowing programs running on those machines to easily and securely assume those roles, without needing to store any kinds of tokens or secrets. This concept nicely ties in with roles for users, making role-based management of both users and services and emerging best practice in industry.</p>\n<h2 id=\"rbac-vs-acl\">RBAC vs ACL</h2>\n<p>The system described above is known as Role Based Access Control, or RBAC. Many people are likely familiar with the related concept known as Access Control Lists, or ACL. With ACLs, administrators typically have more work to do, specifically managing large numbers of resources and assigning users to each of those per-resource lists. Using groups or roles significantly simplifies the job of the operator, and reduces the likelihood of misapplied permissions.</p>\n<h2 id=\"single-sign-on\">Single sign-on</h2>\n<p>Most modern DevOps platforms have multiple systems, each requiring separate authentication. For example, in a modern Kubernetes-based deployment, you're likely to have:</p>\n<ul>\n<li>The underlying cloud vendor\n<ul>\n<li>Both command line and web based access</li>\n</ul>\n</li>\n<li>Kubernetes itself\n<ul>\n<li>Both command line access and the Kubernetes Dashboard</li>\n</ul>\n</li>\n<li>A monitoring dashboard</li>\n<li>A log aggregation system</li>\n<li>Other company-specific services</li>\n</ul>\n<p>That's in addition to maintaining a company's standard directory, such as Active Directory or G Suite. Maintaining this level of duplication among user accounts is time consuming, costly, and dangerous. Furthermore, while it's reasonable to securely lock down a single account via MFA and other mechanisms, expecting users to maintain such information for all of these systems securely is unreasonable. And some of these systems don't even provide such security mechanisms.</p>\n<p>Instead, single sign-on provides a standards-based, secure, and simple method for authenticating to these various systems. In some cases, user accounts still need to be created in each individual system. In those cases, automated user provisioning is ideal. We'll talk about some of that in later posts. In other cases, like AWS's identity provider mechanism, it's possible for temporary identifiers to be generated on-the-fly for each SSO-based login, with roles assigned.</p>\n<p>Deeper questions arise about where permissions management is handled. Should the central directory, like Active Directory, maintain permissions information for all systems? Should a single role in the directory represent permissions information in all of the associated systems? Should a separate set of role mappings be maintained for each service?</p>\n<p>Typically, organizations end up including some of each, depending on the functionality available in the underlying tooling, and organizational discretion on how much information to include in a directory.</p>\n<h2 id=\"going-deeper\">Going deeper</h2>\n<p>What we've covered here sets the stage for understanding many cloud-specific authentication and authorization schemes. Going forward, we're going to cover a look into common auth protocols, followed by a review of specific cloud providers and tools, specifically AWS, Azure, and Kubernetes.</p>\n",
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"content": "<p>While moving to the cloud brings many benefits associated with it, we\nneed to also be aware of the pain points associated with such a move.\nThis post will discuss those pain points, provide ways to mitigate them, and\ngive you a checklist which can be used if you plan to migrate your\napplications to cloud. We will also discuss the advantages of\nmoving to the cloud.</p>\n<h2 id=\"common-pain-points\">Common pain points</h2>\n<p>One of the primary pain points in moving to the cloud is selecting the\nappropriate tools for a specific usecase. We have an abundance of tools\navailable, with many solving the same problem in different ways. To give\nyou a basic idea, this is the CNCF's (Cloud Native Computing\nFoundation) recommended path through the cloud native technologies:</p>\n<img src=\"/images/insights/cloud-prep-checklist/landscape.png\" alt=\"Cloud Native Landscape\" title=\"Cloud Native Landscape\" width=\"100%\">\n<p></p>\n<p>Picking the right tool is hard, and this is where having experience\nwith them comes in handy.</p>\n<p>Also, the existing knowledge of on-premises data centers may not be\ndirectly transferable when you plan to move to the cloud. An individual might\nhave to undergo a basic training to understand the terminology and the\nconcepts used by a particular cloud vendor. An on-premises system\nadministrator might be used to setting up firewalls via\n<a href=\"https://en.wikipedia.org/wiki/Iptables\">Iptables</a>, but he might also\nwant to consider using <a href=\"https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ec2-security-groups.html\">Security\ngroups</a>\nif he plans to accomplish the same goals in the AWS ecosystem (for EC2 instances).</p>\n<p>Another point to consider while moving to the cloud is the ease with which you\ncan easily get locked in to a single vendor. You might start using\nAmazon's <a href=\"https://docs.aws.amazon.com/autoscaling/ec2/userguide/AutoScalingGroup.html\">Auto Scaling\nGroups</a>\nto automatically handle the load of your application. But when you plan\nto switch to another cloud vendor the migration might not be\nstraightforward. Switching between cloud services isn't easy, and if you want portability, you\nneed to make sure that your applications are built with a multi-cloud\nstrategy. This will allow you to easily switch between vendors if such a\nscenario arises. Taking advantage of containers and Kubernetes may give\nyou additional flexibility and ease portability between different cloud\nvendors.</p>\n<h2 id=\"advantages-of-moving\">Advantages of moving</h2>\n<p>Despite the pain points listed above, there are many advantages involved in\nmoving your applications to cloud. Note that even big media services\nprovider like\n<a href=\"https://netflixtechblog.com/four-reasons-we-choose-amazons-cloud-as-our-computing-platform-4aceb692afec\">Netflix</a>\nhas moved on to the cloud instead of building and managing their own\ndata center solution.</p>\n<h3 id=\"cost\">Cost</h3>\n<p>One of the primary advantages of leveraging the cloud is avoiding\nthe cost of building your\nown data center. Building a secure data center is not trivial. By\noffloading this activity to an external cloud provider, you can instead build your\napplications on top of the infrastructure provided by them. This not\nonly saves the initial capital expenditure but also saves headaches from\nreplacing hardware, such as replacing failing network switches. But note that\nswitching to the cloud will not magically save cost. Depending on your\napplication's architecture and workload, you have to be aware of the\nchoices you make and make sure that your choices are cost efficient.</p>\n<h3 id=\"uptime\">Uptime</h3>\n<p>Cloud vendors provide SLAs (Service Level Agreements) where they state\ninformation about uptime and the guarantees they make. This is a\nsnapshot from the Amazon Compute SLA:</p>\n<p><img src=\"/images/insights/cloud-prep-checklist/sla.png\" alt=\"SLA\" title=\"SLA\" /></p>\n<p>All major cloud providers have historically provided excellent uptime,\nespecially for applications that properly leverage availability zones.\nBut depending on a specific\nusecase/applications, you should define the acceptable uptime for your\napplication and make sure that your SLA matches with it. Also depending\non the requirements, you can architect your application such that it has\nmulti region deployments to provide a better uptime in case there is an\noutage in one region.</p>\n<h3 id=\"security-and-compliance\">Security and Compliance</h3>\n<p>Cloud deployments provide an extra benefit when working in regulated industries\nor with government projects. In many cases, cloud vendors provide regulation-compliant\nhardware.\nBy using cloud providers, we can take advantage of the various\ncompliance standards (eg: HIPAA, PCI etc) they meet.\nValidating an on-premises data center against such standards can be a time consuming,\nexpensive process. Relying on already validated hardware can be faster, cheaper, easier,\nand more reliable.</p>\n<p>Broadening the security topic, cloud vendors typically also provide\na wide range of additional security tools.</p>\n<p>Despite these boons,\nproper care must still be taken, and best practices must still be followed,\nto deploy an application securely.\nAlso, be aware that running on compliant hardware does not automatically\nensure compliance of the software. Code and infrastructure must still meet\nvarious standards.</p>\n<h3 id=\"ease-of-scaling\">Ease of scaling</h3>\n<p>With cloud providers, you can easily add and remove machines or add more\npower (RAM, CPU etc) to them. The ease with which you can horizontally and\nvertically scale your application without worrying about your\ninfrastructure is powerful, and can revolutionize how your approach\nhardware allocation. As your applications load increases,\nyou can easily scale up in a few minutes.</p>\n<p>One of the perhaps surprising benefits of this is that you don't need to\npreemptively scale up your hardware. Many cloud deployments are able\nto reduce the total compute capacity available in a cluster, relying\non the speed of cloud providers to scale up in response to increases in demand.</p>\n<h3 id=\"focus-on-problem-solving\">Focus on problem solving</h3>\n<p>With no efforts in maintaining the on-premises data center, you can\ninstead put your effort in your application and the problem it solves.\nThis allows you to focus on your core business problems and your\ncustomers.</p>\n<p>While not technically important, the cloud providers have energy\nefficient data centers and run it on better efficiency. As a case study,\n<a href=\"https://cloud.google.com/blog/topics/google-cloud-next/our-heads-in-the-cloud-but-were-keeping-the-earth-in-mind\">Google even uses machine learning technology to make its data centers\nmore\nefficient</a>.\nHence, it might be environmentally a better decision to run your\napplications on cloud.</p>\n<h2 id=\"getting-ready-for-cloud\">Getting ready for Cloud</h2>\n<p>Once you are ready for migrating to the cloud, you can plan for the next\nsteps and initiate the process. We have the following general checklist\nwhich we usually take and tailor it based on our clients requirements:</p>\n<h3 id=\"checklist\">Checklist</h3>\n<ul>\n<li>Make a list of your applications and dependencies which need to be\nmigrated.</li>\n<li>Benchmark your applications to establish cloud performance\nKPIs (Key Performance Indicators).</li>\n<li>List out any required compliance requirements for your\napplication and plan for ensuring it.</li>\n<li>Onboard relevant team members to the cloud service's use management\nsystem, ideally integrating with existing user directories and\nleveraging features like single sign on and automated user provisioning.</li>\n<li>Establish access controls to your cloud service, relying on role based\nauthorization techniques.</li>\n<li>Evaluate your migration options. You might want to re-architect it\nto take advantage of cloud-native technologies. Or you might simply\ndecide to shift the existing application without any changes.</li>\n<li>Create your migration plan in a Runbook.</li>\n<li>Have a rollback plan in case migration fails.</li>\n<li>Test your migration and rollback plans in a separate environment.</li>\n<li>Communicate about the migration to internal stakeholders and customers.</li>\n<li>Execute your cloud migration.</li>\n<li>Prune your on-premises infrastructure.</li>\n<li>Optimize your cloud infrastructure for your workloads.</li>\n</ul>\n<h2 id=\"conclusion\">Conclusion</h2>\n<p>I hope we were able to present you the challenges involved in\nmigration to cloud and how to prepare for them. We have helped various\ncompanies in migration and other devops services. Free feel to <a href=\"https://www.fpcomplete.com/contact-us/\">reach out to\nus</a> regarding any questions on\ncloud migrations or any of the other services.</p>\n",
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"content": "<p>DevOps Security and Privacy—FP Complete’s\ncomprehensive, easy to understand guide designed\nto help you understand why they’re so critical to\nthe safety of your DevOps strategy.</p>\n<p>The following is a transcription of a live\nwebinar given by <a href=\"https://www.fpcomplete.com/\">FP Complete</a>\nFounder and Chairman Aaron Contorer, on\n<a href=\"https://www.youtube.com/user/FPComplete\">FP Complete's YouTube Channel</a>.</p>\n<h2 id=\"introducing-aaron\">Introducing Aaron</h2>\n<p>I’m the Founder and Chairman of <a href=\"https://www.fpcomplete.com/\">FP Complete</a>,\nwhere we help companies use state-of-the-art tools and\ntechniques to produce secure, lightning-fast,\nfeature-rich software, faster and more often.</p>\n<p>Before founding FP Complete, I was an\nexecutive at Microsoft, where I served as program\nmanager for distributed systems, and general\nmanager of Visual C++, the leading software\ndevelopment tool at that time. Also, I \narchitected MSN’s move to Internet-based server\nsoftware, served as the full-time technology\nadviser to Bill Gates, and I founded and ran the\ncompany’s Productivity Tools Team for complex\nsoftware engineering projects.</p>\n<p>Okay, so enough about me. Let’s begin this\ndiscussion by recognizing our industry’s\nunfortunate—but preventable—reality:</p>\n<h2 id=\"breaches-are-happening-far-too-often\">Breaches are happening far too often</h2>\n<p>We all know how bad the state of the\nworld is within security and privacy\nright now. Projects are getting very\ncomplicated. And I—just as a sample—want\nto point out that this is a very typical\nbreach. Monzo said that for six months\nunauthorized people had access to\npeople’s secret code numbers, their pin\nnumbers. I’m not singling them out at\nall, but rather saying… “This is very\ntypical.” They’re a bank, and they\ncompromised this type of data for months\nand months.</p>\n<p>How does it happen? It’s not only\nbecause of logging and monitoring not\nbeing in place, although that can be a\nbig factor. It’s because of complexity.\nHonestly, we’re all trying very hard to\ndo our jobs, but users keep asking and\nexecutives keep asking for new features.\nAnd that integration just creates point\nafter point where problems can happen,\nand things get overlooked.</p>\n<h2 id=\"opportunities-for-penetration-are-everywhere\">Opportunities for penetration are everywhere</h2>\n<p>I would argue that today’s\napplications are more about assembling\nbuilding blocks than they are about just\nwriting new code. But every time you\nincrease that complexity by adding more\nbuilding blocks, you increase the number\nof interface points between\ncomponents—the number of places where\nsomebody might have done something wrong.\nAnd so we’re really creating a system of\nentry points between component A and\ncomponent B. But entry points—that sounds\nlike something I would compromise if I\nwere a security violator, right?\nFurthermore, we’re manually configuring\nour systems. People aren’t using\ncontinuous deployment. And so there is\nsome wizard who’s supposed to go set up\nthe latest server or integrate it with\nthe database or integrate it with the web\nwith a firewall or whatever they’re\nsupposed to do. Every manual step creates\nfurther opportunities for penetration,\nfor defects, because people are\nimperfect. Even the best person in your\nteam doing a process a hundred times\nmight do it wrong, one or two times. An\nautomated scanner is going to find that\ntime, and it’s going to break into your\nsystem before you know it.</p>\n<h2 id=\"let-s-talk-devsecops\">Let’s talk DevSecOps</h2>\n<p>DevSecOps—DevOps with security stuck\nright in the middle. And I think that’s a\ngood way of looking at this problem. We\nwant to integrate all the different parts\nof our engineering into one pool of\nautomation, and include security and\nquality assurance as part of that\nautomated process. We talked earlier\nabout automated testing being part of our\nbuilds. But we want to go much farther\nthan that, as technical teams. We want to\nstart from the beginning of our projects,\ntalking about how secure they need to be.\nWhat are the risks that they’re supposed\nto defend against or or not create? We\nwant every member of the team to\nunderstand that system downtime—because\nsomebody broke in and trashed it, or even\nworse privacy violations which you can\nnever undo, because when people’s\npersonal information has been published,\nyou can’t unpublish it—we need to let our\nteam members know that these are\npriorities and put them on the to-do list\nfor the project. And we can’t call it\nsomething done if the security part isn’t\ndone. It’s not something we tack on at\nthe end. We don’t build in unsecured,\ncrazy, poorly architected apps, and then\nat the end, ask someone to build a brick\nwall around them. Because as soon as one\nlittle person gets through the brick\nwall, it’s open season. So, we want the\nengineers to know everything they do\nshould be checked for security. That’s a\nculture change to say that it’s\neveryone’s job.</p>\n<p>We need to integrate quality assurance\nwith security, which means somebody is\nchecking the software we wrote for\nweaknesses; somebody is trying to break\nin or, at least, trying to run tools that\nwill show us common ways to break in and\nweather their presence.</p>\n<p>And we need to inspect our cloud\nsystems that are running to make sure\nthat our deployment, and our system\noperations and administration, is as\nsecure as we meant it to be. Did somebody\nomit a step? We want to discover that\nright away and fix it. Or, ideally,\nautomate the way we set up all of our\nsystems using, for example, an\norchestration software package to\nautomatically configure our servers, so\nit isn’t the case that late in the day,\npeople are more likely to make a mistake.\nBecause, well written scripts do just as\ngood a job even when they’re tired.</p>\n<p>And we want to make sure that all of\nour systems are updated and patched and\nnot tell people that security is a waste\nof time and they should get back to work\non features.</p>\n<h2 id=\"process-tips\">Process tips</h2>\n<p>To do all this, we need to have a\nsimple design. And I would encourage\npeople to focus on the idea that\nsimplicity and modular design are great\nways to make a system easier to check for\nsecurity holes.</p>\n<p>We want to make sure that credentials\nthat are used in our modular\nsystems—where one piece of software is\nlogging into another service or another\npiece of software database—are kept in\nproperly secured credential storage. A\ncommon form of security violations is you\nlook at somebody’s source code and… Oh\nlook! There’s the password for the\ndatabase server right there …because the\napp had to connect to the server. That’s\ninappropriate design. There are special\ncredential storage services—your team\nshould use them.</p>\n<p>And we want to make sure that quality\ncontrol remains central to our culture,\nas developers of software, and that\nincludes DevOps, that includes system\nadministration. Too often, we have a good\npiece of software, and then it’s deployed\nincorrectly. And that’s where the problem\noccurs. So if you’re going to test\nwhether your code is written properly,\nmaybe also test whether the servers\nconfigured properly, from time to time.\nIt’s time well spent.</p>\n<h2 id=\"how-to-strengthen-your-security\">How to strengthen your security</h2>\n<p>So how can you move forward on\nsecurity? The good news is, while it may\nsound like a scary and intimidating area,\nthere are lots of practical steps you can\ntake right now, and you don’t even have\nto take them all at the same time, you\ncan take them incrementally. Here are\nsome great steps though that I highly\nrecommend.</p>\n<p>One is that—in your engineering team,\nand if you have multiple teams—in each\nengineering team somebody is explicitly\nthe security person. Somebody knows that\nit’s their job to keep an eye out for\nsecurity issues and prevention and that\nif there’s a problem they’re the person\nwho’s going to hear about it. They should\nhave the power to look into anything they\nneed to make sure there isn’t a security\nhole in the system.</p>\n<p>Use best practices from other\ncompanies. This is a great idea\nthroughout all of DevOps, including\nDevSecOps. You don’t have to reinvent\nanything. You can learn best practices\nand get a checklist together of what\nother companies have found helpful to\nlook for to find opportunities to secure\nyour system incrementally. We just piece\nby piece chip away at the risks that are\npresent in our systems. We don’t have to\nwait until some magic day when all of\nsecurity happens at once.</p>\n<p>Teach your people about security. A\nlot of security problems happen because\none person didn’t realize… Who didn’t\nknow that you’re supposed to not put\npasswords in the source code where\neveryone can see them? Well, one person\ntyped a password into the source code,\nbut now it’s there for everyone. So be\nsure that training and security, and how\nimportant it is, and how to do it is\navailable to everyone in your team. And\nmake sure that there’s a checklist. Who\ntook the security training? Who’s not\nbeen to security training yet?</p>\n<p>Scary but true fact: You should,\naccording to Price Waterhouse Coopers, if\nyou want to be a normal IT operation, be\nspending 11 to 15% of your IT budget on\nsecurity overall. That’s a significant\nnumber. And I think we can all agree that\nwith more internet work and more\nimporting of modules and stuff, we, if\nanything, could be worried that that\nnumber is going to go up. So automation\nthrough DevOps is really a way to keep a\nlid on that number. But I wouldn’t think\nof it as a way to make that number drive\ndown towards zero. Security is everyone’s\njob, and it’s going to remain that\nway.</p>\n<p>Beyond that, I’d say use it use the\nother techniques we talked about earlier\nin this presentation. You don’t have to\nbe the next Equifax, of having no\nmonitoring. You don’t have to allow silly\nmistakes by having no automation. And you\ndon’t have to create more security holes\nby reinventing your own tools and\nprocesses using components. Reuse is your\nfriend.</p>\n<h2 id=\"7-tech-ideas-you-can-start-now\">7 tech ideas you can start now</h2>\n<p>I won’t spend too long on this, but I\nwanted this for people who are more\nhands-on or the people who are\nsupervising hands-on engineers. These are\nsome practical steps that you can take to\nstart turning on pieces of security,\nright now. Every one of these—except\nperhaps service-oriented architecture—is\nsomething that literally you could task\nsomebody to do this week or next\nweek.</p>\n<p>These are straightforward tasks.</p>\n<ol>\n<li>Ensure all databases have firewalls on them. They’re a common data breach source!</li>\n<li>Use a password manager to generate secure passwords; enable two-factor authentication.</li>\n<li>Use roles and policies to assign specific permissions to users and services instead of running everything from root credentials or privileged users.</li>\n<li>Use bastion hosts or VPNs to limit access to internal machines.</li>\n<li>Use service-oriented architecture (SOA) to break off components that need high privilege.</li>\n<li>Include code analysis tools in the dev process and enforce fixes prior to deployment.</li>\n<li>Test your servers with automated scanners for break-in vulnerabilities.</li>\n</ol>\n<h2 id=\"fast-to-market-reliable-and-secure\">Fast to market, reliable, and secure</h2>\n<p>It’s a winning formula!</p>\n<p>So, in short, you have a choice to\nturn on DevOps to use a lot of technology\nthat’s been solved, a lot of best\npractices and engineering techniques that\nhave already been solved and tested at\nnumerous other companies—clients of ours,\nfamous internet companies, everyone. When\nI say “everyone”, the truth is the\nminority of companies are already using\nproper DevOps. But enough companies that\nyou don’t have to be the first, you don’t\nhave to be the Pioneer. DevOps is a\nwinning formula that will get you to\nmarket faster, and more reliable, and\nwith better security. Or you could be the\nnext Equifax and the next Capital One,\nwhich is the default situation.</p>\n<h2 id=\"need-help-with-devops-security-and-privacy\">Need help with DevOps Security and Privacy?</h2>\n<p>FP Complete offers corporations its\nDevOps Success Program which offers\nadvanced Privacy and Security software\nengineering mentoring among many other\nmoving parts in the DevOps world.</p>\n<p>For more information, please <a href=\"https://www.fpcomplete.com/contact-us/\">contact us</a> or see our <a href=\"https://www.fpcomplete.com/devops/\">DevOps homepage</a>.</p>\n",
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"content": "<p>Continuous integration and deployment, monitoring and logging, and security and privacy—FP Complete’s comprehensive, easy to understand guide designed to help you learn why those three DevOps strategies collectively create an environment where high-quality software can be developed quicker and more efficiently than ever before.</p>\n<p>Aaron Contorer, founder and chairman of FP Complete, presented the following webinar. Read below for a transcript of the video.</p>\n<iframe width=\"560\" height=\"315\" src=\"https://www.youtube.com/embed/5U11unR_py0\" frameborder=\"0\" allow=\"accelerometer; autoplay; encrypted-media; gyroscope; picture-in-picture\" allowfullscreen></iframe>\n<h2 id=\"introducing-aaron\">Introducing Aaron</h2>\n<p>I’m the Founder and Chairman of <a href=\"https://www.fpcomplete.com/\">FP Complete</a>,\nwhere we help companies use\nstate-of-the-art tools and techniques to\nproduce secure, lightning-fast,\nfeature-rich software, faster and more\noften.</p>\n<p>Before founding FP Complete, I was an\nexecutive at Microsoft, where I served as\nprogram manager for distributed systems,\nand general manager of Visual C++, the\nleading software development tool at that\ntime. Also, I architected MSN’s move\nto Internet-based server software, served\nas the full-time technology adviser to\nBill Gates, and I founded and ran the\ncompany’s Productivity Tools Team for\ncomplex software engineering\nprojects.</p>\n<p>Okay, so enough about me. Let’s begin\nthis presentation by stating the\nobvious:</p>\n<h2 id=\"software-development-is-complicated\">Software development is complicated</h2>\n<p>As information technology and software\npeople, it’s easy to recognize how things\nare changing at an astonishing speed. To\nkeep pace, we need tools and processes\nthat allow us to rapidly deploy better\ncode more frequently with fewer errors.\nIs that a high bar to reach? Yes, of\ncourse, it is. But it absolutely must be\nmet—that is <em>if</em> you\nwant your company to survive.</p>\n<h2 id=\"inefficiencies-are-everywhere\">Inefficiencies are everywhere</h2>\n<p>In most companies, I would argue that\nthe information technology team and the\nsoftware engineering team are not totally\ntrusted by the rest of the company.</p>\n<p>Of course, I don’t mean they’re not\ntrusted as in they’re not good, smart\npeople. What I mean is that they don’t\nmeet their deadlines, leading to sprints\nbecoming longer than initially expected,\nultimately causing everyone to feel\nrushed and end results lacking in\nquality.</p>\n<h2 id=\"it-has-lost-management-s-trust\">IT has lost management’s trust</h2>\n<p>When management begins to not trust\nengineering and IT, a bad dynamic\ndevelops. No longer does the team get to\nfocus on building great things for their\nend-users. Instead, they’re forced to\nfocus on solving their struggles and\ndealing with interpersonal friction.</p>\n<p>Believe it or not, the problems we’re\nhaving aren’t people-problems. It’s not\nthat they lack good intentions or\nbrainpower.</p>\n<p>Instead, the problem is this:</p>\n<h2 id=\"modern-software-ancient-tech\">Modern software, ancient tech</h2>\n<p><strong>Modern software development can’t be performed using ancient technologies applied within simplistic workflows.</strong></p>\n<p>I often like to say…</p>\n<p><em>“The best craftsperson with a\nhandsaw cannot do woodworking as\nefficiently as a robotic cutting\ntool.”</em></p>\n<p>When we automate our work, it becomes\nfaster and easier to replicate. We don’t\nbuild in lots of mistakes. As a result,\nwe get to move on with our lives instead\nof going back and reworking things over\nand over again.</p>\n<p>When we automate with good tools and\nbetter processes programmed in, and we\nrepeat this same process every time,\neveryone can trust that our work will be\nperformed with quality, and our systems\nwill be more safe and secure.</p>\n<p>Sounds ideal, doesn’t it? Of course,\nit does.</p>\n<p>But how do you do it? How do you\nevolve from the environment you’re\noperating in today to the utopia DevOps\nstrategies will allow you to live and\nwork within well into the future?</p>\n<p>Learn more about how <a href=\"https://www.fpcomplete.com/devops/\">FP Complete does DevOps</a>.</p>\n",
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"content": "<p>On February 2, 2020, one of FP Complete's Lead Software Engineers—Mike McGirr—presented a webinar on using Rust for creating DevOps tooling.</p>\n<h2 id=\"webinar-outline\">Webinar Outline</h2>\n<p>FP Complete is hosting a functional programming\nwebinar on, “Learn Rapid Rust with DevOps Success\nStrategies.” A beginner’s guide including sample Rust\ndemonstration on writing your DevOps tools with Rust\nover Haskell. An introduction to Rust, with basic DevOps\nuse cases, and the library ecosystem, airing on\nFebruary 5th, 2020.</p>\n<p>The webinar will be hosted by Mike McGirr, a DevOps\nSoftware Engineer at FP Complete which will provide an\nabundance of Rust information with respect to\nfunctional programming and DevOps, featuring (safety,\nspeed and accuracy) that make it unique and contributes\nto its popularity, and its possible preference as a\nlanguage of choice for operating systems over Haskell,\nweb browsers and device drivers among others. The\nwebinar offers an interesting opportunity to learn and\nuse Rust in developing real world projects aside from\nHaskell or other functional programming languages\navailable today.</p>\n<h2 id=\"topics-covered\">Topics covered</h2>\n<p>During the webinar we will cover the following\ntopics:</p>\n<ul>\n<li>A quick intro and background into the Rust programming language</li>\n<li>Some scenarios and reasons why you would want to use Rust for writing your DevOps tooling (and some reasons why you wouldn’t)</li>\n<li>A small example of using the existing AWS libraries to create a basic DevOps tool</li>\n<li>How to Integrate FP into your Organization</li>\n</ul>\n<p>Mike Mcgirr, a Lead Software Engineer at FP\nComplete,will help us understand reasoning that\nsupports using Rust over other functional programming\nlanguages offered in the market today.</p>\n<h2 id=\"more-about-your-host\">More about your host</h2>\n<p>The webinar will be hosted by Mike McGirr, a veteran\nDevOps Software Engineer at FP Complete. With years of\nexperience in DevOps software development, Mike will\nwalk us through a first in a series of Rust webinars\ndiscussing why we would, and how we could utilize Rust\nas a functional programming language to build DevOps\nover other functional programming languages available\nin the market today. Mike will also share with us a\nsmall example script written in Rust showing how Rust\nmay be used.</p>\n",
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"content": "<h2 id=\"introduction\">Introduction</h2>\n<p>As the world adopts blockchain technologies, your IT infrastructure — and its\npredictability — become critical. Many companies lack the levels of automation\nand control needed to survive in this high-opportunity, high-threat environment.</p>\n<p>Are your software, cloud, and server systems automated and robust enough? Do you\nhave enough quality control for both your development and your online operations?\nOr will you join the list of companies bruised by huge data breaches and loss o\nf control over their own computer systems? If you are involved in blockchain, or\nany industry for that matter, these are the questions you need to ask yourself.</p>\n<p>Blockchain will require you to put more information online than ever before,\ncreating huge exposures for organizations that do not have a handle on their\nsecurity. Modern DevOps technologies, including many open-source systems, offer\npowerful solutions that can improve your systems to a level suitable for use with\nblockchain.</p>\n<h2 id=\"are-companies-really-ready-for-blockchain-technology\">Are companies REALLY ready for Blockchain technology?</h2>\n<p>The answer to it is most of the companies are NOT and those who are need to audit\nor reevaluate whether they are. The reason is BlockChain puts data to public making\nit prone to outside attacks if systems are not hardenend and updated on timely\nmanner.</p>\n<p>Big companies such as Equifax had millions of records stolen, Heartland credit\nprocessing was hacked and eventually had to pay 110 million and Airbus A400M due \nto wrong installation of manual software patch resulted in death of everyone on\non the plain. These are few of many such big companies that was hacked due to poorly\nimplemented IT technology.</p>\n<p>Once hailed as unhackable, blockchains are now getting hacked. According to a MIT\ntechnology review, hackers have stolen nearly $2 billion worth of cryptocurrency\nsince the beginning of 2017.</p>\n<h2 id=\"big-question-why-companies-are-getting-hacked\">Big Question: Why Companies are getting hacked ?</h2>\n<p>Blockchain itself isn't always the problem. Sometimes the blockchain is secure \nbut the IT infrastructure is not capable to supporting it. There are cases where \nopen firewalls, unencrypted data, poor testing and manual errors were reasons \nbehind the hacking.</p>\n<p>So, the question to ask is: Is the majority of your IT infrastructure secure \nand reliable enough to support Blockchain Technology ?</p>\n<h2 id=\"what-is-an-it-factory\">What is an IT Factory ?</h2>\n<p>IT factory as per <a href=\"https://www.fpcomplete.com/our-team/\">Aaron Contorer</a>, founder \nand Chariman of FP Complete is divided into 3 parts</p>\n<ol>\n<li>Development</li>\n<li>Deployment</li>\n<li>System Operations</li>\n</ol>\n<p>If IT factory is implemented properly at each stage it could result in a new and\nbetter IT services leading to a more reliable, scalable and secure environment.</p>\n<p>Deployment is a bridge that allows software running on a developer laptop all the\nway to a scalable system and running Ops for monitoring. With DevOps practice,\nwe can ensure all the three stages of IT factory implemented.</p>\n<p>But, the key to build a working IT factory is Automation that ensure each step\nin the deployment process is reliable. With microservices architecture ,building\nand testing a reliable containerized based system is much easier now compared to\nthe earlier days.</p>\n<p>The only way to ensure a reliable, reproducible system is if companies start\nautomating each step of their software life cycle journey. Companies that are ensuring\ngood DevOps practices have a robust IT infrastructure compared to those that are\nNOT.</p>\n<h2 id=\"devops-for-blockchain\">DevOps for Blockchain</h2>\n<p>DevOps tools helps BlockChain better as it can ensure all code is tracked, tested,\ndeployed automatically, audited and Quality Assurance tested along each stage of\nthe delivery pipeline.</p>\n<p>The other benefits of having DevOps methods implemented in BlockChain is that it \nreduces the overall operational cost to companies, speeds up the overall pace of \nsoftware development and release cycle, improves the software quality and increases\nthe productivity.</p>\n<p>The following DevOps methods, if implemented in Blockchain, can be very helpful</p>\n<p><strong>1. Engineer for Safety</strong></p>\n<ul>\n<li>With proper version control tool like GITHUB , source code can be viewed,\ntracked with proper history of all changes to the base</li>\n<li>Development tools used by developers should be of the same version, should be\ntracked and should be uniform across the project</li>\n<li>Continuous Integration (CI) pipeline must be implemented at the development\nstage to ensure nothing breaks on each commit. There are tools such as Jenkins,\nBamboo, Code Pipeline and many more that can help in setting up a proper CI .</li>\n<li>Each commit should be properly tested using test case management system with\nproper unit test cases for each commit</li>\n<li>Each Project should also have an Issue tracking system like JIRA, GITLAB etc\nto ensure all requests are properly tracked and closed.</li>\n</ul>\n<p><strong>2. Deploy for Safety</strong></p>\n<ul>\n<li>Continuous Deployment via DevOps tools to ensure code is automatically deployed\nto each environment</li>\n<li>Each environment (Development, Testing, DR, Production) should be a replica\nof each other</li>\n<li>Allow automation to setup all relevant infrastructure related to allow successful\ndeployment of code</li>\n<li>Setup infrastructure as code (IAC) to provision infrastructure that helps in\nreducing manual errors</li>\n<li>Sanity of each deployment by running test cases to ensure each component is\nfunctioning as expected</li>\n<li>Running Security testing after each Deployment on each environment</li>\n<li>Ensure system can be RollBack/Rollforward without any manual intervention like\nCanary/Blue-Green Deployment</li>\n<li>Use container based deployments that provide more reliability for deployments</li>\n</ul>\n<p><strong>3. Operate for Safety</strong></p>\n<ul>\n<li>Set up Continuous Automated Monitoring and Logging</li>\n<li>Set up Anomaly detection and alerting mechanism</li>\n<li>Set up Automated Response and Recovery for any failures</li>\n<li>Ensure a Highly Available and scalable system for reliability</li>\n<li>Ensure data is encrypted for all outbound and inbound communication</li>\n<li>Ensure separation of admin powers, database powers, deployment powers , user \naccess etc. The more the powers are separated the lesser the risk</li>\n</ul>\n<p><strong>4. Separate for Safety</strong></p>\n<ul>\n<li>Separate each system internally from each other by using multiple small networks.\nFor Eg: database/backend on private subnets while UI on public subnets</li>\n<li>Set Internal and MutFirewalls ensure the database systems are protected with no access</li>\n<li>Separate Responsibility and credentials for reduce risk of exposure</li>\n</ul>\n<p><strong>5. Human systems</strong></p>\n<p>Despite keeping hardware and software checks, most the breaking of blockchain\nsystems today has happened because of "People" or "Human Errors".</p>\n<p>Most people try hacks/workaround to get stuff working on production with no knowledge\non the impacts it could do on the system. Sometimes these stuff are not documented\nmaking it hard for the other person to fix it. Sometimes asking others to login\nto unauthorized systems by sharing credentials over calls paves a path for unsecure\nsystems</p>\n<p>To ensure companies must,</p>\n<ul>\n<li>Train people to STOP doing manual efforts to fix a broken system.</li>\n<li>Train people NOT to do "Social Engineering" like asking colleagues \nto login to systems on their behalf, sharing passwords etc.</li>\n</ul>\n<p><strong>6. Quality Assurance</strong></p>\n<ul>\n<li>Need to review the Architectural as well as best practices are ensured in the\nproduct life cycle</li>\n<li>Need to ensure the code deploy pipeline has scope for penetration Testing</li>\n<li>Need to ensure there is weekly/monthly auditing of metrics, logs , systems to\ncheck for threats to the systems</li>\n<li>Each component and patch on system should be tested and approved by QA before\nrolling out to Production</li>\n<li>Companies could also hire third parties to audit their system on their behalf</li>\n</ul>\n<h2 id=\"how-to-get-there\">How to get there ?</h2>\n<p>The good news is "IT IS POSSIBLE". There is no need for giant or all-in-one solutions.</p>\n<p>Companies that are starting fresh need to start at the early phase of development\nto building a reliable system by focussing on above 6 points mentioned above. They\nneed to start thinking on all areas in the "Plan and Design" phase itself.</p>\n<p>For companies who are already on production or nearing production does not need\nto have to start fresh . They can start making incremental progress but it needs\nto start TODAY.</p>\n<p>Automation is the only SCIENCE in IT that can reduce errors and help towards building \na more and more reliable system. It will in the future save money and resources that \ncan be redirected to focus on other areas.</p>\n<p>To conclude, <a href=\"https://www.fpcomplete.com\">FP Complete</a> has been a leading consultant \non providing DevOps services. We excel at what we do and if you are looking to implement \nDevOps in your BlockChain. Please feel free to reach out to us for free consultations.</p>\n",
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"content": "<p>In this blog post, we will learn how to control access to nomad.</p>\n<h2 id=\"introduction\">Introduction</h2>\n<p>Nomad is an application scheduler, that helps you schedule application-processes\nefficiently, across multiple servers, and keep your infrastructure costs low.\nNomad is capable of scheduling containers, virtual machines, as well as isolated\nforked processes.</p>\n<p>There are other schedulers available, such as Kubernetes, Mesos or Docker Swarm,\nbut each has different mechanisms for securing access. By following this post,\nyou will understand the main components in securing your Nomad cluster, but the\noverall idea is valid across any of the other schedulers available.</p>\n<p>One of Nomad's selling points, and why you could consider it over tools like\nKubernetes, is that you can schedule not only containers, but also QEMU\nimages, LXC, isolated <code>fork/exec</code> processes, and even Java applications in a\nchroot(!). All you need is a driver implemented for Nomad. On the other hand,\nits community is smaller than Kubernetes, so the tradeoffs have to be measured\non a project-by-project basis.</p>\n<p>We will start by deploying a test cluster and configuring access control lists\n(ACLs).</p>\n<h2 id=\"overview\">Overview</h2>\n<ul>\n<li>Nomad uses tokens to authenticate client requests.</li>\n<li>Each token is associated with policies.</li>\n<li>Policies are a collection of rules to allow or deny operations on resources.</li>\n</ul>\n<p>In this tutorial, we will:</p>\n<ol>\n<li>Setup our environment to run nomad inside a Vagrant virtual machine for running experiments</li>\n<li>We generate a root/admin token (usually known as the "management" token) and activate ACLs</li>\n<li>Using the management token, we add a new "non-admin" policy and create a token associated with this new policy</li>\n<li>Use the "non-admin" token to demonstrate access control.</li>\n</ol>\n<h2 id=\"setup-the-environment\">Setup the environment</h2>\n<p>Pre-requisites:</p>\n<ul>\n<li>POSIX shell, such as GNU Bash</li>\n<li>Vagrant > <code>2.0.1</code></li>\n<li>Nomad demo <a href=\"https://raw.githubusercontent.com/hashicorp/nomad/master/demo/vagrant/Vagrantfile\"><code>Vagrantfile</code></a></li>\n</ul>\n<p>We will run everything from within a virtual machine with all the necessary\nconfiguration and applications. Execute the following commands on your shell:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code>$ cd $(mktemp --directory)\n$ curl -LO https://raw.githubusercontent.com/hashicorp/nomad/master/demo/vagrant/Vagrantfile\n$ vagrant up\n ...\n lines and lines of Vagrant output\n this might take a while\n ...\n$ vagrant ssh\n ...\n Message of the day greeting from VM\n Anything after this point is being executed inside the virtual machine\n ...\nvagrant@nomad:~$ nomad version\nNomad vX.X.X\nvagrant@nomad:~$ uname -n\nnomad\n</code></pre>\n<p>Depending on your system and the version of <code>Vagrantfile</code> used, the prompt may\nbe different.</p>\n<h2 id=\"setup-nomad\">Setup Nomad</h2>\n<p>We configure nomad to execute both as server and client for convenience, as\nopposed to a production environment where the server is remote and client is\nlocal to each machine or node. Create a <code>nomad-agent.conf</code> with the following\ncontents:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code>bind_addr = "0.0.0.0"\ndata_dir = "/var/lib/nomad"\nregion = "global"\nacl {\n enabled = true\n}\nserver {\n enabled = true\n bootstrap_expect = 1\n authoritative_region = "global"\n}\nclient {\n enabled = true\n}\n</code></pre>\n<p>Then, execute:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code>vagrant@nomad:~$ sudo nomad agent -config=nomad-agent.conf # sudo is needed to run as a client\n</code></pre>\n<p>You should see output indicating that Nomad is running.</p>\n<blockquote>\n<p>Clients need root access to be able to execute processes, while servers only\ncommunicate to synchronize state.</p>\n</blockquote>\n<h2 id=\"acl-bootstrap\">ACL Bootstrap</h2>\n<p>On another terminal, after running <code>vagrant ssh</code> from our temporary working\ndirectory, run the following command:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code>vagrant@nomad:~$ nomad acl bootstrap\n\nAccessor ID = 2f34299b-0403-074d-83e2-60511341a54c\nSecret ID = 9fff6a06-b991-22db-7fed-55f17918e846\nName = Bootstrap Token\nType = management\nGlobal = true\nPolicies = n/a\nCreate Time = 2018-02-14 19:09:23.424119008 +0000 UTC\nCreate Index = 13\nModify Index = 13\n</code></pre>\n<p>This <code>Secret ID</code> is our <code>management</code> (admin) token. This token is valid globally\nand all operations are permitted. No policies are necessary while authenticating\nwith the management token, and so, none are configured by default.</p>\n<p>It is important to copy the <code>Accessor ID</code> and <code>Secret ID</code> to some file, for\nsafekeeping, as we will need these values later. For a production environment,\nit is safest to store these in a separate vault permanently.</p>\n<p>Once ACLs are on, all operations are denied <em>unless</em> a valid token is provided\nwith each request, and the operation we want is allowed by a policy associated\nwith the provided token.</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code>vagrant@nomad:~$ nomad node-status\nError querying node status: Unexpected response code: 403 (Permission denied)\n\nvagrant@nomad:~$ export NOMAD_TOKEN='9fff6a06-b991-22db-7fed-55f17918e846' # Secret ID, above\nvagrant@nomad:~$ nomad node-status\n\nID DC Name Class Drain Status\n1f638a17 dc1 nomad <none> false ready\n</code></pre><h2 id=\"designing-policies\">Designing policies</h2>\n<p>Policies are a collection of (ideally, non-overlapping) roles, that provide\naccess to different operations. The table below shows typical users of a Nomad\ncluster.</p>\n<table><thead><tr><th>Role</th><th>Namespace</th><th>Agent</th><th>Node</th><th>Remarks</th></tr></thead><tbody>\n<tr><td>Anonymous</td><td><code>deny</code></td><td><code>deny</code></td><td><code>deny</code></td><td>Unnecessary, as token-less requests are denied all operations.</td></tr>\n<tr><td>Developer</td><td><code>write</code></td><td><code>deny</code></td><td><code>read</code></td><td>Developers are permitted to debug their applications, but not to perform cluster management</td></tr>\n<tr><td>Logger</td><td><code>list-jobs</code>, <code>read-logs</code></td><td><code>deny</code></td><td><code>read</code></td><td>Automated log aggregators or analyzers that need read access to logs</td></tr>\n<tr><td>Job requester</td><td><code>submit-job</code></td><td><code>deny</code></td><td><code>deny</code></td><td>CI systems create new jobs, but don't interact with running jobs.</td></tr>\n<tr><td>Infrastructure</td><td><code>read</code></td><td><code>write</code></td><td><code>write</code></td><td>DevOps teams perform cluster management but seldom need to interact with running jobs.</td></tr>\n</tbody></table>\n<blockquote>\n<p>For namespace access, <code>read</code> is equivalent to\n<code>[read-job, list-jobs]</code>. <code>write</code> is equivalent to\n<code>[list-jobs, read-job, submit-job, read-logs, read-fs, dispatch-job]</code>.</p>\n</blockquote>\n<blockquote>\n<p>In the event that operators do need to have access to namespaces, one can\nalways create a token that has <em>both</em> Developer and Infrastructure policies\nattached. This is equivalent to having a <code>management</code> token.</p>\n</blockquote>\n<p>We have left out multi-region and multi-namespace setups here. We have assumed\neverything to be running under the <code>default</code> namespace. It should be noted that\non production deployments, with much larger needs, the policies could be\ndesigned per-namespace, and tracked between regions.</p>\n<h2 id=\"policy-specification\">Policy specification</h2>\n<p>Policies are expressed by a combination of rules Note that the <code>deny</code> rule will\npreside over any conflicting capability.</p>\n<p>Nomad accepts a JSON payload with the name and description of a policy, along\nwith a <em>quoted</em> JSON or HCL document with rules, like the following.</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code>{\n "Description": "Agent and node management",\n "Name": "infrastructure",\n "Rules": "{\\"agent\\":{\\"policy\\":\\"write\\"},\\"node\\":{\\"policy\\":\\"write\\"}}"\n}\n</code></pre>\n<p>This policy matches what we have in the table above.\nCreate an <code>infrastructure.json</code> with the content above for use in the next step.</p>\n<blockquote>\n<p>TIP:</p>\n<p>To avoid error-prone quoting, one could write the policies in YAML:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code>Name: infrastructure\nDescription: Agent and node management\nRules:\n agent:\n policy: write\n node:\n policy: write\n</code></pre>\n<p>And then, convert them to JSON with the necessary quoting, by:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code>$ yaml2json < infrastructure.yaml | jq '.Rules = (.Rules | @text)' > infrastructure.json\n</code></pre></blockquote>\n<h2 id=\"adding-a-policy\">Adding a policy</h2>\n<p>To add the policy, simply make an HTTP POST request to the server. The\n<code>NOMAD_TOKEN</code> below is the "management" token that we first created.</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code>vagrant@nomad:~$ curl \\\n --request POST \\\n --data @infrastructure.json \\\n --header "X-Nomad-Token: ${NOMAD_TOKEN}" \\\n https://127.0.0.1:4646/v1/acl/policy/infrastructure\n\nvagrant@nomad:~$ nomad acl policy list\nName Description\ninfrastructure Agent and node management\n\nvagrant@nomad:~$ nomad acl policy info infrastructure\nName = infrastructure\nDescription = Agent and node management\nRules = {"agent":{"policy":"write"},"node":{"policy":"write"}}\nCreateIndex = 425\nModifyIndex = 425\n</code></pre><h2 id=\"creating-a-token-for-a-policy\">Creating a token for a policy</h2>\n<p>We now create a token for the <code>infrastructure</code> policy, and attempt a few operations\nwith it:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code>vagrant@nomad:~$ nomad acl token create \\\n -name='devops-team' \\\n -type='client' \\\n -global='true' \\\n -policy='infrastructure'\n\nAccessor ID = 927ea7a4-e689-037f-be89-54a2cdbd338c\nSecret ID = 26832c8d-9315-c1ef-aabf-2058c8632da8\nName = devops-team\nType = client\nGlobal = true\nPolicies = [infrastructure]\nCreate Time = 2018-02-15 19:53:59.97900843 +0000 UTC\nCreate Index = 432\nModify Index = 432\n\nvagrant@nomad:~$ export NOMAD_TOKEN='26832c8d-9315-c1ef-aabf-2058c8632da8' # change the token to the new one with the "infrastructure" policy attached\nvagrant@nomad:~$ nomad status\nError querying jobs: Unexpected response code: 403 (Permission denied)\n\nvagrant@nomad:~$ nomad node-status\nID DC Name Class Drain Status\n1f638a17 dc1 nomad <none> false ready\n</code></pre>\n<p>As you can see, anyone with the <code>devops-team</code> token will be allowed to\nrun operations on nodes, but not on jobs -- i.e. on namespace resources.</p>\n<h2 id=\"where-to-go-next\">Where to go next</h2>\n<p>The example above demonstrates adding one of the policies from our list at the\nbeginning. Adding the rest of them and trying different commands could be a\ngood exercise.</p>\n<p>As a reference, the FP Complete team maintains a\n<a href=\"https://github.com/fpco/nomad-acl-policies\">repository</a> with\npolicies ready for use.</p>\n<h4 id=\"related-articles\">Related articles</h4>\n<ul>\n<li><a href=\"https://www.fpcomplete.com/blog/2016/11/devops-best-practices-immutability/\">DevOps best practices: immutability</a></li>\n<li><a href=\"https://www.fpcomplete.com/blog/how-to-implement-containers-to-streamline-your-devops-workflow/\">How to implement containers to streamline your DevOps workflow</a></li>\n<li><a href=\"https://www.fpcomplete.com/blog/2016/05/stack-security-gnupg-keys/\">Stack security: GnuPG keys</a></li>\n</ul>\n",
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"content": "<h1 id=\"what-are-docker-containers\">What are Docker Containers?</h1>\n<p>Docker containers are a form of "lightweight" virtualization They allow a\nprocess or process group to run in an environment with its own file system,\nsomewhat like <code>chroot</code> jails , and also with its own process table, users and\ngroups and, optionally, virtual network and resource limits. For most purposes,\nthe processes in a container think they have an entire OS to themselves and do\nnot have access to anything outside the container (unless explicitly granted).\nThis lets you precisely control the environment in which your processes run,\nallows multiple processes on the same (virtual) machine that have completely\ndifferent (even conflicting) requirements, and significantly increases isolation\nand container security.</p>\n<p>In addition to containers, Docker makes it easy to build and distribute images\nthat wrap up an application with its complete runtime environment.</p>\n<p>For more information, see \n<a href=\"https://www.cio.com/article/2924995/software/what-are-containers-and-why-do-you-need-them.html\">What are containers and why do you need them?</a> \nand \n<a href=\"https://containerjournal.com/2017/01/11/containers-devops-anyway/\">What Do Containers Have to Do with DevOps, Anyway?</a>.</p>\n<h1 id=\"containers-vs-virtual-machines-vms\">Containers vs Virtual Machines (VMs)</h1>\n<p>The difference between the "lightweight" virtualization of containers and\n"heavyweight" virtualization of VMs boils down to that, for the former, the\nvirtualization happens at the kernel level while for the latter it happens at\nthe hypervisor level. In other words, all the containers on a machine share the\nsame kernel, and code in the kernel isolates the containers from each other\nwhereas each VM acts like separate hardware and has its own kernel.</p>\n<img alt=\"Docker Carrying Haskell.jpg\" sizes=\"(max-width: 320px) 100vw, 320px\" src=\"/images/hubspot/4536576cadee37e3ea1e0a35a83a97a55015af6773242ecda5f919a7f1628cc5.jpeg\" srcset=\"/images/hubspot/04a7b5b957c890331f8535859d7c8528eadf4d83c82ae65e86ea28fea6f82898.jpeg 160w, /images/hubspot/4536576cadee37e3ea1e0a35a83a97a55015af6773242ecda5f919a7f1628cc5.jpeg 320w, /images/hubspot/1652b04e09bee96b23e47adb5830543a1feac5a48d5488b22602cec12a1b131d.jpeg 480w, /images/hubspot/4a5e5498d817ee00db5fdc27b5827a41a41d07253d95f73e093809cd27d6ea45.jpeg 640w, /images/hubspot/d77567fd61f4146be574d81e707b90ca7f80f3005770d6ef527ff656eb9b913d.jpeg 800w, /images/hubspot/f9971781a2d67ed9b0b30a5798652fdc1975985603d9fde0b60bf89de73faa7a.jpeg 960w\" style=\"width: 320px; margin: 0px 0px 10px 10px; letter-spacing: -0.08px; float: right;\" width=\"320\">\n<p>Containers are much less resource intensive than VMs because they do not need\nto be allocated exclusive memory and file system space or have the overhead of\nrunning an entire operating system. This makes it possible to run many more\ncontainers on a machine than you would VMs. Containers start nearly as fast as\nregular processes (you don't have to wait for the OS to boot), and parts of the\nhost's file system can be easily "mounted" into the container's file system\nwithout any additional overhead of network file system protocols.</p>\n<p>On the other hand, isolation is less guaranteed. If not careful, you can\noversubscribe a machine by running containers that need more resources than the\nmachine has available (this can be mitigated by setting appropriate resource\nlimits on containers). While containers security is an improvement over normal\nprocesses, the shared kernel means the attack surface is greater and there is\nmore risk of leakage between containers than there is between VMs.</p>\n<p>For more information, see <a href=\"https://blog.netapp.com/blogs/containers-vs-vms/\">Docker containers vs. virtual machines: What's the\ndifference?</a> and <a href=\"https://www.fpcomplete.com/blog/2016/11/devops-best-practices-immutability/\">DevOps Best\nPractices: Immutability</a></p>\n<h1 id=\"how-docker-containers-enhance-continuous-delivery-pipelines\">How Docker Containers Enhance Continuous Delivery Pipelines</h1>\n<p>There are, broadly, two areas where containers fit into your devops\nworkflow: for builds, and for deployment. They are often used together,\nbut do not have to be.</p>\n<h3 id=\"builds\">Builds</h3>\n<ul>\n<li>\n<p><strong>Synchronizing build environments:</strong> It can be difficult to keep\nbuild environments synchronized between developers and CI/CD\nservers, which can lead to unexpected build failures or changes in\nbehaviour . Docker images let you specify <em>exactly</em> the build tools,\nlibraries, and other dependencies (including their versions)\nrequired without needing to install them on individual machines, and\ndistribute those images easily. This way you can be sure that\neveryone is using exactly the same build environment.</p>\n</li>\n<li>\n<p><strong>Managing changes to build environments:</strong> Managing changes to\nbuild environments can also be difficult, since you need to roll\nthose out to all developers and build servers at the right time.\nThis can be especially tricky when there are multiple branches of\ndevelopment some of which may need older or newer environments than\neach other. With Docker, you can specify a particular version of the\nbuild image along with the source code, which means a particular\nrevision of the source code will always build in the right\nenvironment.</p>\n</li>\n<li>\n<p><strong>Isolating build environments:</strong> One CI/CD server may have to build\nmultiple projects, which may have conflicting requirements for build\ntools, libraries, and other dependencies. By running each build in\nits own ephemeral container created from potentially different\nDocker images, you can be certain that these builds environments\nwill not interfere with each other.</p>\n</li>\n</ul>\n<h3 id=\"deployment\">Deployment</h3>\n<ul>\n<li>\n<p><strong>Runtime environment bundled with application :</strong> The CD system\nbuilds a complete Docker image which bundles the application's\nenvironment with the application itself and then deploys the whole\nimage as one "atomic" step. There is no chance for configuration\nmanagement scripts to fail at deployment time, and no risk of the\nsystem configuration to be out of sync.</p>\n</li>\n<li>\n<p><strong>Preventing malicious changes:</strong> Container security is improved by\nusing immutable SHA digests to identify Docker images, which means\nthere is no way for a malicious actor to inject malware into your\napplication or its environment.</p>\n</li>\n<li>\n<p><strong>Easily roll back to a previous version:</strong> All it takes to roll\nback is to deploy a previous version of the Docker image. There is\nno worrying about system configuration changes needing to be\nmanually rolled back.</p>\n</li>\n<li>\n<p><strong>Zero downtime rollouts:</strong> In conjunction with container\norchestration tools like Kubernetes, it is easily to roll out new\nimage versions with zero downtime.</p>\n</li>\n<li>\n<p><strong>High availability and horizontal scaling:</strong> Container\norchestration tools like Kubernetes make it easy to distribute the\nsame image to containers on multiple servers, and add/remove\nreplicas at will or automatically.</p>\n</li>\n<li>\n<p><strong>Sharing a server between multiple applications:</strong> Multiple\napplications, or multiple versions of the same application (e.g. a\ndev and qa deployment), can run on the same server even if they have\nconflicting dependencies, since their runtime environments are\ncompletely separate.</p>\n</li>\n<li>\n<p><strong>Isolating applications:</strong> When multiple applications are deployed\nto a server in containers, they are isolated from one another.\nContainer security means each has its own file system, processes,\nand users there is less risk that they interfere with each other\nintentionally. When data <em>does</em> need to be shared between\napplications, parts of the host file system can be mounted into\nmultiple containers, but this is something you have full control\nover.</p>\n</li>\n</ul>\n<p>For more information, see:</p>\n<ul>\n<li><a href=\"https://www.fpcomplete.com/blog/2017/03/continuous-integration/\">Continuous Integration: An Overview</a></li>\n<li><a href=\"https://docs.microsoft.com/en-us/dotnet/standard/containerized-lifecycle-architecture/docker-application-lifecycle/containers-foundation-for-devops-collaboration\">Containers as the foundation for DevOps collaboration</a></li>\n<li><a href=\"https://www.sumologic.com/blog/devops/how-containerization-enables-devops/\">Docker and DevOps -- Enabling DevOps Teams Through Containerization</a>.</li>\n</ul>\n<h1 id=\"implementing-containers-into-your-devops-workflow\">Implementing Containers into Your DevOps Workflow</h1>\n<p>Containers can be integrated into your DevOps toolchain incrementally.\nOften it makes sense to start with the build environment, and then move\non to the deployment environment. This is a very broad overview of the\nsteps for a simple approach, without delving into the technical details\nvery much or covering all the possible variations.</p>\n<h3 id=\"requirements\">Requirements</h3>\n<ul>\n<li>Docker Engine installed on build servers and/or application servers</li>\n<li>Access to a Docker Registry. This is where Docker images are stored\nand pulled. There are numerous services that provide registries, and\nit's also easy to run your own.</li>\n</ul>\n<h3 id=\"containerizing-the-build-environment\">Containerizing the build environment</h3>\n<p>Many CI/CD systems now include built-in Docker support or easily enable\nit through plugins, but <code>docker</code> is a command-line application which\ncan be called from any build script even if your CI/CD system does not\nhave explicit support.</p>\n<ol>\n<li>\n<p>Determine your build environment requirements and write\na <code>Dockerfile</code> based on an existing Docker image, which is the\nspecification used to build an image for build containers. If you\nalready use a configuration management tool, you can use it within\nthe Dockerfile. Always specify precise versions of base images and\ninstalled packages so that image builds are consistent and upgrades\nare deliberate.</p>\n</li>\n<li>\n<p>Build the image using <code>docker build</code> and push it to the Docker\nregistry using <code>docker push</code> .</p>\n</li>\n<li>\n<p>Create a <code>Dockerfile</code> for the application that is based on the build\nimage (specify the exact version of the base build image). This file\nbuilds the application, adds any required runtime dependencies that\naren't in the build image, and tests the application. A multi-stage\n <code>Dockerfile</code> can be used if you don't want the application\ndeployment image to include all the build dependencies.</p>\n</li>\n<li>\n<p>Modify CI build scripts to build the application image and push it\nto the Docker registry. The image should be tagged with the build\nnumber, and possibly additional information such as the name of the\nbranch.</p>\n</li>\n<li>\n<p>If you are not yet ready to deploy with Docker, you can extract the\nbuild artifacts from the resulting Docker image.</p>\n</li>\n</ol>\n<p>It is best to <em>also</em> integrate building the build image itself into your\ndevops automation tools.</p>\n<h3 id=\"containerizing-deployment\">Containerizing deployment</h3>\n<p>This can be easier if your CD tool has support for Docker, but that is\nby no means necessary. We also recommend deploying to a container\norchestration system such as Kubernetes in most cases.</p>\n<p>Half the work has already been done, since the build process creates and\npushes an image containing the application and its environment.</p>\n<ul>\n<li>\n<p>If using Docker directly, now it's a matter of updating deployment\nscripts to use <code>docker run</code> on the application server with the\nimage and tag that was pushed in the previous section (after\nstopping any existing container). Ideally your application accepts\nits configuration via environment variables, in which case you use\nthe <code>-e</code> argument to specify those values depending on which\nstage is being deployed. If a configuration file are used, write it\nto the host file system and then use the <code>-v</code> argument to mount\nit to the correct path in the container.</p>\n</li>\n<li>\n<p>If using a container orchestration system such as Kubernetes, you\nwill typically have the deployment script connect to the\norchestration API endpoint to trigger an image update (e.g. using \n<code>kubectl set image</code> , a Helm chart, or better yet, a\n<code>kustomization</code>.).</p>\n</li>\n</ul>\n<p>Once deployed, tools such as Prometheus are well suited to docker\ncontainer monitoring and alerting, but this can be plugged into existing\nmonitoring systems as well.</p>\n<p>FP Complete has implemented this kind of DevOps workflow, and\nsignificantly more complex ones, for many clients and would love to\ncount you among them! See our <a href=\"https://www.fpcomplete.com/devops/\">Devops Services</a> page.</p>\n<p>For more information, see <a href=\"https://techbeacon.com/how-secure-container-lifecycle\">How to secure the container\nlifecycle</a> and <a href=\"https://www.fpcomplete.com/blog/2017/01/containerize-legacy-app/\">Containerizing\na legacy application: an\noverview</a>.</p>\n",
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"content": "<p>This blog post is the second in the <a href=\"/tags/rust-quickies/\">Rust quickies</a> series. In my <a href=\"https://www.fpcomplete.com/training/\">training sessions</a>, we often come up with quick examples to demonstrate some point. Instead of forgetting about them, I want to put short blog posts together focusing on these examples. Hopefully these will be helpful, enjoy!</p>\n<div class=\"alert alert-secondary text-center\">FP Complete is looking for Rust and DevOps engineers. Interested in working with us? <a href=\"/jobs/\">Check out our jobs page</a>.</div>\n<h2 id=\"hello-hyper\">Hello Hyper!</h2>\n<p>For those not familiar, <a href=\"https://hyper.rs/\">Hyper</a> is an HTTP implementation for Rust, built on top of Tokio. It's a low level library powering frameworks like <a href=\"https://crates.io/crates/warp\">Warp</a> and <a href=\"https://rocket.rs/\">Rocket</a>, as well as the <a href=\"https://lib.rs/crates/reqwest\">reqwest</a> client library. For most people, most of the time, using a higher level wrapper like these is the right thing to do.</p>\n<p>But sometimes we like to get our hands dirty, and sometimes working directly with Hyper is the right choice. And definitely from a learning perspective, it's worth doing so at least once. And what could be easier than following the example from Hyper's homepage? To do so, <code>cargo new</code> a new project, add the following dependencies:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">hyper </span><span style=\"color:#657b83;\">= { </span><span style=\"color:#268bd2;\">version </span><span style=\"color:#657b83;\">= </span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">0.14</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, </span><span style=\"color:#268bd2;\">features </span><span style=\"color:#657b83;\">= [</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">full</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">] }\n</span><span style=\"color:#268bd2;\">tokio </span><span style=\"color:#657b83;\">= { </span><span style=\"color:#268bd2;\">version </span><span style=\"color:#657b83;\">= </span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">1</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, </span><span style=\"color:#268bd2;\">features </span><span style=\"color:#657b83;\">= [</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">full</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">] }\n</span></code></pre>\n<p>And add the following to <code>main.rs</code>:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#859900;\">use </span><span style=\"color:#657b83;\">std::convert::Infallible;\n</span><span style=\"color:#859900;\">use </span><span style=\"color:#657b83;\">std::net::SocketAddr;\n</span><span style=\"color:#859900;\">use </span><span style=\"color:#657b83;\">hyper::{Body, Request, Response, Server};\n</span><span style=\"color:#859900;\">use </span><span style=\"color:#657b83;\">hyper::service::{make_service_fn, service_fn};\n\nasync </span><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">hello_world</span><span style=\"color:#657b83;\">(</span><span style=\"color:#268bd2;\">_req</span><span style=\"color:#657b83;\">: Request<Body>) -> </span><span style=\"color:#859900;\">Result</span><span style=\"color:#657b83;\"><Response<Body>, Infallible> {\n </span><span style=\"color:#859900;\">Ok</span><span style=\"color:#657b83;\">(Response::new(</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">Hello, World</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">.</span><span style=\"color:#859900;\">into</span><span style=\"color:#657b83;\">()))\n}\n\n#[</span><span style=\"color:#268bd2;\">tokio</span><span style=\"color:#657b83;\">::</span><span style=\"color:#268bd2;\">main</span><span style=\"color:#657b83;\">]\nasync </span><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">main</span><span style=\"color:#657b83;\">() {\n </span><span style=\"color:#93a1a1;\">// We'll bind to 127.0.0.1:3000\n </span><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> addr = SocketAddr::from(([</span><span style=\"color:#6c71c4;\">127</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">0</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">0</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">1</span><span style=\"color:#657b83;\">], </span><span style=\"color:#6c71c4;\">3000</span><span style=\"color:#657b83;\">));\n\n </span><span style=\"color:#93a1a1;\">// A `Service` is needed for every connection, so this\n // creates one from our `hello_world` function.\n </span><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> make_svc = </span><span style=\"color:#859900;\">make_service_fn</span><span style=\"color:#657b83;\">(|</span><span style=\"color:#268bd2;\">_conn</span><span style=\"color:#657b83;\">| async {\n </span><span style=\"color:#93a1a1;\">// service_fn converts our function into a `Service`\n </span><span style=\"color:#859900;\">Ok</span><span style=\"color:#657b83;\">::<</span><span style=\"color:#859900;\">_</span><span style=\"color:#657b83;\">, Infallible>(</span><span style=\"color:#859900;\">service_fn</span><span style=\"color:#657b83;\">(hello_world))\n });\n\n </span><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> server = Server::bind(</span><span style=\"color:#859900;\">&</span><span style=\"color:#657b83;\">addr).</span><span style=\"color:#859900;\">serve</span><span style=\"color:#657b83;\">(make_svc);\n\n </span><span style=\"color:#93a1a1;\">// Run this server for... forever!\n </span><span style=\"color:#859900;\">if </span><span style=\"color:#268bd2;\">let </span><span style=\"color:#859900;\">Err</span><span style=\"color:#657b83;\">(e) = server.await {\n </span><span style=\"color:#859900;\">eprintln!</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">server error: </span><span style=\"color:#cb4b16;\">{}</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, e);\n }\n}\n</span></code></pre>\n<p>If you're interested, there's a <a href=\"https://hyper.rs/guides/server/hello-world/\">quick explanation</a> of this code available on Hyper's website. But our focus will be on making an ever-so-minor modification to this code. Let's go!</p>\n<h2 id=\"counter\">Counter</h2>\n<p>Remember the good old days of Geocities websites, where every page had to have a visitor counter? I want that. Let's modify our <code>hello_world</code> function to do just that:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#859900;\">use </span><span style=\"color:#657b83;\">std::sync::{Arc, Mutex};\n\n</span><span style=\"color:#268bd2;\">type </span><span style=\"color:#b58900;\">Counter </span><span style=\"color:#657b83;\">= Arc<Mutex<</span><span style=\"color:#268bd2;\">usize</span><span style=\"color:#657b83;\">>>; </span><span style=\"color:#93a1a1;\">// Bonus points: use an AtomicUsize instead\n</span><span style=\"color:#657b83;\">\nasync </span><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">hello_world</span><span style=\"color:#657b83;\">(</span><span style=\"color:#268bd2;\">counter</span><span style=\"color:#657b83;\">: Counter, </span><span style=\"color:#268bd2;\">_req</span><span style=\"color:#657b83;\">: Request<Body>) -> </span><span style=\"color:#859900;\">Result</span><span style=\"color:#657b83;\"><Response<Body>, Infallible> {\n </span><span style=\"color:#268bd2;\">let </span><span style=\"color:#586e75;\">mut</span><span style=\"color:#657b83;\"> guard = counter.</span><span style=\"color:#859900;\">lock</span><span style=\"color:#657b83;\">().</span><span style=\"color:#859900;\">unwrap</span><span style=\"color:#657b83;\">(); </span><span style=\"color:#93a1a1;\">// unwrap poisoned Mutexes\n </span><span style=\"color:#657b83;\">*guard += </span><span style=\"color:#6c71c4;\">1</span><span style=\"color:#657b83;\">;\n </span><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> message = </span><span style=\"color:#859900;\">format!</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">You are visitor number </span><span style=\"color:#cb4b16;\">{}</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, guard);\n </span><span style=\"color:#859900;\">Ok</span><span style=\"color:#657b83;\">(Response::new(message.</span><span style=\"color:#859900;\">into</span><span style=\"color:#657b83;\">()))\n}\n</span></code></pre>\n<p>That's easy enough, and now we're done with <code>hello_world</code>. The only problem is rewriting <code>main</code> to pass in a <code>Counter</code> value to it. Let's take a first, naive stab at the problem:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> addr = SocketAddr::from(([</span><span style=\"color:#6c71c4;\">127</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">0</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">0</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">1</span><span style=\"color:#657b83;\">], </span><span style=\"color:#6c71c4;\">3000</span><span style=\"color:#657b83;\">));\n</span><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> counter: Counter = Arc::new(Mutex::new(</span><span style=\"color:#6c71c4;\">0</span><span style=\"color:#657b83;\">));\n\n</span><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> make_svc = </span><span style=\"color:#859900;\">make_service_fn</span><span style=\"color:#657b83;\">(|</span><span style=\"color:#268bd2;\">_conn</span><span style=\"color:#657b83;\">| async {\n </span><span style=\"color:#859900;\">Ok</span><span style=\"color:#657b83;\">::<</span><span style=\"color:#859900;\">_</span><span style=\"color:#657b83;\">, Infallible>(</span><span style=\"color:#859900;\">service_fn</span><span style=\"color:#657b83;\">(|</span><span style=\"color:#268bd2;\">req</span><span style=\"color:#657b83;\">| </span><span style=\"color:#859900;\">hello_world</span><span style=\"color:#657b83;\">(counter, req)))\n});\n\n</span><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> server = Server::bind(</span><span style=\"color:#859900;\">&</span><span style=\"color:#657b83;\">addr).</span><span style=\"color:#859900;\">serve</span><span style=\"color:#657b83;\">(make_svc);\n\n</span><span style=\"color:#859900;\">if </span><span style=\"color:#268bd2;\">let </span><span style=\"color:#859900;\">Err</span><span style=\"color:#657b83;\">(e) = server.await {\n </span><span style=\"color:#859900;\">eprintln!</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">server error: </span><span style=\"color:#cb4b16;\">{}</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, e);\n}\n</span></code></pre>\n<p>Unfortunately, this fails due to moving out of captured variables. (That's a topic we cover in detail in our closure training module.)</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#657b83;\">error[E0507]: cannot move out of `counter`, a captured variable in an `FnMut` closure\n --> src\\main.rs:21:58\n |\n18 | let counter: Counter = Arc::new(Mutex::new(0));\n | ------- captured outer variable\n...\n21 | Ok::<_, Infallible>(service_fn(|req| hello_world(counter, req)))\n | ^^^^^^^ move occurs because `counter` has type `Arc<std::sync::Mutex<usize>>`, which does not implement the `Copy` trait\n\nerror[E0507]: cannot move out of `counter`, a captured variable in an `FnMut` closure\n --> src\\main.rs:20:50\n |\n18 | let counter: Counter = Arc::new(Mutex::new(0));\n | ------- captured outer variable\n19 |\n20 | let make_svc = make_service_fn(|_conn| async {\n | __________________________________________________^\n21 | | Ok::<_, Infallible>(service_fn(|req| hello_world(counter, req)))\n | | -------------------------------\n | | |\n | | move occurs because `counter` has type `Arc<std::sync::Mutex<usize>>`, which does not implement the `Copy` trait\n | | move occurs due to use in generator\n22 | | });\n | |_____^ move out of `counter` occurs here\n</span></code></pre><h2 id=\"clone\">Clone</h2>\n<p>That error isn't terribly surprising. We put our <code>Mutex</code> inside an <code>Arc</code> for a reason: we'll need to make multiple clones of it and pass those around to each new request handler. But we haven't called <code>clone</code> once yet! Again, let's do the most naive thing possible, and change:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#859900;\">Ok</span><span style=\"color:#657b83;\">::<</span><span style=\"color:#859900;\">_</span><span style=\"color:#657b83;\">, Infallible>(</span><span style=\"color:#859900;\">service_fn</span><span style=\"color:#657b83;\">(|</span><span style=\"color:#268bd2;\">req</span><span style=\"color:#657b83;\">| </span><span style=\"color:#859900;\">hello_world</span><span style=\"color:#657b83;\">(counter, req)))\n</span></code></pre>\n<p>into</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#859900;\">Ok</span><span style=\"color:#657b83;\">::<</span><span style=\"color:#859900;\">_</span><span style=\"color:#657b83;\">, Infallible>(</span><span style=\"color:#859900;\">service_fn</span><span style=\"color:#657b83;\">(|</span><span style=\"color:#268bd2;\">req</span><span style=\"color:#657b83;\">| </span><span style=\"color:#859900;\">hello_world</span><span style=\"color:#657b83;\">(counter.</span><span style=\"color:#859900;\">clone</span><span style=\"color:#657b83;\">(), req)))\n</span></code></pre>\n<p>This is where the error messages begin to get more interesting:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#657b83;\">error[E0597]: `counter` does not live long enough\n --> src\\main.rs:21:58\n |\n20 | let make_svc = make_service_fn(|_conn| async {\n | ____________________________________-------_-\n | | |\n | | value captured here\n21 | | Ok::<_, Infallible>(service_fn(|req| hello_world(counter.clone(), req)))\n | | ^^^^^^^ borrowed value does not live long enough\n22 | | });\n | |_____- returning this value requires that `counter` is borrowed for `'static`\n...\n29 | }\n | - `counter` dropped here while still borrowed\n</span></code></pre>\n<p>Both <code>async</code> blocks and closures will, by default, capture variables from their environment by reference, instead of taking ownership. Our closure needs to have a <code>'static</code> lifetime, and therefore can't hold onto a reference to data in our <code>main</code> function.</p>\n<h2 id=\"move-all-the-things\"><code>move</code> all the things!</h2>\n<p>The standard solution to this is to simply sprinkle <code>move</code>s on each <code>async</code> block and closure. This will force each closure to own the <code>Arc</code> itself, not a reference to it. Doing so looks simple:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> make_svc = </span><span style=\"color:#859900;\">make_service_fn</span><span style=\"color:#657b83;\">(</span><span style=\"color:#586e75;\">move </span><span style=\"color:#859900;\">|</span><span style=\"color:#657b83;\">_conn</span><span style=\"color:#859900;\">|</span><span style=\"color:#657b83;\"> async </span><span style=\"color:#586e75;\">move </span><span style=\"color:#657b83;\">{\n </span><span style=\"color:#859900;\">Ok</span><span style=\"color:#657b83;\">::<</span><span style=\"color:#859900;\">_</span><span style=\"color:#657b83;\">, Infallible>(</span><span style=\"color:#859900;\">service_fn</span><span style=\"color:#657b83;\">(</span><span style=\"color:#586e75;\">move </span><span style=\"color:#859900;\">|</span><span style=\"color:#657b83;\">req</span><span style=\"color:#859900;\">| hello_world</span><span style=\"color:#657b83;\">(counter.</span><span style=\"color:#859900;\">clone</span><span style=\"color:#657b83;\">(), req)))\n});\n</span></code></pre>\n<p>And this does in fact fix the error above. But it gives us a new error instead:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#657b83;\">error[E0507]: cannot move out of `counter`, a captured variable in an `FnMut` closure\n --> src\\main.rs:20:60\n |\n18 | let counter: Counter = Arc::new(Mutex::new(0));\n | ------- captured outer variable\n19 |\n20 | let make_svc = make_service_fn(move |_conn| async move {\n | ____________________________________________________________^\n21 | | Ok::<_, Infallible>(service_fn(move |req| hello_world(counter.clone(), req)))\n | | --------------------------------------------\n | | |\n | | move occurs because `counter` has type `Arc<std::sync::Mutex<usize>>`, which does not implement the `Copy` trait\n | | move occurs due to use in generator\n22 | | });\n | |_____^ move out of `counter` occurs here\n</span></code></pre><h2 id=\"double-the-closure-double-the-clone\">Double the closure, double the clone!</h2>\n<p>Well, even <em>this</em> error makes a lot of sense. Let's understand better what our code is doing:</p>\n<ul>\n<li>Creates a closure to pass to <code>make_service_fn</code>, which will be called for each new incoming connection</li>\n<li>Within <em>that</em> closure, creates a new closure to pass to <code>service_fn</code>, which will be called for each new incoming request on an existing connection</li>\n</ul>\n<p>This is where the trickiness of working directly with Hyper comes into play. Each of those layers of closure need to own their own clone of the <code>Arc</code>. And in our code above, we're trying to move the <code>Arc</code> from the outer closure's captured variable into the inner closure's captured variable. If you squint hard enough, that's what the error message above is saying. Our outer closure is an <code>FnMut</code>, which must be callable multiple times. Therefore, we cannot move out of its captured variable.</p>\n<p>It seems like this should be an easy fix: just <code>clone</code> again!</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> make_svc = </span><span style=\"color:#859900;\">make_service_fn</span><span style=\"color:#657b83;\">(</span><span style=\"color:#586e75;\">move </span><span style=\"color:#859900;\">|</span><span style=\"color:#657b83;\">_conn</span><span style=\"color:#859900;\">|</span><span style=\"color:#657b83;\"> async </span><span style=\"color:#586e75;\">move </span><span style=\"color:#657b83;\">{\n </span><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> counter_clone = counter.</span><span style=\"color:#859900;\">clone</span><span style=\"color:#657b83;\">();\n </span><span style=\"color:#859900;\">Ok</span><span style=\"color:#657b83;\">::<</span><span style=\"color:#859900;\">_</span><span style=\"color:#657b83;\">, Infallible>(</span><span style=\"color:#859900;\">service_fn</span><span style=\"color:#657b83;\">(</span><span style=\"color:#586e75;\">move </span><span style=\"color:#859900;\">|</span><span style=\"color:#657b83;\">req</span><span style=\"color:#859900;\">| hello_world</span><span style=\"color:#657b83;\">(counter_clone.</span><span style=\"color:#859900;\">clone</span><span style=\"color:#657b83;\">(), req)))\n});\n</span></code></pre>\n<p>And this is the point at which we hit a real head scratcher: we get almost exactly the same error message:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#657b83;\">error[E0507]: cannot move out of `counter`, a captured variable in an `FnMut` closure\n --> src\\main.rs:20:60\n |\n18 | let counter: Counter = Arc::new(Mutex::new(0));\n | ------- captured outer variable\n19 |\n20 | let make_svc = make_service_fn(move |_conn| async move {\n | ____________________________________________________________^\n21 | | let counter_clone = counter.clone();\n | | -------\n | | |\n | | move occurs because `counter` has type `Arc<std::sync::Mutex<usize>>`, which does not implement the `Copy` trait\n | | move occurs due to use in generator\n22 | | Ok::<_, Infallible>(service_fn(move |req| hello_world(counter_clone.clone(), req)))\n23 | | });\n | |_____^ move out of `counter` occurs here\n</span></code></pre><h2 id=\"the-paradigm-shift\">The paradigm shift</h2>\n<p>What we need to do is to rewrite our code ever so slightly so reveal what the problem is. Let's add a bunch of unnecessary braces. We'll convert the code above:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> make_svc = </span><span style=\"color:#859900;\">make_service_fn</span><span style=\"color:#657b83;\">(</span><span style=\"color:#586e75;\">move </span><span style=\"color:#859900;\">|</span><span style=\"color:#657b83;\">_conn</span><span style=\"color:#859900;\">|</span><span style=\"color:#657b83;\"> async </span><span style=\"color:#586e75;\">move </span><span style=\"color:#657b83;\">{\n </span><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> counter_clone = counter.</span><span style=\"color:#859900;\">clone</span><span style=\"color:#657b83;\">();\n </span><span style=\"color:#859900;\">Ok</span><span style=\"color:#657b83;\">::<</span><span style=\"color:#859900;\">_</span><span style=\"color:#657b83;\">, Infallible>(</span><span style=\"color:#859900;\">service_fn</span><span style=\"color:#657b83;\">(</span><span style=\"color:#586e75;\">move </span><span style=\"color:#859900;\">|</span><span style=\"color:#657b83;\">req</span><span style=\"color:#859900;\">| hello_world</span><span style=\"color:#657b83;\">(counter_clone.</span><span style=\"color:#859900;\">clone</span><span style=\"color:#657b83;\">(), req)))\n});\n</span></code></pre>\n<p>into this semantically identical code:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> make_svc = </span><span style=\"color:#859900;\">make_service_fn</span><span style=\"color:#657b83;\">(</span><span style=\"color:#586e75;\">move </span><span style=\"color:#859900;\">|</span><span style=\"color:#657b83;\">_conn</span><span style=\"color:#859900;\">| </span><span style=\"color:#657b83;\">{ </span><span style=\"color:#93a1a1;\">// outer closure\n</span><span style=\"color:#657b83;\"> async </span><span style=\"color:#586e75;\">move </span><span style=\"color:#657b83;\">{ </span><span style=\"color:#93a1a1;\">// async block\n </span><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> counter_clone = counter.</span><span style=\"color:#859900;\">clone</span><span style=\"color:#657b83;\">();\n </span><span style=\"color:#859900;\">Ok</span><span style=\"color:#657b83;\">::<</span><span style=\"color:#859900;\">_</span><span style=\"color:#657b83;\">, Infallible>(</span><span style=\"color:#859900;\">service_fn</span><span style=\"color:#657b83;\">(</span><span style=\"color:#586e75;\">move </span><span style=\"color:#859900;\">|</span><span style=\"color:#657b83;\">req</span><span style=\"color:#859900;\">| </span><span style=\"color:#657b83;\">{ </span><span style=\"color:#93a1a1;\">// inner closure\n </span><span style=\"color:#859900;\">hello_world</span><span style=\"color:#657b83;\">(counter_clone.</span><span style=\"color:#859900;\">clone</span><span style=\"color:#657b83;\">(), req)\n }))\n }\n});\n</span></code></pre>\n<p>The error message is basically identical, just slightly different source locations. But now I can walk through the ownership of <code>counter</code> more correctly. I've added comments to highlight three different entities in the code above that can take ownership of values via some kind of environment:</p>\n<ul>\n<li>The outer closure, which handles each connection</li>\n<li>An <code>async</code> block, which forms the body of the outer closure</li>\n<li>The inner closure, which handles each request</li>\n</ul>\n<p>In the original structuring of the code, we put <code>move |_conn| async move</code> next to each other on one line, which—at least for me—obfuscated the fact that the closure and <code>async</code> block were two completely separate entities. With that change in place, let's track the ownership of <code>counter</code>:</p>\n<ol>\n<li>We create the <code>Arc</code> in the <code>main</code> function; it's owned by the <code>counter</code> variable.</li>\n<li>We move the <code>Arc</code> from the <code>main</code> function's <code>counter</code> variable into the outer closure's captured variables.</li>\n<li>We move the <code>counter</code> variable out of the outer closure and into the <code>async</code> block's captured variables.</li>\n<li>Within the body of the <code>async</code> block, we create a clone of <code>counter</code>, called <code>counter_clone</code>. This does not move out of the <code>async</code> block, since the <code>clone</code> method only requires a reference to the <code>Arc</code>.</li>\n<li>We move the <code>Arc</code> out of the <code>counter_clone</code> variable and into the inner closure.</li>\n<li>Within the body of the inner closure, we clone the <code>Arc</code> (which, as explained in (4), doesn't move) and pass it into the <code>hello_world</code> function.</li>\n</ol>\n<p>Based on this breakdown, can you see where the problem is? It's at step (3). We don't want to move out of the outer closure's captured variables. We try to avoid that move by cloning <code>counter</code>. But we clone too late! By using <code>counter</code> from inside an <code>async move</code> block, we're forcing the compiler to move. Hurray, we've identified the problem!</p>\n<h2 id=\"non-solution-non-move-async\">Non-solution: non-move <code>async</code></h2>\n<p>It seems like we were simply over-ambitious with our "sprinkling <code>move</code>" attempt above. The problem is that the <code>async</code> block is taking ownership of <code>counter</code>. Let's try simply removing the <code>move</code> keyword there:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> make_svc = </span><span style=\"color:#859900;\">make_service_fn</span><span style=\"color:#657b83;\">(</span><span style=\"color:#586e75;\">move </span><span style=\"color:#859900;\">|</span><span style=\"color:#657b83;\">_conn</span><span style=\"color:#859900;\">| </span><span style=\"color:#657b83;\">{\n async {\n </span><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> counter_clone = counter.</span><span style=\"color:#859900;\">clone</span><span style=\"color:#657b83;\">();\n </span><span style=\"color:#859900;\">Ok</span><span style=\"color:#657b83;\">::<</span><span style=\"color:#859900;\">_</span><span style=\"color:#657b83;\">, Infallible>(</span><span style=\"color:#859900;\">service_fn</span><span style=\"color:#657b83;\">(</span><span style=\"color:#586e75;\">move </span><span style=\"color:#859900;\">|</span><span style=\"color:#657b83;\">req</span><span style=\"color:#859900;\">| </span><span style=\"color:#657b83;\">{\n </span><span style=\"color:#859900;\">hello_world</span><span style=\"color:#657b83;\">(counter_clone.</span><span style=\"color:#859900;\">clone</span><span style=\"color:#657b83;\">(), req)\n }))\n }\n});\n</span></code></pre>\n<p>Unfortunately, this isn't a solution:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#657b83;\">error: captured variable cannot escape `FnMut` closure body\n --> src\\main.rs:21:9\n |\n18 | let counter: Counter = Arc::new(Mutex::new(0));\n | ------- variable defined here\n19 |\n20 | let make_svc = make_service_fn(move |_conn| {\n | - inferred to be a `FnMut` closure\n21 | / async {\n22 | | let counter_clone = counter.clone();\n | | ------- variable captured here\n23 | | Ok::<_, Infallible>(service_fn(move |req| {\n24 | | hello_world(counter_clone.clone(), req)\n25 | | }))\n26 | | }\n | |_________^ returns an `async` block that contains a reference to a captured variable, which then escapes the closure body\n |\n = note: `FnMut` closures only have access to their captured variables while they are executing...\n = note: ...therefore, they cannot allow references to captured variables to escape\n</span></code></pre>\n<p>The problem here is that the outer closure will return the <code>Future</code> generated by the <code>async</code> block. And if the <code>async</code> block doesn't <code>move</code> the <code>counter</code>, it will be holding a reference to the outer closure's captured variables. And that's not allowed.</p>\n<h2 id=\"real-solution-clone-early-clone-often\">Real solution: clone early, clone often</h2>\n<p>OK, undo the <code>async move</code> to <code>async</code> transformation, it's a dead end. It turns out that all we've got to do is clone the <code>counter</code> before we start the <code>async move</code> block, like so:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> make_svc = </span><span style=\"color:#859900;\">make_service_fn</span><span style=\"color:#657b83;\">(</span><span style=\"color:#586e75;\">move </span><span style=\"color:#859900;\">|</span><span style=\"color:#657b83;\">_conn</span><span style=\"color:#859900;\">| </span><span style=\"color:#657b83;\">{\n </span><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> counter_clone = counter.</span><span style=\"color:#859900;\">clone</span><span style=\"color:#657b83;\">(); </span><span style=\"color:#93a1a1;\">// this moved one line earlier\n</span><span style=\"color:#657b83;\"> async </span><span style=\"color:#586e75;\">move </span><span style=\"color:#657b83;\">{\n </span><span style=\"color:#859900;\">Ok</span><span style=\"color:#657b83;\">::<</span><span style=\"color:#859900;\">_</span><span style=\"color:#657b83;\">, Infallible>(</span><span style=\"color:#859900;\">service_fn</span><span style=\"color:#657b83;\">(</span><span style=\"color:#586e75;\">move </span><span style=\"color:#859900;\">|</span><span style=\"color:#657b83;\">req</span><span style=\"color:#859900;\">| </span><span style=\"color:#657b83;\">{\n </span><span style=\"color:#859900;\">hello_world</span><span style=\"color:#657b83;\">(counter_clone.</span><span style=\"color:#859900;\">clone</span><span style=\"color:#657b83;\">(), req)\n }))\n }\n});\n</span></code></pre>\n<p>Now, we create a temporary <code>counter_clone</code> within the outer closure. This works by reference, and therefore doesn't move anything. We then move the new, temporary <code>counter_clone</code> into the <code>async move</code> block via a capture, and from there move it into the inner closure. With this, all of our closure captured variables remain unmoved, and therefore the requirements of <code>FnMut</code> are satisfied.</p>\n<p>And with that, we can finally enjoy the glory days of Geocities visitor counters!</p>\n<h2 id=\"async-closures\">Async closures</h2>\n<p>The formatting recommended by <code>rustfmt</code> hides away the fact that there are two different environments at play between the outer closure and the <code>async block</code>, by moving the two onto a single line with <code>move |_conn| async move</code>. That makes it feel like the two entities are somehow one and the same. But as we've demonstrated, they aren't.</p>\n<p>Theoretically this could be solved by having an async closure. I tested with <code>#![feature(async_closure)]</code> on <code>nightly-2021-03-02</code>, but couldn't figure out a way to use an async closure to solve this problem differently than I solved it above. But that may be my own lack of familiarity with <code>async_closure</code>.</p>\n<p>For now, the main takeaway is that closures and <code>async</code> blocks are two different entities, each with their own environment.</p>\n<p>If you liked this post you may also be interested in:</p>\n<ul>\n<li><a href=\"https://www.fpcomplete.com/rust/crash-course/\">Rust Crash Course</a></li>\n<li><a href=\"https://www.fpcomplete.com/training/\">Training</a></li>\n<li><a href=\"https://www.fpcomplete.com/rust/\">FP Complete Rust home page</a></li>\n<li><a href=\"/tags/rust/\">Rust tagged blog posts</a></li>\n<li><a href=\"https://www.fpcomplete.com/jobs/\">Jobs at FP Complete</a></li>\n</ul>\n",
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"relative_path": "blog/short-circuit-sum-rust.md",
"content": "<p>This blog post is the first in a planned series I'm calling "Rust quickies." In my <a href=\"https://www.fpcomplete.com/training/\">training sessions</a>, we often come up with quick examples to demonstrate some point. Instead of forgetting about them, I want to put short blog posts together focusing on these examples. Hopefully these will be helpful, enjoy!</p>\n<div class=\"alert alert-secondary text-center\">FP Complete is looking for Rust and DevOps engineers. Interested in working with us? <a href=\"/jobs/\">Check out our jobs page</a>.</div>\n<h2 id=\"short-circuiting-a-for-loop\">Short circuiting a <code>for</code> loop</h2>\n<p>Let's say I've got an <code>Iterator</code> of <code>u32</code>s. I want to double each value and print it. Easy enough:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">weird_function</span><span style=\"color:#657b83;\">(</span><span style=\"color:#268bd2;\">iter</span><span style=\"color:#657b83;\">: impl </span><span style=\"color:#859900;\">IntoIterator</span><span style=\"color:#657b83;\"><Item=</span><span style=\"color:#268bd2;\">u32</span><span style=\"color:#657b83;\">>) {\n </span><span style=\"color:#859900;\">for</span><span style=\"color:#657b83;\"> x </span><span style=\"color:#859900;\">in</span><span style=\"color:#657b83;\"> iter.</span><span style=\"color:#859900;\">into_iter</span><span style=\"color:#657b83;\">().</span><span style=\"color:#859900;\">map</span><span style=\"color:#657b83;\">(|</span><span style=\"color:#268bd2;\">x</span><span style=\"color:#657b83;\">| x * </span><span style=\"color:#6c71c4;\">2</span><span style=\"color:#657b83;\">) {\n </span><span style=\"color:#859900;\">println!</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#cb4b16;\">{}</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, x);\n }\n}\n\n</span><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">main</span><span style=\"color:#657b83;\">() {\n </span><span style=\"color:#859900;\">weird_function</span><span style=\"color:#657b83;\">(</span><span style=\"color:#6c71c4;\">1</span><span style=\"color:#859900;\">..</span><span style=\"color:#6c71c4;\">10</span><span style=\"color:#657b83;\">);\n}\n</span></code></pre>\n<p>And now let's say we hate the number 8, and want to stop when we hit it. That's a simple one-line change:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">weird_function</span><span style=\"color:#657b83;\">(</span><span style=\"color:#268bd2;\">iter</span><span style=\"color:#657b83;\">: impl </span><span style=\"color:#859900;\">IntoIterator</span><span style=\"color:#657b83;\"><Item=</span><span style=\"color:#268bd2;\">u32</span><span style=\"color:#657b83;\">>) {\n </span><span style=\"color:#859900;\">for</span><span style=\"color:#657b83;\"> x </span><span style=\"color:#859900;\">in</span><span style=\"color:#657b83;\"> iter.</span><span style=\"color:#859900;\">into_iter</span><span style=\"color:#657b83;\">().</span><span style=\"color:#859900;\">map</span><span style=\"color:#657b83;\">(|</span><span style=\"color:#268bd2;\">x</span><span style=\"color:#657b83;\">| x * </span><span style=\"color:#6c71c4;\">2</span><span style=\"color:#657b83;\">) {\n </span><span style=\"color:#859900;\">if</span><span style=\"color:#657b83;\"> x == </span><span style=\"color:#6c71c4;\">8 </span><span style=\"color:#657b83;\">{ </span><span style=\"color:#859900;\">return </span><span style=\"color:#657b83;\">} </span><span style=\"color:#93a1a1;\">// added this line\n </span><span style=\"color:#859900;\">println!</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#cb4b16;\">{}</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, x);\n }\n}\n</span></code></pre>\n<p>Easy, done, end of story. And for this reason, I <em>recommend</em> using <code>for</code> loops when possible. Even though, from a functional programming background, it feels overly imperative. However, some people out there want to be more functional, so let's explore that.</p>\n<h2 id=\"for-each-vs-map\">for_each vs map</h2>\n<p>Let's forget about the short-circuiting for a moment. And now we want to go back to the original version of the program, but <em>without</em> using a <code>for</code> loop. Easy enough with the method <code>for_each</code>. It takes a closure, which it runs for each value in the <code>Iterator</code>. Let's check it out:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">weird_function</span><span style=\"color:#657b83;\">(</span><span style=\"color:#268bd2;\">iter</span><span style=\"color:#657b83;\">: impl </span><span style=\"color:#859900;\">IntoIterator</span><span style=\"color:#657b83;\"><Item=</span><span style=\"color:#268bd2;\">u32</span><span style=\"color:#657b83;\">>) {\n iter.</span><span style=\"color:#859900;\">into_iter</span><span style=\"color:#657b83;\">().</span><span style=\"color:#859900;\">map</span><span style=\"color:#657b83;\">(|</span><span style=\"color:#268bd2;\">x</span><span style=\"color:#657b83;\">| x * </span><span style=\"color:#6c71c4;\">2</span><span style=\"color:#657b83;\">).</span><span style=\"color:#859900;\">for_each</span><span style=\"color:#657b83;\">(|</span><span style=\"color:#268bd2;\">x</span><span style=\"color:#657b83;\">| {\n </span><span style=\"color:#859900;\">println!</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#cb4b16;\">{}</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, x);\n })\n}\n</span></code></pre>\n<p>But why, exactly do we need <code>for_each</code>? That seems awfully similar to <code>map</code>, which <em>also</em> applies a function over every value in an <code>Iterator</code>. Trying to make that change, however, demonstrates the problem. With this code:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">weird_function</span><span style=\"color:#657b83;\">(</span><span style=\"color:#268bd2;\">iter</span><span style=\"color:#657b83;\">: impl </span><span style=\"color:#859900;\">IntoIterator</span><span style=\"color:#657b83;\"><Item=</span><span style=\"color:#268bd2;\">u32</span><span style=\"color:#657b83;\">>) {\n iter.</span><span style=\"color:#859900;\">into_iter</span><span style=\"color:#657b83;\">().</span><span style=\"color:#859900;\">map</span><span style=\"color:#657b83;\">(|</span><span style=\"color:#268bd2;\">x</span><span style=\"color:#657b83;\">| x * </span><span style=\"color:#6c71c4;\">2</span><span style=\"color:#657b83;\">).</span><span style=\"color:#859900;\">map</span><span style=\"color:#657b83;\">(|</span><span style=\"color:#268bd2;\">x</span><span style=\"color:#657b83;\">| {\n </span><span style=\"color:#859900;\">println!</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#cb4b16;\">{}</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, x);\n })\n}\n</span></code></pre>\n<p>we get an error message:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#657b83;\">error[E0308]: mismatched types\n --> src\\main.rs:2:5\n |\n2 | / iter.into_iter().map(|x| x * 2).map(|x| {\n3 | | println!("{}", x);\n4 | | })\n | |______^ expected `()`, found struct `Map`\n</span></code></pre>\n<p>Undaunted, I fix this error by sticking a semicolon at the end of that expression. That generates a warning of <code>unused `Map` that must be used</code>. And sure enough, running this program produces no output.</p>\n<p>The problem is that <code>map</code> doesn't drain the <code>Iterator</code>. Said another way, <code>map</code> is <em>lazy</em>. It adapts one <code>Iterator</code> into a new <code>Iterator</code>. But unless something comes along and <em>drains</em> or <em>forces</em> the <code>Iterator</code>, no actions will occur. By contrast, <code>for_each</code> will always drain an <code>Iterator</code>.</p>\n<p>One easy trick to force draining of an <code>Iterator</code> is with the <code>count()</code> method. This will perform some unnecessary work of counting how many values are in the <code>Iterator</code>, but it's not that expensive. Another approach would be to use <code>collect</code>. This one is a little trickier, since <code>collect</code> typically needs some type annotations. But thanks to a fun trick of how <code>FromIterator</code> is implemented for the unit type, we can collect a stream of <code>()</code>s into a single <code>()</code> value. Meaning, this code works:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">weird_function</span><span style=\"color:#657b83;\">(</span><span style=\"color:#268bd2;\">iter</span><span style=\"color:#657b83;\">: impl </span><span style=\"color:#859900;\">IntoIterator</span><span style=\"color:#657b83;\"><Item=</span><span style=\"color:#268bd2;\">u32</span><span style=\"color:#657b83;\">>) {\n iter.</span><span style=\"color:#859900;\">into_iter</span><span style=\"color:#657b83;\">().</span><span style=\"color:#859900;\">map</span><span style=\"color:#657b83;\">(|</span><span style=\"color:#268bd2;\">x</span><span style=\"color:#657b83;\">| x * </span><span style=\"color:#6c71c4;\">2</span><span style=\"color:#657b83;\">).</span><span style=\"color:#859900;\">map</span><span style=\"color:#657b83;\">(|</span><span style=\"color:#268bd2;\">x</span><span style=\"color:#657b83;\">| {\n </span><span style=\"color:#859900;\">println!</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#cb4b16;\">{}</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, x);\n }).</span><span style=\"color:#859900;\">collect</span><span style=\"color:#657b83;\">()\n}\n</span></code></pre>\n<p>Note the lack of a semicolon at the end there. What do you think will happen if we add in the semicolon?</p>\n<h2 id=\"short-circuiting\">Short circuiting</h2>\n<p><strong>EDIT</strong> Enough people have asked "why not use <code>take_while</code>?" that I thought I'd address it. Yes, below, <code>take_while</code> will work for "short circuiting." It's probably even a good idea. But the main goal in this post is to explore some funny implementation approaches, not recommend a best practice. And overall, despite some good arguments for <code>take_while</code> being a good choice here, I still stand by the overall recommendation to prefer <code>for</code> loops for simplicity.</p>\n<p>With the <code>for</code> loop approach, stopping at the first 8 was a trivial, 1 line addition. Let's do the same thing here:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">weird_function</span><span style=\"color:#657b83;\">(</span><span style=\"color:#268bd2;\">iter</span><span style=\"color:#657b83;\">: impl </span><span style=\"color:#859900;\">IntoIterator</span><span style=\"color:#657b83;\"><Item=</span><span style=\"color:#268bd2;\">u32</span><span style=\"color:#657b83;\">>) {\n iter.</span><span style=\"color:#859900;\">into_iter</span><span style=\"color:#657b83;\">().</span><span style=\"color:#859900;\">map</span><span style=\"color:#657b83;\">(|</span><span style=\"color:#268bd2;\">x</span><span style=\"color:#657b83;\">| x * </span><span style=\"color:#6c71c4;\">2</span><span style=\"color:#657b83;\">).</span><span style=\"color:#859900;\">map</span><span style=\"color:#657b83;\">(|</span><span style=\"color:#268bd2;\">x</span><span style=\"color:#657b83;\">| {\n </span><span style=\"color:#859900;\">if</span><span style=\"color:#657b83;\"> x == </span><span style=\"color:#6c71c4;\">8 </span><span style=\"color:#657b83;\">{ </span><span style=\"color:#859900;\">return </span><span style=\"color:#657b83;\">}\n </span><span style=\"color:#859900;\">println!</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#cb4b16;\">{}</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, x);\n }).</span><span style=\"color:#859900;\">collect</span><span style=\"color:#657b83;\">()\n}\n</span></code></pre>\n<p>Take a guess at what the output will be. Ready? OK, here's the real thing:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#657b83;\">2\n4\n6\n10\n12\n14\n16\n18\n</span></code></pre>\n<p>We <em>skipped</em> 8, but we didn't stop. It's the difference between a <code>continue</code> and a <code>break</code> inside the <code>for</code> loop. Why did this happen?</p>\n<p>It's important to think about the scope of a <code>return</code>. It will exit the current function. And in this case, the current function isn't <code>weird_function</code>, but the <em>closure inside the <code>map</code> call</em>. This is what makes short-circuiting inside <code>map</code> so difficult.</p>\n<p>The same exact comment will apply to <code>for_each</code>. The only way to stop a <code>for_each</code> from continuing is to panic (or abort the program, if you want to get really aggressive).</p>\n<p>But with <code>map</code>, we have some ingenious ways of working around this and short-circuiting. Let's see it in action.</p>\n<h2 id=\"collect-an-option\">collect an <code>Option</code></h2>\n<p><code>map</code> needs some draining method to drive it. We've been using <code>collect</code>. I've <a href=\"https://www.fpcomplete.com/blog/collect-rust-traverse-haskell-scala/\">previously discussed the intricacies of this method</a>. One cool feature of <code>collect</code> is that, for <code>Option</code> and <code>Result</code>, it provides short-circuit capabilities. We can modify our program to take advantage of that:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">weird_function</span><span style=\"color:#657b83;\">(</span><span style=\"color:#268bd2;\">iter</span><span style=\"color:#657b83;\">: impl </span><span style=\"color:#859900;\">IntoIterator</span><span style=\"color:#657b83;\"><Item=</span><span style=\"color:#268bd2;\">u32</span><span style=\"color:#657b83;\">>) -> </span><span style=\"color:#859900;\">Option</span><span style=\"color:#657b83;\"><()> {\n iter.</span><span style=\"color:#859900;\">into_iter</span><span style=\"color:#657b83;\">().</span><span style=\"color:#859900;\">map</span><span style=\"color:#657b83;\">(|</span><span style=\"color:#268bd2;\">x</span><span style=\"color:#657b83;\">| x * </span><span style=\"color:#6c71c4;\">2</span><span style=\"color:#657b83;\">).</span><span style=\"color:#859900;\">map</span><span style=\"color:#657b83;\">(|</span><span style=\"color:#268bd2;\">x</span><span style=\"color:#657b83;\">| {\n </span><span style=\"color:#859900;\">if</span><span style=\"color:#657b83;\"> x == </span><span style=\"color:#6c71c4;\">8 </span><span style=\"color:#657b83;\">{ </span><span style=\"color:#859900;\">return None </span><span style=\"color:#657b83;\">} </span><span style=\"color:#93a1a1;\">// short circuit!\n </span><span style=\"color:#859900;\">println!</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#cb4b16;\">{}</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, x);\n </span><span style=\"color:#859900;\">Some</span><span style=\"color:#657b83;\">(()) </span><span style=\"color:#93a1a1;\">// keep going!\n </span><span style=\"color:#657b83;\">}).</span><span style=\"color:#859900;\">collect</span><span style=\"color:#657b83;\">()\n}\n</span></code></pre>\n<p>I put a return type of <code>weird_function</code>, though we could also use turbofish on <code>collect</code> and throw away the result. We just need some type annotation to say what we're trying to collect. Since collecting the underlying <code>()</code> values doesn't take up extra memory, this is even pretty efficient! The only cost is the extra <code>Option</code>. But that extra <code>Option</code> is (arguably) useful; it lets us know if we short-circuited or not.</p>\n<p>But the story isn't so rosy with other types. Let's say our closure within <code>map</code> returns the <code>x</code> value. In other words, replace the last line with <code>Some(x)</code> instead of <code>Some(())</code>. Now we need to somehow collect up those <code>u32</code>s. Something like this would work:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">weird_function</span><span style=\"color:#657b83;\">(</span><span style=\"color:#268bd2;\">iter</span><span style=\"color:#657b83;\">: impl </span><span style=\"color:#859900;\">IntoIterator</span><span style=\"color:#657b83;\"><Item=</span><span style=\"color:#268bd2;\">u32</span><span style=\"color:#657b83;\">>) -> </span><span style=\"color:#859900;\">Option</span><span style=\"color:#657b83;\"><</span><span style=\"color:#859900;\">Vec</span><span style=\"color:#657b83;\"><</span><span style=\"color:#268bd2;\">u32</span><span style=\"color:#657b83;\">>> {\n iter.</span><span style=\"color:#859900;\">into_iter</span><span style=\"color:#657b83;\">().</span><span style=\"color:#859900;\">map</span><span style=\"color:#657b83;\">(|</span><span style=\"color:#268bd2;\">x</span><span style=\"color:#657b83;\">| x * </span><span style=\"color:#6c71c4;\">2</span><span style=\"color:#657b83;\">).</span><span style=\"color:#859900;\">map</span><span style=\"color:#657b83;\">(|</span><span style=\"color:#268bd2;\">x</span><span style=\"color:#657b83;\">| {\n </span><span style=\"color:#859900;\">if</span><span style=\"color:#657b83;\"> x == </span><span style=\"color:#6c71c4;\">8 </span><span style=\"color:#657b83;\">{ </span><span style=\"color:#859900;\">return None </span><span style=\"color:#657b83;\">} </span><span style=\"color:#93a1a1;\">// short circuit!\n </span><span style=\"color:#859900;\">println!</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#cb4b16;\">{}</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, x);\n </span><span style=\"color:#859900;\">Some</span><span style=\"color:#657b83;\">(x) </span><span style=\"color:#93a1a1;\">// keep going!\n </span><span style=\"color:#657b83;\">}).</span><span style=\"color:#859900;\">collect</span><span style=\"color:#657b83;\">()\n}\n</span></code></pre>\n<p>But that incurs a heap allocation that we don't want! And using <code>count()</code> from before is useless too, since it won't even short circuit.</p>\n<p>But we do have one other trick.</p>\n<h2 id=\"sum\">sum</h2>\n<p>It turns out there's another draining method on <code>Iterator</code> that performs short circuiting: <code>sum</code>. This program works perfectly well:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">weird_function</span><span style=\"color:#657b83;\">(</span><span style=\"color:#268bd2;\">iter</span><span style=\"color:#657b83;\">: impl </span><span style=\"color:#859900;\">IntoIterator</span><span style=\"color:#657b83;\"><Item=</span><span style=\"color:#268bd2;\">u32</span><span style=\"color:#657b83;\">>) -> </span><span style=\"color:#859900;\">Option</span><span style=\"color:#657b83;\"><</span><span style=\"color:#268bd2;\">u32</span><span style=\"color:#657b83;\">> {\n iter.</span><span style=\"color:#859900;\">into_iter</span><span style=\"color:#657b83;\">().</span><span style=\"color:#859900;\">map</span><span style=\"color:#657b83;\">(|</span><span style=\"color:#268bd2;\">x</span><span style=\"color:#657b83;\">| x * </span><span style=\"color:#6c71c4;\">2</span><span style=\"color:#657b83;\">).</span><span style=\"color:#859900;\">map</span><span style=\"color:#657b83;\">(|</span><span style=\"color:#268bd2;\">x</span><span style=\"color:#657b83;\">| {\n </span><span style=\"color:#859900;\">if</span><span style=\"color:#657b83;\"> x == </span><span style=\"color:#6c71c4;\">8 </span><span style=\"color:#657b83;\">{ </span><span style=\"color:#859900;\">return None </span><span style=\"color:#657b83;\">} </span><span style=\"color:#93a1a1;\">// short circuit!\n </span><span style=\"color:#859900;\">println!</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#cb4b16;\">{}</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, x);\n </span><span style=\"color:#859900;\">Some</span><span style=\"color:#657b83;\">(x) </span><span style=\"color:#93a1a1;\">// keep going!\n </span><span style=\"color:#657b83;\">}).</span><span style=\"color:#859900;\">sum</span><span style=\"color:#657b83;\">()\n}\n</span></code></pre>\n<p>The downside is that it's unnecessarily summing up the values. And maybe that could be a real problem if some kind of overflow occurs. But this mostly works. But is there some way we can stay functional, short circuit, and get no performance overhead? Sure!</p>\n<h2 id=\"short\">Short</h2>\n<p>The final trick here is to create a new helper type for summing up an <code>Iterator</code>. But this thing won't really sum. Instead, it will throw away all of the values, and stop as soon as it sees an <code>Option</code>. Let's see it in practice:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#657b83;\">#[</span><span style=\"color:#268bd2;\">derive</span><span style=\"color:#657b83;\">(Debug)]\n</span><span style=\"color:#268bd2;\">enum </span><span style=\"color:#b58900;\">Short </span><span style=\"color:#657b83;\">{\n Stopped,\n Completed,\n}\n\n</span><span style=\"color:#268bd2;\">impl</span><span style=\"color:#657b83;\"><T> std::iter::Sum<</span><span style=\"color:#859900;\">Option</span><span style=\"color:#657b83;\"><T>> </span><span style=\"color:#859900;\">for </span><span style=\"color:#b58900;\">Short </span><span style=\"color:#657b83;\">{\n </span><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">sum</span><span style=\"color:#657b83;\"><I: </span><span style=\"color:#859900;\">Iterator</span><span style=\"color:#657b83;\"><Item = </span><span style=\"color:#859900;\">Option</span><span style=\"color:#657b83;\"><T>>>(</span><span style=\"color:#268bd2;\">iter</span><span style=\"color:#657b83;\">: I) -> </span><span style=\"color:#268bd2;\">Self </span><span style=\"color:#657b83;\">{\n </span><span style=\"color:#859900;\">for</span><span style=\"color:#657b83;\"> x </span><span style=\"color:#859900;\">in</span><span style=\"color:#657b83;\"> iter {\n </span><span style=\"color:#859900;\">if </span><span style=\"color:#268bd2;\">let </span><span style=\"color:#859900;\">None </span><span style=\"color:#657b83;\">= x { </span><span style=\"color:#859900;\">return </span><span style=\"color:#657b83;\">Short::Stopped }\n }\n Short::Completed\n }\n}\n</span><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">weird_function</span><span style=\"color:#657b83;\">(</span><span style=\"color:#268bd2;\">iter</span><span style=\"color:#657b83;\">: impl </span><span style=\"color:#859900;\">IntoIterator</span><span style=\"color:#657b83;\"><Item=</span><span style=\"color:#268bd2;\">u32</span><span style=\"color:#657b83;\">>) -> Short {\n iter.</span><span style=\"color:#859900;\">into_iter</span><span style=\"color:#657b83;\">().</span><span style=\"color:#859900;\">map</span><span style=\"color:#657b83;\">(|</span><span style=\"color:#268bd2;\">x</span><span style=\"color:#657b83;\">| x * </span><span style=\"color:#6c71c4;\">2</span><span style=\"color:#657b83;\">).</span><span style=\"color:#859900;\">map</span><span style=\"color:#657b83;\">(|</span><span style=\"color:#268bd2;\">x</span><span style=\"color:#657b83;\">| {\n </span><span style=\"color:#859900;\">if</span><span style=\"color:#657b83;\"> x == </span><span style=\"color:#6c71c4;\">8 </span><span style=\"color:#657b83;\">{ </span><span style=\"color:#859900;\">return None </span><span style=\"color:#657b83;\">} </span><span style=\"color:#93a1a1;\">// short circuit!\n </span><span style=\"color:#859900;\">println!</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#cb4b16;\">{}</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, x);\n </span><span style=\"color:#859900;\">Some</span><span style=\"color:#657b83;\">(x) </span><span style=\"color:#93a1a1;\">// keep going!\n </span><span style=\"color:#657b83;\">}).</span><span style=\"color:#859900;\">sum</span><span style=\"color:#657b83;\">()\n}\n\n</span><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">main</span><span style=\"color:#657b83;\">() {\n </span><span style=\"color:#859900;\">println!</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#cb4b16;\">{:?}</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, </span><span style=\"color:#859900;\">weird_function</span><span style=\"color:#657b83;\">(</span><span style=\"color:#6c71c4;\">1</span><span style=\"color:#859900;\">..</span><span style=\"color:#6c71c4;\">10</span><span style=\"color:#657b83;\">));\n}\n</span></code></pre>\n<p>And voila! We're done!</p>\n<p><strong>Exercise</strong> It's pretty cheeky to use <code>sum</code> here. <code>collect</code> makes more sense. Replace <code>sum</code> with <code>collect</code>, and then change the <code>Sum</code> implementation into something else. Solution at the end.</p>\n<h2 id=\"conclusion\">Conclusion</h2>\n<p>That's a lot of work to be functional. Rust has a great story around short circuiting. And it's not just with <code>return</code>, <code>break</code>, and <code>continue</code>. It's with the <code>?</code> try operator, which forms the basis of error handling in Rust. There are times when you'll want to use <code>Iterator</code> adapters, async streaming adapters, and functional-style code. But unless you have a pressing need, my recommendation is to stick to <code>for</code> loops.</p>\n<p>If you liked this post, and would like to see more Rust quickies, <a href=\"https://twitter.com/snoyberg\">let me know</a>. You may also like these other pages:</p>\n<ul>\n<li><a href=\"https://www.fpcomplete.com/rust/crash-course/\">Rust Crash Course</a></li>\n<li><a href=\"https://www.fpcomplete.com/training/\">Training</a></li>\n<li><a href=\"https://www.fpcomplete.com/rust/\">FP Complete Rust home page</a></li>\n<li><a href=\"/tags/rust/\">Rust tagged blog posts</a></li>\n<li><a href=\"https://www.fpcomplete.com/jobs/\">Jobs at FP Complete</a></li>\n</ul>\n<h2 id=\"solution\">Solution</h2>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#859900;\">use </span><span style=\"color:#657b83;\">std::iter::FromIterator;\n\n#[</span><span style=\"color:#268bd2;\">derive</span><span style=\"color:#657b83;\">(Debug)]\n</span><span style=\"color:#268bd2;\">enum </span><span style=\"color:#b58900;\">Short </span><span style=\"color:#657b83;\">{\n Stopped,\n Completed,\n}\n\n</span><span style=\"color:#268bd2;\">impl</span><span style=\"color:#657b83;\"><T> FromIterator<</span><span style=\"color:#859900;\">Option</span><span style=\"color:#657b83;\"><T>> </span><span style=\"color:#859900;\">for </span><span style=\"color:#b58900;\">Short </span><span style=\"color:#657b83;\">{\n </span><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">from_iter</span><span style=\"color:#657b83;\"><I: </span><span style=\"color:#859900;\">IntoIterator</span><span style=\"color:#657b83;\"><Item = </span><span style=\"color:#859900;\">Option</span><span style=\"color:#657b83;\"><T>>>(</span><span style=\"color:#268bd2;\">iter</span><span style=\"color:#657b83;\">: I) -> </span><span style=\"color:#268bd2;\">Self </span><span style=\"color:#657b83;\">{\n </span><span style=\"color:#859900;\">for</span><span style=\"color:#657b83;\"> x </span><span style=\"color:#859900;\">in</span><span style=\"color:#657b83;\"> iter {\n </span><span style=\"color:#859900;\">if </span><span style=\"color:#268bd2;\">let </span><span style=\"color:#859900;\">None </span><span style=\"color:#657b83;\">= x { </span><span style=\"color:#859900;\">return </span><span style=\"color:#657b83;\">Short::Stopped }\n }\n Short::Completed\n }\n}\n</span><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">weird_function</span><span style=\"color:#657b83;\">(</span><span style=\"color:#268bd2;\">iter</span><span style=\"color:#657b83;\">: impl </span><span style=\"color:#859900;\">IntoIterator</span><span style=\"color:#657b83;\"><Item=</span><span style=\"color:#268bd2;\">u32</span><span style=\"color:#657b83;\">>) -> Short {\n iter.</span><span style=\"color:#859900;\">into_iter</span><span style=\"color:#657b83;\">().</span><span style=\"color:#859900;\">map</span><span style=\"color:#657b83;\">(|</span><span style=\"color:#268bd2;\">x</span><span style=\"color:#657b83;\">| x * </span><span style=\"color:#6c71c4;\">2</span><span style=\"color:#657b83;\">).</span><span style=\"color:#859900;\">map</span><span style=\"color:#657b83;\">(|</span><span style=\"color:#268bd2;\">x</span><span style=\"color:#657b83;\">| {\n </span><span style=\"color:#859900;\">if</span><span style=\"color:#657b83;\"> x == </span><span style=\"color:#6c71c4;\">8 </span><span style=\"color:#657b83;\">{ </span><span style=\"color:#859900;\">return None </span><span style=\"color:#657b83;\">} </span><span style=\"color:#93a1a1;\">// short circuit!\n </span><span style=\"color:#859900;\">println!</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#cb4b16;\">{}</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, x);\n </span><span style=\"color:#859900;\">Some</span><span style=\"color:#657b83;\">(x) </span><span style=\"color:#93a1a1;\">// keep going!\n </span><span style=\"color:#657b83;\">}).</span><span style=\"color:#859900;\">collect</span><span style=\"color:#657b83;\">()\n}\n\n</span><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">main</span><span style=\"color:#657b83;\">() {\n </span><span style=\"color:#859900;\">println!</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#cb4b16;\">{:?}</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, </span><span style=\"color:#859900;\">weird_function</span><span style=\"color:#657b83;\">(</span><span style=\"color:#6c71c4;\">1</span><span style=\"color:#859900;\">..</span><span style=\"color:#6c71c4;\">10</span><span style=\"color:#657b83;\">));\n}\n</span></code></pre>",
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"content": "<p>Rust is a systems programming language following fairly standard imperative approaches and a C-style syntax. Haskell is a purely functional programming language, innovating in areas such as type theory and effect management. Viewed that way, these languages are polar opposites.</p>\n<p>And yet, these two languages attract many of the same people, including the engineering team at FP Complete. Putting on a different set of lenses, both languages provide powerful abstractions, enforce different kinds of correctness via static analysis in the compiler, and favor powerful features over quick adoption.</p>\n<p>In this post, I want to look at some of the philosophical underpinnings that explain some of the similarities and differences in the languages. Some of these are inherent. Rust's status as a systems programming language essentially requires some different approaches to Haskell's purely functional nature. But some of these are not. It wasn't strictly necessary for both languages to converge on similar systems for Algebraic Data Types (ADTs) and ad hoc polymorphism (via traits/type classes).</p>\n<p>Keep in mind that in writing this post, I'm viewing it as a <em>consumer</em> of the languages, not a designer. The designers themselves may have different motivations than those I describe. It would certainly be interesting to see if others have different takes on this topic.</p>\n<h2 id=\"rust-ownership\">Rust: ownership</h2>\n<p>This is so obvious that I almost forgot to include it. If there's one thing that defines Rust versus any other language, it's ownership and the borrow checker. This speaks to two core pieces of Rust:</p>\n<ul>\n<li>The goal of serving as a systems programming language, where garbage collection is not an option</li>\n<li>The goal of providing a safe subset of the language, where undefined behavior cannot occur</li>\n</ul>\n<p>The concept of ownership achieves both of these. Many additions have been made to the language to make it easier to work with ownership overall. This hints at the concept of ergonomics, which is fundamental to Rust philosophy. But ownership and borrow checking are also known as the harder parts of the language. Putting it together, we see a philosophy of striving to meet our goals safely, while making the usage of the features as easy as possible. However, if there's a conflict between the goals and ease of use, the goals win out.</p>\n<p>All of this stands in stark contrast to Haskell, which is explicitly <em>not</em> a systems language, and does not attempt in any way to address those cases. Instead, it leverages garbage collection quite happily, with the trade-offs between performance and ease-of-use inherent in that choice.</p>\n<h2 id=\"haskell-purely-functional\">Haskell: purely functional</h2>\n<p>The underlying goal of Haskell is ultimately to create a purely functional programming language. Many of the most notable and unusual features of Haskell directly derive from this goal, such as using monads to explicitly track effects.</p>\n<p>Other parts of the language follow from this less directly. For example, Haskell strongly embraces Higher Order Functions, currying, and partial function application. This combination turns many common structures in other languages (like loops) into normal functions. But in order to make this feel natural, Haskell uses slightly odd (compared to other languages) syntax for function application.</p>\n<p>And this gets into a more fundamental piece of philosophy. Haskell is willing to be quite dramatically different from other programming languages in its pursuit of its goals. In my opinion, Rust has been less willing to diverge from mainstream approaches, veering away only out of absolute necessity.</p>\n<p>This results in a world where Haskell feels quite a bit more foreign to others, but has more freedom to innovate. Rust, on the other hand, has stuck to existing solutions when possible, such as eschewing monadic futures in favor of <code>async</code>/<code>.await</code> syntax.</p>\n<h2 id=\"expression-oriented\">Expression oriented</h2>\n<p>I undervalued how important this feature was for a while, but recently I've realized that it's one of the most important features in both languages for me.</p>\n<blockquote class=\"twitter-tweet\"><p lang=\"en\" dir=\"ltr\">I used to think that the reason I loved both Haskell and Rust so much was their shared strong typing, ADTs, and pattern matching combination.<br><br>After a recent discussion, I think it may be more about being expression-oriented languages.</p>— Michael Snoyman (@snoyberg) <a href=\"https://twitter.com/snoyberg/status/1348486654017855489?ref_src=twsrc%5Etfw\">January 11, 2021</a></blockquote> <script async src=\"https://platform.twitter.com/widgets.js\" charset=\"utf-8\"></script>\n<p>Instead of relying on declare-then-assign patterns, both languages allow conditionals and other constructs to evaluate to values. This reduces the frequency of seeing mutable assignment and avoids cases of uninitialized variables. By restricting mutable assignment to cases where it's actual mutation, we get to free up a lot of head space to focus on the trickier parts of programming.</p>\n<h2 id=\"type-system\">Type system</h2>\n<p>Rust and Haskell have very similar type systems. Both make it easy to create new types, provide for features like newtypes, provide type aliases, and offer a combination of product (<code>struct</code>) and sum (<code>enum</code>) types. Both allow labeling fields or accessing values positionally. Both offer <a href=\"https://www.fpcomplete.com/blog/pattern-matching/\">pattern matching</a> constructs. Overall, the similarities between the two languages far outweigh the differences.</p>\n<p>I place a large part of the shared interest between these languages at the feet of the type system. Since I started using Haskell, I feel strongly hampered using any language without a rich, flexible, and powerful type system. Rust's embrace of Algebraic Data Types (ADTs) feels natural.</p>\n<p>There are some differences between the languages in these topics, but they are <em>mostly</em> superficial. For example, Haskell uses the single keyword <code>data</code> for introducing both product and sum types, while Rust uses <code>struct</code> and <code>enum</code>, respectively. Haskell will allow creation of partial field accessors in sum types, while Rust does not. Haskell allows for partial pattern matches (with an optional warning), and Rust does not.</p>\n<p>These are meaningful and affect the way you use the languages, but I don't see them as deeply philosophical. Instead, I see both languages embracing the idea that encouraging programmers to define and use strong typing mechanisms leads to better code. And it's a message I wholeheartedly endorse.</p>\n<h2 id=\"traits-and-type-classes\">Traits and type classes</h2>\n<p>In the wide world of inheritance and polymorphism, there are a lot of different approaches. Within that, Rust's traits and Haskell's type classes are far more similar than different. Both of them allow you to separate out functionality (methods) from data (<code>struct</code>/<code>data</code>). Both allow you to create new types or traits/classes yourself and add them on to existing types/traits/classes. Both of them support a concept of associated types, and multiple parameters (either via parameterized traits or multi-param type classes).</p>\n<p>There are some differences between the two. For one, Rust doesn't allow orphans. An implementation must appear in the same crate as either the type definition or the trait definition. (The fact that Rust treats an entire crate as a compilation unit instead of a single module makes this restriction less of an imposition.) Also, Haskell supports functional dependencies, but that's not terribly interesting, since that can be closely approximated with associated types. And there are other, more subtle differences, around issues like overlapping instances. Rust's lack of orphans allows it to make some closed world assumptions that Haskell cannot.</p>\n<p>Ultimately, the distinctions above don't lend themselves to a deep philosophical difference, but rather minor variations on a theme. There is, however, one major distinction in this area between the two languages: Higher Kinded Types (HKTs). In Haskell, HKTs provide the basis for such typeclasses as <code>Functor</code>, <code>Applicative</code>, <code>Monad</code>, <code>Foldable</code>, and <code>Traversable</code>. In Rust, implementing some kind of traits around these concepts is <a href=\"https://www.fpcomplete.com/blog/monads-gats-nightly-rust/\">a bit more complicated</a>.</p>\n<p>And this is one of the deeper philosophical differences between the two languages. Haskellers readily embrace concepts like HKTs. The Rust community has adamantly avoided embracing them, due to their perceived complexity. Instead, in Rust, alternative and arguably simpler approaches have been used to solve the same problems these typeclasses solve in Haskell. Which leads us to probably the biggest philosophical difference between the languages.</p>\n<h2 id=\"general-vs-specific\">General vs specific</h2>\n<p>Let's say I want to have early termination in the case of an error. Or asynchronous coding capabilities. Or the ability to pass information to the rest of a computation. How would I achieve this?</p>\n<p>In Haskell, the answer is <em>obviously</em> <code>Monad</code>s. <code>do</code>-notation is a general purpose "programmable semicolon." It generally solves all of these cases. And many, many more. Writing a parser? <code>Monad</code>. Concurrency? Maybe <code>Monad</code>, or maybe <code>Applicative</code> with <code>ApplicativeDo</code> turned on. But the common factor: we can express large classes of problems as <code>do</code>-notation.</p>\n<p>How about Rust? Well, if you want early termination for errors, you'll use a <code>Result</code> return type and the <code>?</code> try operator. Async? <code>async</code>/<code>.await</code> syntax. Pass in information? Maybe use method syntax, maybe use thread-local state, maybe something else.</p>\n<p>The point is that the Haskell community overall reaches for generalizing a solution as far as possible, usually along the lines of some abstract mathematical underpinning. There are huge advantages to this. We build out solutions to problems we didn't even know we had. We are able to rely on mathematical laws to guide our designs and ensure concepts compose nicely.</p>\n<p>The Rust community, instead, favors specific, ergonomic solutions. Error handling is <em>really</em> common, so give it a single character operator. Make sure that it handles common cases, like unifying error types via the <code>From</code> trait. Make sure error messages are as clear as possible. Optimize for the 95%, and don't worry about the 5% yet. (And see the next section for the 5%.)</p>\n<p>To me, this is the deepest non-inherent divide between the languages. Sure, ownership versus purity is huge, but it's right there on the label of the languages. <strong>This distinction ends up impacting how new language features are added, how people generally think about solutions, and how libraries are designed.</strong></p>\n<p>One final point. As much as I've implied that the Rust and Haskell communities are in two camps here, that's not quite fair. There are people in the Haskell community looking to make more specific solutions to some problems. (I'm probably one of them with things like <code>RIO</code>.) And while I can't think of a concrete Rust example to the contrary, I have no doubt that there are cases where people design general solutions when a more specific one would suffice.</p>\n<h2 id=\"code-generation-metaprogramming-macros\">Code generation/metaprogramming/macros</h2>\n<p>Haskell has metaprogramming via Template Haskell (TH). It's almost universally viewed as a necessary evil, but evil nonetheless. It screws up compilation in some cases via stage restrictions, it requires a language pragma to enable, and introduces awkward syntax. Features like deriving serialization instances are generally moving towards in-language features via the <code>Generic</code> typeclass.</p>\n<p>Rust's "Hello World" sticks a macro call on the second line via <code>println!</code>. The syntax for calling macros looks almost identical to function calls. Common libraries encourage macro usage all over the place. <code>serde</code> serialization deriving, <code>structopt</code> command line parsing, and <code>snafu</code>/<code>thiserror</code> error type creation all leverage macro attributes and deriving.</p>\n<p>This is a fascinating distinction to me. I've been on both sides of the TH divide. Yesod famously uses TH for a lot of code generation, which has earned the ire of many Haskellers. I've since generally avoided using TH when possible in the past few years. And when I picked up Rust, I studiously avoided learning how to create macros until relatively recently, lest I be tempted to slip back into my old, evil ways.</p>\n<p>Metaprogramming definitely complicates some things. It makes it harder to debug some problems. Rust does a pretty good job at making sure error messages can be comprehensible. But documentation on macro arguments and return types is still not as nice as functions and methods.</p>\n<p>I think I'm still mostly in the Haskell camp of avoiding unnecessary metaprogramming in my API design, but I'm beginning to be more free with it. And I have no reservations in Rust about <em>using</em> macros; they're wonderful. I do wonder if the main issue in Haskell isn't the overall concept of metaprogramming, but the specific implementation with Template Haskell.</p>\n<h2 id=\"backwards-compatibility\">Backwards compatibility</h2>\n<p>Rust overall has a more coherent and consistent story around backwards compatibility. It's almost always painless to upgrade to new versions of the Rust compiler. This puts an extra burden on the compiler team, and constrains changes that can be made to the language. And in one case (the module system update), it required a new <code>edition</code> system to allow for full backwards compatibility.</p>\n<p>The Haskell community overall cares less about backwards compatibility. New versions of the compiler regularly break code. New versions of libraries will get released to smooth out rough edges in the APIs. (I used to do this regularly, and now regret that. I've tried hard to keep backwards compatibility in my libraries.)</p>\n<p>Overall, I think the Rust community's approach here is better for producing production software. Arguably the Haskell approach allows for much more exploration and attainment of some higher level of beauty. Or as they say, "avoid (success at all costs)."</p>\n<h2 id=\"optimistic-optimizations\">Optimistic optimizations</h2>\n<p>GHC has a powerful rewrite rules system, which can rewrite less efficient combinations of functions to more optimized ones. This plays in a big way in the <code>vector</code> package, where rewrite rules implement stream fusion, allowing many classes of vector pipelines to completely avoid allocation. This is a massive optimization. At least when it works. As I've personally experienced, and many others have too, rewrite rules can be finicky. The Haskell approach is to be happy that our code sometimes gets much faster, and that we get to keep elegant, easy-to-understand code.</p>\n<p>The Rust approach is the polar opposite. Either code will <em>definitely</em> be fast or <em>definitely</em> be slow. I learned this a while ago when looking into recursive functions and tail call optimization (TCO). The Rust compiler will <em>not</em> perform a TCO, because it's so easy to accidentally change a TCO-able implementation into something that eats up stack space. There are plans to make explicit tail calls possible with the <code>become</code> keyword someday.</p>\n<p>More generally, Rust embraces the concept of zero cost abstractions. The idea is that you should be able to abstract and simplify code, when we can guarantee that there is no cost. In the Haskell world, we tend to focus on the elegant abstraction, even if a cost will be involved.</p>\n<h2 id=\"learning-curve\">Learning curve</h2>\n<p>A short one here. Both languages have a higher-than-average learning curve compared with other languages. Both languages embrace their learning curves. As much as possible, we try to make learning and using the languages easy. But neither language shies away from powerful features, even if it will make the language a bit harder to learn.</p>\n<p>To quote a Perlism: you'll only learn the language once, you'll use it for the rest of your life.</p>\n<h2 id=\"explicitly-mark-things\">Explicitly mark things</h2>\n<p>Both languages embrace the idea of explicitly marking things. For example, both languages encourage (in Haskell's case) or enforce (in Rust's case) marking the type signature of all functions. But that's pretty common. Haskell goes further, and requires that you mark all effectful computations with the <code>IO</code> type (or something similar, like <code>MonadIO</code>). Rust requires than anything which may fail be marked with a <code>Result</code> return value.</p>\n<p>You may argue that these are actually a <em>difference</em> in the language, and to some extent that's true. But I think the difference is about what the language considers important. Haskell, for reasons of purity, values deeply the idea that an effect may be performed. It then lumps errors and exceptions into the contract of <code>IO</code> and the concept of laziness (for better or worse). Rust, on the other hand, doesn't care if you may perform an effect, but deeply cares about whether an error may occur.</p>\n<h2 id=\"type-enforce-everything\">Type enforce <em>everything</em>?</h2>\n<p>When I initially implemented Haskell's <code>monad-logger</code>, I provided an instance for <code>IO</code> which performed no output. I received many complaints that people would rather get a compile time error if they forgot to initialize the logging system, and I removed the <code>IO</code> instance. (Without getting into details: this was <em>definitely</em> the right decision for the API, regardless of the distinction with Rust.)</p>\n<p>That's why I was so amused when I first used the <code>log</code> crate in Rust, and realized that if you don't initialize the logging system, it produces no output. There's no runtime error, just silence.</p>\n<p>Similarly, many functions in the Tokio crate will fail at runtime if run from outside of the context of a Tokio runtime. But nothing in the type system enforces this idea.</p>\n<p>And finally, I've been bitten a few times by <code>actix-web</code>'s state management. If you mismatch the type of the state between your handlers and your service declaration, you'll end up with a runtime error instead of a compile time bug.</p>\n<p>In the Haskell world, the overall philosophy is generally to approach "if it compiles, it works." Haskellers love enforcing almost every invariant at the type level.</p>\n<p>I haven't discussed this much with Rustaceans, but it seems to me that the overall Rust philosophy here is slightly different. Instead, we like to express <em>tricky</em> invariants at the type level. But if something is so obviously going to fail or behave incorrectly in the most basic smoke testing, such as a Tokio function crashing, there's no need to develop type-level protections against it.</p>\n<h2 id=\"conclusion\">Conclusion</h2>\n<p>I hope this laundry list comparison was interesting. I've been meaning to write it down for a while, so I kind of feel like I checked off a New Year's Resolution in doing so. I'd be curious to hear any other points of comparison people have, or disagreements about my assessments.</p>\n<p>You may also like:</p>\n<ul>\n<li><a href=\"https://www.fpcomplete.com/blog/rust-at-fpco-2020/\">Rust at FP Complete, 2020 update</a></li>\n<li><a href=\"https://www.fpcomplete.com/blog/collect-rust-traverse-haskell-scala/\">Collect in Rust, traverse in Haskell and Scala</a></li>\n<li><a href=\"https://www.fpcomplete.com/blog/error-handling-is-hard/\">Error handling is hard</a></li>\n<li><a href=\"https://www.fpcomplete.com/training/\">Training at FP Complete</a></li>\n<li><a href=\"https://www.fpcomplete.com/rust/crash-course/\">Rust Crash Course</a></li>\n<li><a href=\"https://www.fpcomplete.com/haskell/syllabus/\">Applied Haskell syllabus</a></li>\n</ul>\n",
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"content": "<p>This semi-surprising corner case came up in some recent <a href=\"https://www.fpcomplete.com/training/\">Rust training</a> I was giving. I figured a short write-up may help some others in the future.</p>\n<p>Rust's language design focuses on ergonomics. The goal is to make common patterns easy to write on a regular basis. This overall works out very well. But occasionally, you end up with a surprising outcome. And I think this situation is a good example.</p>\n<p>Let's start off by pretending that method syntax doesn't exist at all. Let's say I've got a <code>String</code>, and I want to clone it. I know that there's a <code>Clone::clone</code> method, which takes a <code>&String</code> and returns a <code>String</code>. We can leverage that like so:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">uses_string</span><span style=\"color:#657b83;\">(</span><span style=\"color:#268bd2;\">x</span><span style=\"color:#657b83;\">: String) {\n </span><span style=\"color:#859900;\">println!</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">I consumed the String! </span><span style=\"color:#cb4b16;\">{}</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, x);\n}\n\n</span><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">main</span><span style=\"color:#657b83;\">() {\n </span><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> name = </span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">Alice</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">.</span><span style=\"color:#859900;\">to_owned</span><span style=\"color:#657b83;\">();\n </span><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> name_clone = </span><span style=\"color:#859900;\">Clone</span><span style=\"color:#657b83;\">::clone(</span><span style=\"color:#859900;\">&</span><span style=\"color:#657b83;\">name);\n </span><span style=\"color:#859900;\">uses_string</span><span style=\"color:#657b83;\">(name);\n </span><span style=\"color:#859900;\">uses_string</span><span style=\"color:#657b83;\">(name_clone);\n}\n</span></code></pre>\n<p>Notice that I needed to pass <code>&name</code> to <code>clone</code>, not simply <code>name</code>. If I did the latter, I would end up with a type error:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#657b83;\">error[E0308]: mismatched types\n --> src\\main.rs:7:35\n |\n7 | let name_clone = Clone::clone(name);\n | ^^^^\n | |\n | expected reference, found struct `String`\n | help: consider borrowing here: `&name`\n</span></code></pre>\n<p>And that's because Rust won't automatically borrow a reference from function arguments. You need to explicit say that you want to borrow the value. Cool.</p>\n<p>But now I've remembered that method syntax <em>is</em>, in fact, a thing. So let's go ahead and use it!</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> name_clone = (</span><span style=\"color:#859900;\">&</span><span style=\"color:#657b83;\">name).</span><span style=\"color:#859900;\">clone</span><span style=\"color:#657b83;\">();\n</span></code></pre>\n<p>Remembering that <code>clone</code> takes a <code>&String</code> and not a <code>String</code>, I've gone ahead and helpfully borrowed from <code>name</code> before calling the <code>clone</code> method. And I needed to wrap up that whole expression in parentheses, otherwise it will be parsed incorrectly by the compiler.</p>\n<p>That all works, but it's clearly not the way we want to write code in general. Instead, we'd like to forgo the parentheses and the <code>&</code> symbol. And fortunately, we can! Most Rustaceans early on learn that you can simply do this:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> name_clone = name.</span><span style=\"color:#859900;\">clone</span><span style=\"color:#657b83;\">();\n</span></code></pre>\n<p>In other words, when we use method syntax, we can call <code>.clone()</code> on either a <code>String</code> <em>or</em> a <code>&String</code>. That's because with a <a href=\"https://doc.rust-lang.org/stable/reference/expressions/method-call-expr.html\">method call expression</a>, "the receiver may be automatically dereferenced or borrowed in order to call a method." Essentially, the compiler follows these steps:</p>\n<ul>\n<li>What's the type of <code>name</code>? OK, it's a <code>String</code></li>\n<li>Is there a method available that takes a <code>String</code> as the receiver? Nope.</li>\n<li>OK, try borrowing it. Is there a method available that takes a <code>&String</code> as the receiver? Yes. Use that!</li>\n</ul>\n<p>And, for the most part, this works exactly as you'd expect. Until it doesn't. Let's start off with a confusing error message. Let's say I've got a helper function to loudly clone a <code>String</code>:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">clone_loudly</span><span style=\"color:#657b83;\">(</span><span style=\"color:#268bd2;\">x</span><span style=\"color:#657b83;\">: </span><span style=\"color:#859900;\">&</span><span style=\"color:#657b83;\">String) -> String {\n </span><span style=\"color:#859900;\">println!</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">Cloning </span><span style=\"color:#cb4b16;\">{}</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, x);\n x.</span><span style=\"color:#859900;\">clone</span><span style=\"color:#657b83;\">()\n}\n\n</span><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">uses_string</span><span style=\"color:#657b83;\">(</span><span style=\"color:#268bd2;\">x</span><span style=\"color:#657b83;\">: String) {\n </span><span style=\"color:#859900;\">println!</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">I consumed the String! </span><span style=\"color:#cb4b16;\">{}</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, x);\n}\n\n</span><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">main</span><span style=\"color:#657b83;\">() {\n </span><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> name = </span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">Alice</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">.</span><span style=\"color:#859900;\">to_owned</span><span style=\"color:#657b83;\">();\n </span><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> name_clone = </span><span style=\"color:#859900;\">clone_loudly</span><span style=\"color:#657b83;\">(</span><span style=\"color:#859900;\">&</span><span style=\"color:#657b83;\">name);\n </span><span style=\"color:#859900;\">uses_string</span><span style=\"color:#657b83;\">(name);\n </span><span style=\"color:#859900;\">uses_string</span><span style=\"color:#657b83;\">(name_clone);\n}\n</span></code></pre>\n<p>Looking at <code>clone_loudly</code>, I realize that I can easily generalize this to more than just a <code>String</code>. The only two requirements are that the type must implement <code>Display</code> (for the <code>println!</code> call) and <code>Clone</code>. Let's go ahead and implement that, accidentally forgetting about the <code>Clone</code>:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#859900;\">use </span><span style=\"color:#657b83;\">std::fmt::Display;\n</span><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">clone_loudly</span><span style=\"color:#657b83;\"><T: Display>(</span><span style=\"color:#268bd2;\">x</span><span style=\"color:#657b83;\">: </span><span style=\"color:#859900;\">&</span><span style=\"color:#657b83;\">T) -> T {\n </span><span style=\"color:#859900;\">println!</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">Cloning </span><span style=\"color:#cb4b16;\">{}</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, x);\n x.</span><span style=\"color:#859900;\">clone</span><span style=\"color:#657b83;\">()\n}\n</span></code></pre>\n<p>As you'd expect, this doesn't compile. However, the error message given may be surprising. If you're like me, you were probably expecting an error message about missing a <code>Clone</code> bound on <code>T</code>. In fact, we get something else entirely:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#657b83;\">error[E0308]: mismatched types\n --> src\\main.rs:4:5\n |\n2 | fn clone_loudly<T: Display>(x: &T) -> T {\n | - this type parameter - expected `T` because of return type\n3 | println!("Cloning {}", x);\n4 | x.clone()\n | ^^^^^^^^^ expected type parameter `T`, found `&T`\n |\n = note: expected type parameter `T`\n found reference `&T`\n</span></code></pre>\n<p>Strangely enough, the <code>.clone()</code> seems to have succeeded, but returned a <code>&T</code> instead of a <code>T</code>. That's because the method call expression is following the same steps as above with <code>String</code>, namely:</p>\n<ul>\n<li>What's the type of <code>x</code>? OK, it's a <code>&T</code></li>\n<li>Is there a <code>clone</code> method available that takes a <code>&T</code> as the receiver? Nope, since we don't know that <code>T</code> implements the <code>Clone</code> trait.</li>\n<li>OK, try borrowing it. Is there a method available that takes a <code>&&T</code> as the receiver? <a href=\"https://doc.rust-lang.org/1.48.0/src/core/clone.rs.html#222-227\">Interestingly yes</a>.</li>\n</ul>\n<p>Let's dig in on that <code>Clone</code> implementation a bit. Removing a bit of noise so we can focus on the important bits:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">impl</span><span style=\"color:#657b83;\"><T> Clone </span><span style=\"color:#859900;\">for &</span><span style=\"color:#b58900;\">T </span><span style=\"color:#657b83;\">{\n </span><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">clone</span><span style=\"color:#657b83;\">(</span><span style=\"color:#268bd2;\">self</span><span style=\"color:#657b83;\">: </span><span style=\"color:#859900;\">&&</span><span style=\"color:#657b83;\">T) -> </span><span style=\"color:#859900;\">&</span><span style=\"color:#657b83;\">T {\n *</span><span style=\"color:#d33682;\">self\n </span><span style=\"color:#657b83;\">}\n}\n</span></code></pre>\n<p>Since references are <code>Copy</code>able, derefing a reference to a reference results in copying the inner reference value. What I find fascinating, and slightly concerning, is that we have two orthogonal features in the language:</p>\n<ul>\n<li>Method call syntax automatically causing borrows</li>\n<li>The ability to implement traits for both a type and a reference to that type</li>\n</ul>\n<p>When combined, there's some level of ambiguity about <em>which</em> trait implementation will end up being used.</p>\n<p>In this example, we're fortunate that the code didn't compile. We ended up with nothing more than a confusing error message. I haven't yet run into a real life issue where this behavior can result in code which compiles but does the wrong thing. It's certainly theoretically possible, but seems unlikely to occur unintentionally. That said, if anyone has been bitten by this, I'd be very interested to hear the details.</p>\n<p>So the takeaway: autoborrowing and derefing as part of method call syntax is a great feature of the language. It would be a major pain to use Rust without it. I'm glad it's present. Having traits implemented for references is a great feature, and I wouldn't want to use the language without it.</p>\n<p>But every once in a while, these two things bite us. Caveat emptor.</p>\n",
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"content": "<p>I first started writing Haskell about 15 years ago. My learning curve for the language was haphazard at best. In many cases, I learnt concepts by osmosis, and only later learned the proper terminology and details around them. One of the prime examples of this is pattern matching. Using a <code>case</code> expression in Haskell, or a <code>match</code> expression in Rust, always felt natural. But it took years to realize that patterns appeared in other parts of the languages than just these expressions, and what terms like <em>irrefutable</em> meant.</p>\n<p>It's quite possible most Haskellers and Rustaceans will consider this content obvious. But maybe there are a few others like me out there who never had a chance to realize how ubiquitous patterns are in these languages. This post may also be a fun glimpse into either Haskell or Rust if you're only familiar with one of the languages.</p>\n<h2 id=\"language-references\">Language references</h2>\n<p>Both Haskell and Rust have language references available online. The caveats are that the Rust reference is marked as incomplete, and the Haskell language reference is for Haskell2010, which GHC does not strictly adhere to. That said, both are readily understandable and complete enough to get a very good intuition. If you've never looked at either of these documents, I highly recommend having a peek.</p>\n<ul>\n<li><a href=\"https://www.haskell.org/onlinereport/haskell2010/haskellch3.html#x8-580003.17\">Haskell 2010 Language Report, section 3.17 Pattern Matching</a></li>\n<li><a href=\"https://doc.rust-lang.org/stable/reference/patterns.html#range-patterns\">Rust language reference, Patterns</a></li>\n</ul>\n<h2 id=\"case-and-match\">case and match</h2>\n<p>The first place most of us hear the term "pattern matching" is in Haskell's <code>case</code> expression, or Rust's <code>match</code> expression. And it makes perfect sense here. We can provide multiple <em>patterns</em>, typically based on a data constructor/variant, and the language will match the most appropriate one. Slightly tying in with <a href=\"https://www.fpcomplete.com/blog/error-handling-is-hard/\">my previous post on errors</a>, let's look at a common example: pattern matching on an <code>Either</code> value in Haskell.</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#b58900;\">mightFail </span><span style=\"color:#859900;\">:: </span><span style=\"color:#268bd2;\">Either String Int\n\n</span><span style=\"color:#657b83;\">main </span><span style=\"color:#859900;\">=\n case</span><span style=\"color:#657b83;\"> mightFail </span><span style=\"color:#859900;\">of\n </span><span style=\"color:#cb4b16;\">Left</span><span style=\"color:#657b83;\"> err </span><span style=\"color:#859900;\">-></span><span style=\"color:#657b83;\"> putStrLn </span><span style=\"color:#859900;\">$ </span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">Error occurred: </span><span style=\"color:#839496;\">" </span><span style=\"color:#859900;\">++</span><span style=\"color:#657b83;\"> err\n </span><span style=\"color:#cb4b16;\">Right</span><span style=\"color:#657b83;\"> x </span><span style=\"color:#859900;\">-></span><span style=\"color:#657b83;\"> putStrLn </span><span style=\"color:#859900;\">$ </span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">Successful result: </span><span style=\"color:#839496;\">" </span><span style=\"color:#859900;\">++</span><span style=\"color:#657b83;\"> show x\n</span></code></pre>\n<p>Or a <code>Result</code> value in Rust:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">might_fail</span><span style=\"color:#657b83;\">() -> </span><span style=\"color:#859900;\">Result</span><span style=\"color:#657b83;\"><</span><span style=\"color:#268bd2;\">i32</span><span style=\"color:#657b83;\">, </span><span style=\"color:#859900;\">String</span><span style=\"color:#657b83;\">> { </span><span style=\"color:#859900;\">... </span><span style=\"color:#657b83;\">}\n\n</span><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">main</span><span style=\"color:#657b83;\">() {\n </span><span style=\"color:#859900;\">match might_fail</span><span style=\"color:#657b83;\">() {\n </span><span style=\"color:#859900;\">Err</span><span style=\"color:#657b83;\">(err) </span><span style=\"color:#859900;\">=> println!</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">Error occurred: </span><span style=\"color:#cb4b16;\">{}</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, err),\n </span><span style=\"color:#859900;\">Ok</span><span style=\"color:#657b83;\">(x) </span><span style=\"color:#859900;\">=> println!</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">Successful result: </span><span style=\"color:#cb4b16;\">{}</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, x),\n }\n}\n</span></code></pre>\n<p>I think most programmers, even those unfamiliar with these languages, could intuit to some extent what these expressions do. <code>mightFail</code> and <code>might_fail()</code> return some kind of value. The value may be in multiple different "states." The patterns match, and we branch our behavior depending on which state. Easy enough.</p>\n<p>Already here, though, there's an important detail many of us gloss over. Or at least I did. Our patterns not only <em>match a constructor</em>, they also <em>bind a variable</em>. In the examples above, we bind the variables <code>err</code> and <code>x</code> to values contained by the data constructors. And that's pretty interesting, because both Haskell and Rust <em>also</em> use <code>let</code> bindings for defining variables. I wonder if there's some kind of connection there.</p>\n<p><em>Narrator: there was a connection</em></p>\n<h2 id=\"functions-in-haskell\">Functions in Haskell</h2>\n<p>Haskell immediately adds a curve ball (in a good way) to this story. Let's take a classic recursive definition of a factorial function (note: this isn't a <em>good</em> definition since it has a space leak).</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#b58900;\">fact </span><span style=\"color:#859900;\">:: </span><span style=\"color:#268bd2;\">Int </span><span style=\"color:#859900;\">-> </span><span style=\"color:#268bd2;\">Int\n</span><span style=\"color:#657b83;\">fact i </span><span style=\"color:#859900;\">=\n case</span><span style=\"color:#657b83;\"> i </span><span style=\"color:#859900;\">of\n </span><span style=\"color:#6c71c4;\">0 </span><span style=\"color:#859900;\">-> </span><span style=\"color:#6c71c4;\">1</span><span style=\"color:#657b83;\">\n _ </span><span style=\"color:#859900;\">-></span><span style=\"color:#657b83;\"> i * fact (i </span><span style=\"color:#859900;\">- </span><span style=\"color:#6c71c4;\">1</span><span style=\"color:#657b83;\">)\n</span></code></pre>\n<p>This feels a bit verbose. We capture the variable <code>i</code>, only to immediately pattern match on it. We also have a <em>new</em> kind of pattern, <code>_</code>. When I first learned Haskell, I thought of <code>_</code> as "a variable I don't care about." But it's actually more specialized than this: a wildcard pattern, something which matches anything. (We'll get into what variables match later.)</p>\n<p>Anyway, to make this kind of code a bit terser, Haskell offers a different way of writing this function:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#b58900;\">fact </span><span style=\"color:#859900;\">:: </span><span style=\"color:#268bd2;\">Int </span><span style=\"color:#859900;\">-> </span><span style=\"color:#268bd2;\">Int\n</span><span style=\"color:#657b83;\">fact </span><span style=\"color:#6c71c4;\">0 </span><span style=\"color:#859900;\">= </span><span style=\"color:#6c71c4;\">1</span><span style=\"color:#657b83;\">\nfact i </span><span style=\"color:#859900;\">=</span><span style=\"color:#657b83;\"> i * fact (i </span><span style=\"color:#859900;\">- </span><span style=\"color:#6c71c4;\">1</span><span style=\"color:#657b83;\">)\n</span></code></pre>\n<p>These two versions of the code are identical. It's just a syntactic trick. Let's see another more interesting syntactic trick.</p>\n<h2 id=\"what-about-let\">What about let?</h2>\n<p>We use <code>let</code> expressions (and <code>let</code> bindings in <code>do</code>-notation) in Haskell to create new variables, e.g.:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#657b83;\">main </span><span style=\"color:#859900;\">=\n let</span><span style=\"color:#657b83;\"> name </span><span style=\"color:#859900;\">= </span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">Alice</span><span style=\"color:#839496;\">"\n </span><span style=\"color:#859900;\">in</span><span style=\"color:#657b83;\"> putStrLn </span><span style=\"color:#859900;\">$ </span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">Hello, </span><span style=\"color:#839496;\">" </span><span style=\"color:#859900;\">++</span><span style=\"color:#657b83;\"> name\n</span></code></pre>\n<p>And we do the same with <code>let</code> statements in Rust:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">main</span><span style=\"color:#657b83;\">() {\n </span><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> name = </span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">Alice</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">;\n </span><span style=\"color:#859900;\">println!</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">Hello, </span><span style=\"color:#cb4b16;\">{}</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, name);\n}\n</span></code></pre>\n<p>But here's where we begin to get a bit fancy. We already saw that we can bind variables in <code>case</code> and <code>match</code> expressions. Does that mean we can do away with the <code>let</code>s? Yes we can!</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#657b83;\">main </span><span style=\"color:#859900;\">=\n case </span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">Alice</span><span style=\"color:#839496;\">" </span><span style=\"color:#859900;\">of</span><span style=\"color:#657b83;\">\n name </span><span style=\"color:#859900;\">-></span><span style=\"color:#657b83;\"> putStrLn </span><span style=\"color:#859900;\">$ </span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">Hello, </span><span style=\"color:#839496;\">" </span><span style=\"color:#859900;\">++</span><span style=\"color:#657b83;\"> name\n</span></code></pre>\n<p>And</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">main</span><span style=\"color:#657b83;\">() {\n </span><span style=\"color:#859900;\">match </span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">Alice</span><span style=\"color:#839496;\">" </span><span style=\"color:#657b83;\">{\n name </span><span style=\"color:#859900;\">=> println!</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">Hello, </span><span style=\"color:#cb4b16;\">{}</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, name)\n }\n}\n</span></code></pre>\n<p>This isn't <em>good</em> code per se. In fact, <code>cargo clippy</code> will complain about it. But it does hint at the fact that there's a deeper connection between two constructs. And the connection is this: the left hand side of the equals sign in a <code>let</code> statement/expression/binding is a <em>pattern</em>.</p>\n<h2 id=\"ditch-the-case-ditch-the-match\">Ditch the case! Ditch the match!</h2>\n<p>Alright, so we can technically get rid of <code>let</code>s if we wanted to (which we don't). Can we get rid of the <code>case</code> expressions in Haskell? The real answer is "definitely not." But interestingly, this code compiles!</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#b58900;\">mightFail </span><span style=\"color:#859900;\">:: </span><span style=\"color:#268bd2;\">Either String Int\n</span><span style=\"color:#657b83;\">mightFail </span><span style=\"color:#859900;\">= </span><span style=\"color:#cb4b16;\">Left </span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">It failed</span><span style=\"color:#839496;\">"\n\n</span><span style=\"color:#b58900;\">main </span><span style=\"color:#859900;\">:: </span><span style=\"color:#268bd2;\">IO </span><span style=\"color:#859900;\">()\n</span><span style=\"color:#657b83;\">main </span><span style=\"color:#859900;\">=\n let </span><span style=\"color:#cb4b16;\">Right</span><span style=\"color:#657b83;\"> x </span><span style=\"color:#859900;\">=</span><span style=\"color:#657b83;\"> mightFail\n </span><span style=\"color:#859900;\">in</span><span style=\"color:#657b83;\"> putStrLn </span><span style=\"color:#859900;\">$ </span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">Successful result: </span><span style=\"color:#839496;\">" </span><span style=\"color:#859900;\">++</span><span style=\"color:#657b83;\"> show x\n</span></code></pre>\n<p>As mentioned, we can put a pattern on the left hand side of the equals sign. And we've done just that here. But what on Earth does this code <em>do</em>? As you can see, the <code>mightFail</code> expression will evaluate to a <code>Left</code> value. But our pattern only matches on <code>Right</code> values! Running this code gives us:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#657b83;\">Main.hs:10:9-27: Non-exhaustive patterns in Right x\n</span></code></pre>\n<p>Haskell is a non-strict language. Performing this binding is allowed. But evaluating the result of this binding blows up.</p>\n<p>Rust, however, <strong>is</strong> a strict language. We can do something very similar in Rust:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">main</span><span style=\"color:#657b83;\">() {\n </span><span style=\"color:#268bd2;\">let </span><span style=\"color:#859900;\">Ok</span><span style=\"color:#657b83;\">(x) = </span><span style=\"color:#859900;\">might_fail</span><span style=\"color:#657b83;\">();\n </span><span style=\"color:#859900;\">println!</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">Successful result: </span><span style=\"color:#cb4b16;\">{}</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, x);\n}\n</span></code></pre>\n<p>But this code won't even compile:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#657b83;\">error[E0005]: refutable pattern in local binding: `Err(_)` not covered\n...\n = note: `let` bindings require an "irrefutable pattern", like a `struct` or an `enum` with only one variant\n = note: for more information, visit https://doc.rust-lang.org/book/ch18-02-refutability.html\n = note: the matched value is of type `std::result::Result<i32, std::string::String>`\nhelp: you might want to use `if let` to ignore the variant that isn't matched\n</span></code></pre>\n<p>Let's dive into those "exhaustive" and "refutable" concepts, and then round out this post with a glance at where else patterns appear in these languages.</p>\n<p>Side note: it's true that the Haskell code above compiles. However, if you turn on the <a href=\"https://ghc.gitlab.haskell.org/ghc/doc/users_guide/using-warnings.html#ghc-flag--Wincomplete-uni-patterns\"><code>-Wincomplete-uni-patterns</code> warning</a>, you'll get a warning about this. I personally think this warning should be included in <code>-Wall</code>.</p>\n<h2 id=\"refutable-and-irrefutable-exhaustive-and-non-exhaustive\">Refutable and irrefutable, exhaustive and non-exhaustive</h2>\n<p>This topic is quite a bit more complicated in Haskell due to non-strictness. How matching works in the presence of "bottom" or undefined values is an entire extra wrench of complication. I'm going to ignore those cases entirely here. If you're interested in more information on this, my article <a href=\"https://www.fpcomplete.com/haskell/tutorial/all-about-strictness/\">All about strictness</a> discusses some of these points.</p>\n<p>Some patterns will <em>always</em> match a value. The simplest example of this is a wildcard. In fact, that's basically its definition. Quoting the Rust reference:</p>\n<blockquote>\n<p>The <em>wildcard pattern</em> (an underscore symbol) matches any value.</p>\n</blockquote>\n<p>And fortunately for us, things behave exactly the same way in Haskell.</p>\n<p>Another pattern that matches any value given is variable. <code>let x = blah</code> is a valid binding, regardless of what <code>blah</code> is. Both of these are known as <em>irrefutable</em> patterns.</p>\n<p>By contrast, some patterns are refutable. They are patterns that only match some possible cases of the value, not all. The simplest example is the one we saw before: matching on one of many data constructors/variants in a data type (Haskell) or enum (Rust).</p>\n<p>Contrasting yet again: if you have a <code>struct</code> in Rust, or a Rust <code>enum</code> with only variant, or a Haskell <code>data</code> with only one data constructor, or a Haskell <code>newtype</code>, the pattern will always match. That is, of course, assuming any patterns nested within will <em>also</em> always match. To demonstrate, this pattern match is irrefutable:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#859900;\">data </span><span style=\"color:#cb4b16;\">Foo </span><span style=\"color:#859900;\">= </span><span style=\"color:#cb4b16;\">Foo Bar\n</span><span style=\"color:#859900;\">data </span><span style=\"color:#cb4b16;\">Bar </span><span style=\"color:#859900;\">= </span><span style=\"color:#cb4b16;\">Baz Int\n\n</span><span style=\"color:#b58900;\">main </span><span style=\"color:#859900;\">:: </span><span style=\"color:#268bd2;\">IO </span><span style=\"color:#859900;\">()\n</span><span style=\"color:#657b83;\">main </span><span style=\"color:#859900;\">=\n let </span><span style=\"color:#cb4b16;\">Foo</span><span style=\"color:#657b83;\"> (</span><span style=\"color:#cb4b16;\">Baz</span><span style=\"color:#657b83;\"> x) </span><span style=\"color:#859900;\">= </span><span style=\"color:#cb4b16;\">Foo</span><span style=\"color:#657b83;\"> (</span><span style=\"color:#cb4b16;\">Baz </span><span style=\"color:#6c71c4;\">5</span><span style=\"color:#657b83;\">)\n </span><span style=\"color:#859900;\">in</span><span style=\"color:#657b83;\"> putStrLn </span><span style=\"color:#859900;\">$ </span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">x == </span><span style=\"color:#839496;\">" </span><span style=\"color:#859900;\">++</span><span style=\"color:#657b83;\"> show x\n</span></code></pre>\n<p>However, if I add another data constructor to <code>Bar</code>, it becomes refutable:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#859900;\">data </span><span style=\"color:#cb4b16;\">Foo </span><span style=\"color:#859900;\">= </span><span style=\"color:#cb4b16;\">Foo Bar\n</span><span style=\"color:#859900;\">data </span><span style=\"color:#cb4b16;\">Bar </span><span style=\"color:#859900;\">= </span><span style=\"color:#cb4b16;\">Baz Int </span><span style=\"color:#859900;\">| </span><span style=\"color:#cb4b16;\">Bin Char\n\n</span><span style=\"color:#b58900;\">main </span><span style=\"color:#859900;\">:: </span><span style=\"color:#268bd2;\">IO </span><span style=\"color:#859900;\">()\n</span><span style=\"color:#657b83;\">main </span><span style=\"color:#859900;\">=\n let </span><span style=\"color:#cb4b16;\">Foo</span><span style=\"color:#657b83;\"> (</span><span style=\"color:#cb4b16;\">Baz</span><span style=\"color:#657b83;\"> x) </span><span style=\"color:#859900;\">= </span><span style=\"color:#cb4b16;\">Foo</span><span style=\"color:#657b83;\"> (</span><span style=\"color:#cb4b16;\">Bin </span><span style=\"color:#839496;\">'</span><span style=\"color:#2aa198;\">c</span><span style=\"color:#839496;\">'</span><span style=\"color:#657b83;\">)\n </span><span style=\"color:#859900;\">in</span><span style=\"color:#657b83;\"> putStrLn </span><span style=\"color:#859900;\">$ </span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">x == </span><span style=\"color:#839496;\">" </span><span style=\"color:#859900;\">++</span><span style=\"color:#657b83;\"> show x\n</span></code></pre>\n<p>In both Haskell and Rust, tuples behave like data types with one constructor, and therefore as long as the patterns inside of them are irrefutable, they are irrefutable too.</p>\n<p>The final case I want to point out is <em>literal patterns</em>. Literal patterns are very much refutable. This code thankfully does not compile:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">main</span><span style=\"color:#657b83;\">() {\n </span><span style=\"color:#268bd2;\">let </span><span style=\"color:#839496;\">'</span><span style=\"color:#2aa198;\">x</span><span style=\"color:#839496;\">' </span><span style=\"color:#657b83;\">= </span><span style=\"color:#839496;\">'</span><span style=\"color:#2aa198;\">a</span><span style=\"color:#839496;\">'</span><span style=\"color:#657b83;\">;\n}\n</span></code></pre>\n<p>But the really interesting thing for someone not used to pattern matching is that you can do this at all! We've already done pattern matching on literal values above, in our definition of <code>fact</code>. It's very convenient to be able to build up complex case/match expressions using literal syntax (like list/slice syntax).</p>\n<p>Alright, let's see a few more examples of where patterns are used in these languages, then tie it up.</p>\n<h2 id=\"function-arguments\">Function arguments</h2>\n<p>Function arguments are patterns in both languages. In Haskell we saw that you can use <em>refutable</em> patterns, and provide multiple function clauses. The same doesn't apply to Rust functions. You'll need to use an irrefutable pattern in the function, and then do some pattern matching or other kind of branching in the body of the functions. For example, the poorly written <code>fact</code> function can be rewritten in Rust as:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">fact</span><span style=\"color:#657b83;\">(</span><span style=\"color:#268bd2;\">i</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">u32</span><span style=\"color:#657b83;\">) -> </span><span style=\"color:#268bd2;\">u32 </span><span style=\"color:#657b83;\">{\n </span><span style=\"color:#859900;\">if</span><span style=\"color:#657b83;\"> i == </span><span style=\"color:#6c71c4;\">0 </span><span style=\"color:#657b83;\">{\n </span><span style=\"color:#6c71c4;\">1\n </span><span style=\"color:#657b83;\">} </span><span style=\"color:#859900;\">else </span><span style=\"color:#657b83;\">{\n i * </span><span style=\"color:#859900;\">fact</span><span style=\"color:#657b83;\">(i - </span><span style=\"color:#6c71c4;\">1</span><span style=\"color:#657b83;\">)\n }\n}\n\n</span><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">main</span><span style=\"color:#657b83;\">() {\n </span><span style=\"color:#859900;\">println!</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">5! == </span><span style=\"color:#cb4b16;\">{}</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, </span><span style=\"color:#859900;\">fact</span><span style=\"color:#657b83;\">(</span><span style=\"color:#6c71c4;\">5</span><span style=\"color:#657b83;\">));\n}\n</span></code></pre>\n<p>Perhaps more interestingly in both languages, you can use a pattern matching a data structure in the function argument. For example, in Rust:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">struct </span><span style=\"color:#b58900;\">Person </span><span style=\"color:#657b83;\">{\n </span><span style=\"color:#268bd2;\">name</span><span style=\"color:#657b83;\">: String,\n </span><span style=\"color:#268bd2;\">age</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">u32</span><span style=\"color:#657b83;\">,\n}\n\n</span><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">greet</span><span style=\"color:#657b83;\">(Person { name, age }: </span><span style=\"color:#859900;\">&</span><span style=\"color:#657b83;\">Person) {\n </span><span style=\"color:#859900;\">println!</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#cb4b16;\">{}</span><span style=\"color:#2aa198;\"> is </span><span style=\"color:#cb4b16;\">{}</span><span style=\"color:#2aa198;\"> years old</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, name, age);\n}\n\n</span><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">main</span><span style=\"color:#657b83;\">() {\n </span><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> alice = Person {\n name: </span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">Alice</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">.</span><span style=\"color:#859900;\">to_owned</span><span style=\"color:#657b83;\">(),\n age: </span><span style=\"color:#6c71c4;\">30</span><span style=\"color:#657b83;\">,\n };\n </span><span style=\"color:#859900;\">greet</span><span style=\"color:#657b83;\">(</span><span style=\"color:#859900;\">&</span><span style=\"color:#657b83;\">alice);\n}\n</span></code></pre>\n<p>Or in Haskell, using positional instead of named fields:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#859900;\">data </span><span style=\"color:#cb4b16;\">Person </span><span style=\"color:#859900;\">= </span><span style=\"color:#cb4b16;\">Person String Int\n\n</span><span style=\"color:#b58900;\">greet </span><span style=\"color:#859900;\">:: </span><span style=\"color:#268bd2;\">Person </span><span style=\"color:#859900;\">-> </span><span style=\"color:#268bd2;\">IO </span><span style=\"color:#859900;\">()\n</span><span style=\"color:#657b83;\">greet (</span><span style=\"color:#cb4b16;\">Person</span><span style=\"color:#657b83;\"> name age) </span><span style=\"color:#859900;\">=</span><span style=\"color:#657b83;\"> putStrLn </span><span style=\"color:#859900;\">$</span><span style=\"color:#657b83;\"> name </span><span style=\"color:#859900;\">++ </span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\"> is </span><span style=\"color:#839496;\">" </span><span style=\"color:#859900;\">++</span><span style=\"color:#657b83;\"> show age </span><span style=\"color:#859900;\">++ </span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\"> years old</span><span style=\"color:#839496;\">"\n\n</span><span style=\"color:#b58900;\">main </span><span style=\"color:#859900;\">:: </span><span style=\"color:#268bd2;\">IO </span><span style=\"color:#859900;\">()\n</span><span style=\"color:#657b83;\">main </span><span style=\"color:#859900;\">=</span><span style=\"color:#657b83;\"> greet </span><span style=\"color:#859900;\">$ </span><span style=\"color:#cb4b16;\">Person </span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">Alice</span><span style=\"color:#839496;\">" </span><span style=\"color:#6c71c4;\">30\n</span></code></pre><h2 id=\"closures-functions-and-lambdas\">Closures, functions, and lambdas</h2>\n<p>The arguments to closures (Rust) and lambdas (Haskell) are patterns. That means we can match on irrefutable things like tuples fairly easily:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">main</span><span style=\"color:#657b83;\">() {\n </span><span style=\"color:#268bd2;\">let </span><span style=\"color:#b58900;\">greet </span><span style=\"color:#859900;\">= </span><span style=\"color:#657b83;\">|(</span><span style=\"color:#268bd2;\">name</span><span style=\"color:#657b83;\">, </span><span style=\"color:#268bd2;\">age</span><span style=\"color:#657b83;\">)| </span><span style=\"color:#859900;\">println!</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#cb4b16;\">{}</span><span style=\"color:#2aa198;\"> is </span><span style=\"color:#cb4b16;\">{}</span><span style=\"color:#2aa198;\"> years old</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, name, age);\n </span><span style=\"color:#859900;\">greet</span><span style=\"color:#657b83;\">((</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">Alice</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">30</span><span style=\"color:#657b83;\">));\n}\n</span></code></pre>\n<p>The big difference is that, in Rust, the pattern must be irrefutable. This is again due to strictness. The following code will compile in Haskell, but fail at runtime:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#b58900;\">main </span><span style=\"color:#859900;\">:: </span><span style=\"color:#268bd2;\">IO </span><span style=\"color:#859900;\">()\n</span><span style=\"color:#657b83;\">main </span><span style=\"color:#859900;\">=\n let</span><span style=\"color:#657b83;\"> mylambda </span><span style=\"color:#859900;\">= \\</span><span style=\"color:#657b83;\">(</span><span style=\"color:#cb4b16;\">Right</span><span style=\"color:#657b83;\"> x) </span><span style=\"color:#859900;\">-></span><span style=\"color:#657b83;\"> putStrLn x\n </span><span style=\"color:#859900;\">in</span><span style=\"color:#657b83;\"> mylambda (</span><span style=\"color:#cb4b16;\">Left </span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">Error!</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">)\n</span></code></pre>\n<p>Again, <code>-Wincomplete-uni-patterns</code> will warn about this. But again, it's not on by default.</p>\n<p>By contrast, in Rust, the equivalent code will fail to compile:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">main</span><span style=\"color:#657b83;\">() {\n </span><span style=\"color:#268bd2;\">let </span><span style=\"color:#b58900;\">myclosure </span><span style=\"color:#859900;\">= </span><span style=\"color:#657b83;\">|Ok(</span><span style=\"color:#268bd2;\">x</span><span style=\"color:#657b83;\">): Result<</span><span style=\"color:#268bd2;\">i32</span><span style=\"color:#657b83;\">, </span><span style=\"color:#859900;\">&</span><span style=\"color:#657b83;\">str>| </span><span style=\"color:#859900;\">println!</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#cb4b16;\">{}</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, x);\n </span><span style=\"color:#859900;\">myclosure</span><span style=\"color:#657b83;\">(</span><span style=\"color:#859900;\">Err</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">Hello</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">));\n}\n</span></code></pre>\n<p>This produces:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#657b83;\">error[E0005]: refutable pattern in function argument: `Err(_)` not covered\n</span></code></pre>\n<p>And if you're wondering: I needed to add the explicit <code>: Result<i32, &str></code> type annotation to help type inference get to that error message. Without it, it just complained that it couldn't infer the type of <code>x</code>.</p>\n<h2 id=\"if-let-while-let-and-for-rust\">if let, while let, and for (Rust)</h2>\n<p>The <code>if let</code> and <code>while let</code> expressions are all about refutable pattern matches. "Only do this if the pattern matches" and "keep doing this while the pattern matches." <code>if let</code> looks something like this:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">main</span><span style=\"color:#657b83;\">() {\n </span><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> result: </span><span style=\"color:#859900;\">Result</span><span style=\"color:#657b83;\"><(), </span><span style=\"color:#859900;\">String</span><span style=\"color:#657b83;\">> = </span><span style=\"color:#859900;\">Err</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">Something happened</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">.</span><span style=\"color:#859900;\">to_owned</span><span style=\"color:#657b83;\">());\n </span><span style=\"color:#859900;\">if </span><span style=\"color:#268bd2;\">let </span><span style=\"color:#859900;\">Err</span><span style=\"color:#657b83;\">(e) = result {\n </span><span style=\"color:#859900;\">eprintln!</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">Something went wrong: </span><span style=\"color:#cb4b16;\">{}</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, e);\n }\n}\n</span></code></pre>\n<p>And with <code>while let</code>, you can make something close to a <code>for</code> loop:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">main</span><span style=\"color:#657b83;\">() {\n </span><span style=\"color:#268bd2;\">let </span><span style=\"color:#586e75;\">mut</span><span style=\"color:#657b83;\"> iter = </span><span style=\"color:#6c71c4;\">1</span><span style=\"color:#859900;\">..</span><span style=\"color:#657b83;\">=</span><span style=\"color:#6c71c4;\">10</span><span style=\"color:#657b83;\">;\n </span><span style=\"color:#859900;\">while </span><span style=\"color:#268bd2;\">let </span><span style=\"color:#859900;\">Some</span><span style=\"color:#657b83;\">(i) = iter.</span><span style=\"color:#859900;\">next</span><span style=\"color:#657b83;\">() {\n </span><span style=\"color:#859900;\">println!</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">i == </span><span style=\"color:#cb4b16;\">{}</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, i);\n }\n}\n</span></code></pre>\n<p>And speaking of <code>for</code> loops, the left hand side of the <code>in</code> keyword is a pattern. This can be really nice for cases like destructuring the tuple generated by the <code>enumerate()</code> method:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">main</span><span style=\"color:#657b83;\">() {\n </span><span style=\"color:#859900;\">for </span><span style=\"color:#657b83;\">(idx, c) </span><span style=\"color:#859900;\">in </span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">Hello, world!</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">.</span><span style=\"color:#859900;\">chars</span><span style=\"color:#657b83;\">().</span><span style=\"color:#859900;\">enumerate</span><span style=\"color:#657b83;\">() {\n </span><span style=\"color:#859900;\">println!</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#cb4b16;\">{}</span><span style=\"color:#2aa198;\">: </span><span style=\"color:#cb4b16;\">{}</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, idx, c);\n }\n}\n</span></code></pre>\n<p>The patterns in a <code>for</code> loop must be irrefutable. This code won't compile:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">main</span><span style=\"color:#657b83;\">() {\n </span><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> array = [</span><span style=\"color:#859900;\">Ok</span><span style=\"color:#657b83;\">(</span><span style=\"color:#6c71c4;\">1</span><span style=\"color:#657b83;\">), </span><span style=\"color:#859900;\">Ok</span><span style=\"color:#657b83;\">(</span><span style=\"color:#6c71c4;\">2</span><span style=\"color:#657b83;\">), </span><span style=\"color:#859900;\">Err</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">something</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">), </span><span style=\"color:#859900;\">Ok</span><span style=\"color:#657b83;\">(</span><span style=\"color:#6c71c4;\">3</span><span style=\"color:#657b83;\">)];\n </span><span style=\"color:#859900;\">for Ok</span><span style=\"color:#657b83;\">(x) </span><span style=\"color:#859900;\">in &</span><span style=\"color:#657b83;\">array {\n </span><span style=\"color:#859900;\">println!</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">x == </span><span style=\"color:#cb4b16;\">{}</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, x);\n }\n}\n</span></code></pre>\n<p>Instead, if you want to exit the <code>for</code> loop at the first <code>Err</code> value, you would need to do something like this:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">main</span><span style=\"color:#657b83;\">() {\n </span><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> array = [</span><span style=\"color:#859900;\">Ok</span><span style=\"color:#657b83;\">(</span><span style=\"color:#6c71c4;\">1</span><span style=\"color:#657b83;\">), </span><span style=\"color:#859900;\">Ok</span><span style=\"color:#657b83;\">(</span><span style=\"color:#6c71c4;\">2</span><span style=\"color:#657b83;\">), </span><span style=\"color:#859900;\">Err</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">something</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">), </span><span style=\"color:#859900;\">Ok</span><span style=\"color:#657b83;\">(</span><span style=\"color:#6c71c4;\">3</span><span style=\"color:#657b83;\">)];\n </span><span style=\"color:#859900;\">for</span><span style=\"color:#657b83;\"> x </span><span style=\"color:#859900;\">in &</span><span style=\"color:#657b83;\">array {\n </span><span style=\"color:#859900;\">match</span><span style=\"color:#657b83;\"> x {\n </span><span style=\"color:#859900;\">Ok</span><span style=\"color:#657b83;\">(x) </span><span style=\"color:#859900;\">=> println!</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">x == </span><span style=\"color:#cb4b16;\">{}</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, x),\n </span><span style=\"color:#859900;\">Err</span><span style=\"color:#657b83;\">(</span><span style=\"color:#859900;\">_</span><span style=\"color:#657b83;\">) </span><span style=\"color:#859900;\">=> break</span><span style=\"color:#657b83;\">,\n }\n }\n}\n</span></code></pre><h2 id=\"where-they-re-used\">Where they're used</h2>\n<p>This was not intended to be a complete explanation of all examples of patterns in these languages. However, for a bit of completeness, let me quote the Haskell language specification for where patterns are part of the language:</p>\n<blockquote>\n<p>Patterns appear in lambda abstractions, function definitions, pattern bindings, list comprehensions, do expressions, and case expressions. However, the first five of these ultimately translate into case expressions, so defining the semantics of pattern matching for case expressions is sufficient.</p>\n</blockquote>\n<p>And similarly for Rust:</p>\n<blockquote>\n<ul>\n<li>let declarations</li>\n<li>Function and closure parameters</li>\n<li>match expressions</li>\n<li>if let expressions</li>\n<li>while let expressions</li>\n<li>for expressions</li>\n</ul>\n</blockquote>\n<p>There are also more advanced examples of patterns that I haven't touched on at all. Reference patterns in Rust would be relevant here, as would lazy patterns in Haskell.</p>\n<h2 id=\"conclusion\">Conclusion</h2>\n<p>I hoped this gave a little bit of insight into the value of patterns. For me, the important takeaway is:</p>\n<ul>\n<li>Patterns appear in lots of places</li>\n<li>The difference between refutable and irrefutable patterns</li>\n<li>There are some places where you must use irrefutable patterns</li>\n<li>There are some places where Haskell lets you use refutable patterns, but you shouldn't</li>\n<li>Variable binding is just one special case of patterns</li>\n</ul>\n<p>If you're interested in learning more about either Haskell or Rust, check out our <a href=\"https://www.fpcomplete.com/haskell/syllabus/\">Haskell syllabus</a> or our <a href=\"https://www.fpcomplete.com/rust/crash-course/\">Rust Crash Course</a>. FP Complete also offers both corporate and public training classes on both Haskell and Rust. If you're interested in learning more, please <a href=\"https://www.fpcomplete.com/contact-us/\">contact us for details</a>.</p>\n<div class=\"text-center\">\n<a class=\"button-coral\" href=\"/training/\">Learn about Rust training</a>\n</div>\n",
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"content": "<p>This blog post was entirely inspired by reading the <a href=\"https://www.reddit.com/r/rust/comments/k4vzvp/gats_on_nightly/\">GATs on Nightly!</a> Reddit post by /u/C5H5N5O. I just decided to take things a little bit too far, and thought a blog post on it would be fun. I want to be clear from the start: I'm introducing some advanced concepts in this post that rely on unstable features in Rust. I'm not advocating their usage <em>at all</em>. I'm just exploring what may and may not be possible with GATs.</p>\n<p>Rust shares many similarities with Haskell at the type system level. Both have types, generic types, associated types, and traits/type classes (which are basically equivalent). However, Haskell has one important additional feature that is lacking in Rust: Higher Kinded Types (HKTs). This isn't an accidental limitation in Rust, or some gap that should be filled in. It's an intentional design decision, at least as far as I know. But as a result, some things until now can't really be implemented in Rust.</p>\n<p>Take, for instance, a <code>Functor</code> in Haskell. For all of its scary sounding name, almost all developers today are familiar with the concept of a <code>Functor</code>. A <code>Functor</code> provides a general purpose interface for "map a function over this structure." Many different structures in Rust can provide such mapping functionality, including <code>Option</code>, <code>Result</code>, <code>Iterator</code>, and <code>Future</code>.</p>\n<p>However, it hasn't been possible to write a general purpose <code>Functor</code> trait that can be implemented by multiple types. Instead, individual types can implement them as methods on that type. For example, we can write our own custom <code>MyOption</code> and <code>MyResult</code> enums and provide <code>map</code> methods:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#657b83;\">#[</span><span style=\"color:#268bd2;\">derive</span><span style=\"color:#657b83;\">(Debug, PartialEq)]\n</span><span style=\"color:#268bd2;\">enum </span><span style=\"color:#b58900;\">MyOption</span><span style=\"color:#657b83;\"><A> {\n </span><span style=\"color:#859900;\">Some</span><span style=\"color:#657b83;\">(A),\n </span><span style=\"color:#859900;\">None</span><span style=\"color:#657b83;\">,\n}\n\n</span><span style=\"color:#268bd2;\">impl</span><span style=\"color:#657b83;\"><A> </span><span style=\"color:#b58900;\">MyOption</span><span style=\"color:#657b83;\"><A> {\n </span><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">map</span><span style=\"color:#657b83;\"><F: </span><span style=\"color:#859900;\">FnOnce</span><span style=\"color:#657b83;\">(A) -> B, B>(</span><span style=\"color:#268bd2;\">self</span><span style=\"color:#657b83;\">, </span><span style=\"color:#268bd2;\">f</span><span style=\"color:#657b83;\">: F) -> MyOption<B> {\n </span><span style=\"color:#859900;\">match </span><span style=\"color:#d33682;\">self </span><span style=\"color:#657b83;\">{\n MyOption::Some(a) </span><span style=\"color:#859900;\">=> </span><span style=\"color:#657b83;\">MyOption::Some(</span><span style=\"color:#859900;\">f</span><span style=\"color:#657b83;\">(a)),\n MyOption::None </span><span style=\"color:#859900;\">=> </span><span style=\"color:#657b83;\">MyOption::None,\n }\n }\n}\n\n#[</span><span style=\"color:#268bd2;\">test</span><span style=\"color:#657b83;\">]\n</span><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">test_option_map</span><span style=\"color:#657b83;\">() {\n </span><span style=\"color:#859900;\">assert_eq!</span><span style=\"color:#657b83;\">(MyOption::Some(</span><span style=\"color:#6c71c4;\">5</span><span style=\"color:#657b83;\">).</span><span style=\"color:#859900;\">map</span><span style=\"color:#657b83;\">(|</span><span style=\"color:#268bd2;\">x</span><span style=\"color:#657b83;\">| x + </span><span style=\"color:#6c71c4;\">1</span><span style=\"color:#657b83;\">), MyOption::Some(</span><span style=\"color:#6c71c4;\">6</span><span style=\"color:#657b83;\">));\n </span><span style=\"color:#859900;\">assert_eq!</span><span style=\"color:#657b83;\">(MyOption::None.</span><span style=\"color:#859900;\">map</span><span style=\"color:#657b83;\">(|</span><span style=\"color:#268bd2;\">x</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">i32</span><span style=\"color:#657b83;\">| x + </span><span style=\"color:#6c71c4;\">1</span><span style=\"color:#657b83;\">), MyOption::None);\n}\n\n#[</span><span style=\"color:#268bd2;\">derive</span><span style=\"color:#657b83;\">(Debug, PartialEq)]\n</span><span style=\"color:#268bd2;\">enum </span><span style=\"color:#b58900;\">MyResult</span><span style=\"color:#657b83;\"><A, E> {\n </span><span style=\"color:#859900;\">Ok</span><span style=\"color:#657b83;\">(A),\n </span><span style=\"color:#859900;\">Err</span><span style=\"color:#657b83;\">(E),\n}\n\n</span><span style=\"color:#268bd2;\">impl</span><span style=\"color:#657b83;\"><A, E> </span><span style=\"color:#b58900;\">MyResult</span><span style=\"color:#657b83;\"><A, E> {\n </span><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">map</span><span style=\"color:#657b83;\"><F: </span><span style=\"color:#859900;\">FnOnce</span><span style=\"color:#657b83;\">(A) -> B, B>(</span><span style=\"color:#268bd2;\">self</span><span style=\"color:#657b83;\">, </span><span style=\"color:#268bd2;\">f</span><span style=\"color:#657b83;\">: F) -> MyResult<B, E> {\n </span><span style=\"color:#859900;\">match </span><span style=\"color:#d33682;\">self </span><span style=\"color:#657b83;\">{\n MyResult::Ok(a) </span><span style=\"color:#859900;\">=> </span><span style=\"color:#657b83;\">MyResult::Ok(</span><span style=\"color:#859900;\">f</span><span style=\"color:#657b83;\">(a)),\n MyResult::Err(e) </span><span style=\"color:#859900;\">=> </span><span style=\"color:#657b83;\">MyResult::Err(e),\n }\n }\n}\n\n#[</span><span style=\"color:#268bd2;\">test</span><span style=\"color:#657b83;\">]\n</span><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">test_result_map</span><span style=\"color:#657b83;\">() {\n </span><span style=\"color:#859900;\">assert_eq!</span><span style=\"color:#657b83;\">(MyResult::Ok(</span><span style=\"color:#6c71c4;\">5</span><span style=\"color:#657b83;\">).</span><span style=\"color:#859900;\">map</span><span style=\"color:#657b83;\">(|</span><span style=\"color:#268bd2;\">x</span><span style=\"color:#657b83;\">| x + </span><span style=\"color:#6c71c4;\">1</span><span style=\"color:#657b83;\">), MyResult::Ok::<</span><span style=\"color:#268bd2;\">i32</span><span style=\"color:#657b83;\">, ()>(</span><span style=\"color:#6c71c4;\">6</span><span style=\"color:#657b83;\">));\n </span><span style=\"color:#859900;\">assert_eq!</span><span style=\"color:#657b83;\">(MyResult::Err(</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">hello</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">).</span><span style=\"color:#859900;\">map</span><span style=\"color:#657b83;\">(|</span><span style=\"color:#268bd2;\">x</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">i32</span><span style=\"color:#657b83;\">| x + </span><span style=\"color:#6c71c4;\">1</span><span style=\"color:#657b83;\">), MyResult::Err(</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">hello</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">));\n}\n</span></code></pre>\n<p>However, it hasn't been possible without GATs to define <code>map</code> as a trait method. Let's see why. Here's a naive approach to a "monomorphic functor" trait, and an implementation for <code>Option</code>:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#93a1a1;\">/// Monomorphic functor trait\n</span><span style=\"color:#268bd2;\">trait </span><span style=\"color:#b58900;\">MonoFunctor </span><span style=\"color:#657b83;\">{\n </span><span style=\"color:#268bd2;\">type </span><span style=\"color:#b58900;\">Unwrapped</span><span style=\"color:#657b83;\">; </span><span style=\"color:#93a1a1;\">// value "contained inside"\n </span><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">map</span><span style=\"color:#657b83;\"><F>(</span><span style=\"color:#268bd2;\">self</span><span style=\"color:#657b83;\">, </span><span style=\"color:#268bd2;\">f</span><span style=\"color:#657b83;\">: F) -> </span><span style=\"color:#268bd2;\">Self\n </span><span style=\"color:#859900;\">where</span><span style=\"color:#657b83;\">\n F: FnMut(</span><span style=\"color:#268bd2;\">Self::</span><span style=\"color:#657b83;\">Unwrapped) -> </span><span style=\"color:#268bd2;\">Self::</span><span style=\"color:#657b83;\">Unwrapped;\n}\n\nimpl<A> MonoFunctor </span><span style=\"color:#859900;\">for Option</span><span style=\"color:#657b83;\"><A> {\n </span><span style=\"color:#268bd2;\">type </span><span style=\"color:#b58900;\">Unwrapped </span><span style=\"color:#657b83;\">= A;\n </span><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">map</span><span style=\"color:#657b83;\"><F: </span><span style=\"color:#859900;\">FnMut</span><span style=\"color:#657b83;\">(A) -> A>(</span><span style=\"color:#268bd2;\">self</span><span style=\"color:#657b83;\">, </span><span style=\"color:#586e75;\">mut </span><span style=\"color:#268bd2;\">f</span><span style=\"color:#657b83;\">: F) -> </span><span style=\"color:#859900;\">Option</span><span style=\"color:#657b83;\"><A> {\n </span><span style=\"color:#859900;\">match </span><span style=\"color:#d33682;\">self </span><span style=\"color:#657b83;\">{\n </span><span style=\"color:#859900;\">Some</span><span style=\"color:#657b83;\">(a) </span><span style=\"color:#859900;\">=> Some</span><span style=\"color:#657b83;\">(</span><span style=\"color:#859900;\">f</span><span style=\"color:#657b83;\">(a)),\n </span><span style=\"color:#859900;\">None => None</span><span style=\"color:#657b83;\">,\n }\n }\n}\n</span></code></pre>\n<p>In our trait definition, we define an associated type <code>Unwrapped</code>, for the value that lives "inside" the <code>MonoFunctor</code>. In the case of <code>Option<A></code>, that would be <code>A</code>. And herein lies the problem. We're hard-coding the <code>Unwrapped</code> to just one type, <code>A</code>. Usually with a <code>map</code> function, we want to change the type to <code>B</code>. But we have no way in current, stable Rust to say "I want a type that's associated with this <code>MonoFunctor</code>, but also a little bit different in what lives inside of it."</p>\n<p>That's where Generic Associated Types come in.</p>\n<h2 id=\"polymorphic-functor\">Polymorphic Functor</h2>\n<p>In order to get a polymorphic functor, we need to be able to say "here's how my type would look if I wrapped up a <em>different</em> type inside of it." For example, with <code>Option</code>, we'd like to say "hey, I've got <code>Option<A></code>, and it contains an <code>A</code> type, but if it contained a <code>B</code> type instead, it would be <code>Option<B></code>." To do this, we're going to use the generic associated type <code>Wrapped<B></code>:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">trait </span><span style=\"color:#b58900;\">Functor </span><span style=\"color:#657b83;\">{\n </span><span style=\"color:#268bd2;\">type </span><span style=\"color:#b58900;\">Unwrapped</span><span style=\"color:#657b83;\">;\n </span><span style=\"color:#268bd2;\">type </span><span style=\"color:#b58900;\">Wrapped</span><span style=\"color:#657b83;\"><B>: Functor;\n\n </span><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">map</span><span style=\"color:#657b83;\"><F, B>(</span><span style=\"color:#268bd2;\">self</span><span style=\"color:#657b83;\">, </span><span style=\"color:#268bd2;\">f</span><span style=\"color:#657b83;\">: F) -> </span><span style=\"color:#268bd2;\">Self::</span><span style=\"color:#657b83;\">Wrapped<B>\n </span><span style=\"color:#859900;\">where</span><span style=\"color:#657b83;\">\n F: FnMut(</span><span style=\"color:#268bd2;\">Self::</span><span style=\"color:#657b83;\">Unwrapped) -> B;\n}\n</span></code></pre>\n<p>So what we're saying is:</p>\n<ul>\n<li>Each functor has an associated type <code>Unwrapped</code>, which is the thing it contains</li>\n<li>When we know a functor, we can also figure out another, associated type <code>Wrapped<B></code> which is "like <code>Self</code>, but has a different wrapped up value underneath"</li>\n<li>Like before, <code>map</code> is a method that takes two parameters: <code>self</code> and a function</li>\n<li>The function parameter will map from the current underlying <code>Unwrapped</code> value to some new type <code>B</code></li>\n<li>And the output of <code>map</code> will be a <code>Wrapped<B></code></li>\n</ul>\n<p>That's a bit abstract. Let's see what this looks like for the <code>Option</code> type:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">impl</span><span style=\"color:#657b83;\"><A> Functor </span><span style=\"color:#859900;\">for </span><span style=\"color:#b58900;\">Option</span><span style=\"color:#657b83;\"><A> {\n </span><span style=\"color:#268bd2;\">type </span><span style=\"color:#b58900;\">Unwrapped </span><span style=\"color:#657b83;\">= A;\n </span><span style=\"color:#268bd2;\">type </span><span style=\"color:#b58900;\">Wrapped</span><span style=\"color:#657b83;\"><B> = </span><span style=\"color:#859900;\">Option</span><span style=\"color:#657b83;\"><B>;\n\n </span><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">map</span><span style=\"color:#657b83;\"><F: </span><span style=\"color:#859900;\">FnMut</span><span style=\"color:#657b83;\">(A) -> B, B>(</span><span style=\"color:#268bd2;\">self</span><span style=\"color:#657b83;\">, </span><span style=\"color:#586e75;\">mut </span><span style=\"color:#268bd2;\">f</span><span style=\"color:#657b83;\">: F) -> </span><span style=\"color:#859900;\">Option</span><span style=\"color:#657b83;\"><B> {\n </span><span style=\"color:#859900;\">match </span><span style=\"color:#d33682;\">self </span><span style=\"color:#657b83;\">{\n </span><span style=\"color:#859900;\">Some</span><span style=\"color:#657b83;\">(x) </span><span style=\"color:#859900;\">=> Some</span><span style=\"color:#657b83;\">(</span><span style=\"color:#859900;\">f</span><span style=\"color:#657b83;\">(x)),\n </span><span style=\"color:#859900;\">None => None</span><span style=\"color:#657b83;\">,\n }\n }\n}\n\n#[</span><span style=\"color:#268bd2;\">test</span><span style=\"color:#657b83;\">]\n</span><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">test_option_map</span><span style=\"color:#657b83;\">() {\n </span><span style=\"color:#859900;\">assert_eq!</span><span style=\"color:#657b83;\">(</span><span style=\"color:#859900;\">Some</span><span style=\"color:#657b83;\">(</span><span style=\"color:#6c71c4;\">5</span><span style=\"color:#657b83;\">).</span><span style=\"color:#859900;\">map</span><span style=\"color:#657b83;\">(|</span><span style=\"color:#268bd2;\">x</span><span style=\"color:#657b83;\">| x + </span><span style=\"color:#6c71c4;\">1</span><span style=\"color:#657b83;\">), </span><span style=\"color:#859900;\">Some</span><span style=\"color:#657b83;\">(</span><span style=\"color:#6c71c4;\">6</span><span style=\"color:#657b83;\">));\n </span><span style=\"color:#859900;\">assert_eq!</span><span style=\"color:#657b83;\">(</span><span style=\"color:#859900;\">None</span><span style=\"color:#657b83;\">.</span><span style=\"color:#859900;\">map</span><span style=\"color:#657b83;\">(|</span><span style=\"color:#268bd2;\">x</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">i32</span><span style=\"color:#657b83;\">| x + </span><span style=\"color:#6c71c4;\">1</span><span style=\"color:#657b83;\">), </span><span style=\"color:#859900;\">None</span><span style=\"color:#657b83;\">);\n}\n</span></code></pre>\n<p>And if you play with all of the type gymnastics, you'll see that this ends up being identical to the <code>map</code> method we special-cased for <code>MyOption</code> above (sans a difference between <code>FnOnce</code> and <code>FnMut</code>). Cool!</p>\n<h3 id=\"side-note-hkts\">Side note: HKTs</h3>\n<p>In Haskell, none of this generic associated type business is needed. In fact, Haskell <code>Functor</code>s don't use <em>any</em> associated types. The typeclass for <code>Functor</code> in Haskell far predates the presence of associated types in the language. For comparison, let's see what that looks like, renaming a bit to match up with Rust:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#859900;\">class Functor </span><span style=\"color:#268bd2;\">f </span><span style=\"color:#859900;\">where\n </span><span style=\"color:#b58900;\">map </span><span style=\"color:#859900;\">::</span><span style=\"color:#657b83;\"> (</span><span style=\"color:#268bd2;\">a </span><span style=\"color:#859900;\">-> </span><span style=\"color:#268bd2;\">b</span><span style=\"color:#657b83;\">) </span><span style=\"color:#859900;\">-> </span><span style=\"color:#268bd2;\">f a </span><span style=\"color:#859900;\">-> </span><span style=\"color:#268bd2;\">f b\n</span><span style=\"color:#859900;\">instance </span><span style=\"color:#268bd2;\">Functor Option </span><span style=\"color:#859900;\">where</span><span style=\"color:#657b83;\">\n map f option </span><span style=\"color:#859900;\">=\n case</span><span style=\"color:#657b83;\"> option </span><span style=\"color:#859900;\">of\n </span><span style=\"color:#cb4b16;\">Some</span><span style=\"color:#657b83;\"> x </span><span style=\"color:#859900;\">-> </span><span style=\"color:#cb4b16;\">Some</span><span style=\"color:#657b83;\"> (f x)\n </span><span style=\"color:#cb4b16;\">None </span><span style=\"color:#859900;\">-> </span><span style=\"color:#cb4b16;\">None\n</span></code></pre>\n<p>Or, to translate it into Rust-like syntax:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">trait </span><span style=\"color:#b58900;\">HktFunctor </span><span style=\"color:#657b83;\">{\n </span><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">map</span><span style=\"color:#657b83;\"><A, B, F: </span><span style=\"color:#859900;\">FnMut</span><span style=\"color:#657b83;\">(A) -> B>(</span><span style=\"color:#268bd2;\">self</span><span style=\"color:#657b83;\">: Self<A>, </span><span style=\"color:#268bd2;\">f</span><span style=\"color:#657b83;\">: F) -> Self<B>;\n\n</span><span style=\"color:#268bd2;\">impl </span><span style=\"color:#657b83;\">HktFunctor </span><span style=\"color:#859900;\">for </span><span style=\"color:#b58900;\">Option </span><span style=\"color:#657b83;\">{\n </span><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">map</span><span style=\"color:#657b83;\"><A, B, F: </span><span style=\"color:#859900;\">FnMut</span><span style=\"color:#657b83;\">(A) -> B>(</span><span style=\"color:#268bd2;\">self</span><span style=\"color:#657b83;\">: </span><span style=\"color:#859900;\">Option</span><span style=\"color:#657b83;\"><A>, </span><span style=\"color:#268bd2;\">f</span><span style=\"color:#657b83;\">: F) -> </span><span style=\"color:#859900;\">Option</span><span style=\"color:#657b83;\"><B> {\n </span><span style=\"color:#859900;\">match </span><span style=\"color:#d33682;\">self </span><span style=\"color:#657b83;\">{\n </span><span style=\"color:#859900;\">Some</span><span style=\"color:#657b83;\">(a) </span><span style=\"color:#859900;\">=> Some</span><span style=\"color:#657b83;\">(</span><span style=\"color:#859900;\">f</span><span style=\"color:#657b83;\">(a)),\n </span><span style=\"color:#859900;\">None => None</span><span style=\"color:#657b83;\">,\n }\n }\n}\n</span></code></pre>\n<p>But this isn't valid Rust! That's because we're trying to provide type parameters to <code>Self</code>. But in Rust, <code>Option</code> isn't a type. <code>Option</code> must be applied to a single type parameter before it becomes a type. <code>Option<i32></code> is a type. <code>Option</code> on its own is not.</p>\n<p>By contrast, in Haskell, <code>Maybe Int</code> is a type of <em>kind</em> <code>Type</code>. <code>Maybe</code> is a <em>type constructor</em>, of <em>kind</em> <code>Type -> Type</code>. But you can treat <code>Maybe</code> has a type of its own for purposes of creating type classes and instance. <code>Functor</code> in Haskell works on the kind <code>Type -> Type</code>. This is what we mean by "higher kinded types": we can have types whose <em>kind</em> is higher than just <code>Type</code>.</p>\n<p>GATs in Rust are a workaround for this lack of HKTs for the examples below. But as we'll ultimately see, they are more brittle and more verbose. That's not to say that GATs are a Bad Thing, far from it. It <em>is</em> to say that trying to write Haskell in Rust is probably not a good idea.</p>\n<p>OK, now that we've thoroughly established that what we're about to do isn't a great idea... let's do it!</p>\n<h2 id=\"pointed\">Pointed</h2>\n<p>There's a controversial typeclass in Haskell called <code>Pointed</code>. It's controversial because it introduces a typeclass without any laws associated with it, which is often not very liked. But since I already told you this is all a bad idea, let's implement <code>Pointed</code>.</p>\n<p>The idea of <code>Pointed</code> is simple: wrap up a value into a <code>Functor</code>-like thing. So in the case of <code>Option</code>, it would be like wrapping it with <code>Some</code>. In a <code>Result</code>, it's using <code>Ok</code>. And for a <code>Vec</code>, it would be a single-element vector. Unlike <code>Functor</code>, this will be a static method, since we don't have an existing <code>Pointed</code> value to change. Let's see it in action:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">trait </span><span style=\"color:#b58900;\">Pointed</span><span style=\"color:#657b83;\">: Functor {\n </span><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">wrap</span><span style=\"color:#657b83;\"><T>(</span><span style=\"color:#268bd2;\">t</span><span style=\"color:#657b83;\">: T) -> </span><span style=\"color:#268bd2;\">Self::</span><span style=\"color:#657b83;\">Wrapped<T>;\n}\n\n</span><span style=\"color:#268bd2;\">impl</span><span style=\"color:#657b83;\"><A> Pointed </span><span style=\"color:#859900;\">for </span><span style=\"color:#b58900;\">Option</span><span style=\"color:#657b83;\"><A> {\n </span><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">wrap</span><span style=\"color:#657b83;\"><T>(</span><span style=\"color:#268bd2;\">t</span><span style=\"color:#657b83;\">: T) -> </span><span style=\"color:#859900;\">Option</span><span style=\"color:#657b83;\"><T> {\n </span><span style=\"color:#859900;\">Some</span><span style=\"color:#657b83;\">(t)\n }\n}\n</span></code></pre>\n<p>What's particularly interesting about this is that we don't use the <code>A</code> type parameter in the <code>Option</code> implementation at all.</p>\n<p>There's one more thing worth noting. The result of calling <code>wrap</code> is a <code>Self::Wrapped<T></code> value. What exactly do we know about <code>Self::Wrapped<T></code>? Well, from the <code>Functor</code> trait definition, we know exactly one thing: that <code>Wrapped<T></code> must be a <code>Functor</code>. Interestingly, we have <em>lost the knowledge</em> here that <code>Self::Wrapped<T></code> is also a <code>Pointed</code>. That's going to be a recurring theme for the next few traits.</p>\n<p>But let me reiterate this a different way. When we're working with a general <code>Functor</code> trait implementation, we don't know <em>anything at all</em> about the <code>Wrapped</code> associated type except that it implements <code>Functor</code> itself. Logically, we know that for a <code>Option<A></code> implementation, we'd like <code>Wrapped</code> to be a <code>Option<B></code> kind of thing. But the GAT implementation does not enforce it. (By contrast, the HKT approach in Haskell <em>does</em> enforce this.) Nothing prevents us from writing a horrifically non-sensible implementation such as:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">impl</span><span style=\"color:#657b83;\"><A> Functor </span><span style=\"color:#859900;\">for </span><span style=\"color:#b58900;\">MyOption</span><span style=\"color:#657b83;\"><A> {\n </span><span style=\"color:#268bd2;\">type </span><span style=\"color:#b58900;\">Unwrapped </span><span style=\"color:#657b83;\">= A;\n </span><span style=\"color:#268bd2;\">type </span><span style=\"color:#b58900;\">Wrapped</span><span style=\"color:#657b83;\"><B> = </span><span style=\"color:#859900;\">Result</span><span style=\"color:#657b83;\"><B, </span><span style=\"color:#859900;\">String</span><span style=\"color:#657b83;\">>; </span><span style=\"color:#93a1a1;\">// wut?\n\n </span><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">map</span><span style=\"color:#657b83;\"><F: </span><span style=\"color:#859900;\">FnMut</span><span style=\"color:#657b83;\">(A) -> B, B>(</span><span style=\"color:#268bd2;\">self</span><span style=\"color:#657b83;\">, </span><span style=\"color:#586e75;\">mut </span><span style=\"color:#268bd2;\">f</span><span style=\"color:#657b83;\">: F) -> </span><span style=\"color:#859900;\">Result</span><span style=\"color:#657b83;\"><B, </span><span style=\"color:#859900;\">String</span><span style=\"color:#657b83;\">> {\n </span><span style=\"color:#859900;\">match </span><span style=\"color:#d33682;\">self </span><span style=\"color:#657b83;\">{\n MyOption::Some(a) </span><span style=\"color:#859900;\">=> Ok</span><span style=\"color:#657b83;\">(</span><span style=\"color:#859900;\">f</span><span style=\"color:#657b83;\">(a)),\n MyOption::None </span><span style=\"color:#859900;\">=> Err</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">Well this is weird, isn't it?</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">.</span><span style=\"color:#859900;\">to_owned</span><span style=\"color:#657b83;\">()),\n }\n }\n}\n\n</span></code></pre>\n<p>You may be thinking, "So what, no one would write something like that. And it's their own fault if they do." That's not the point here. The point is that the compiler can't know that there's a connection between <code>Self</code> and <code>Wrapped<B></code>. And since it can't know that, there are some things we can't get to type check. I'll show you one of those at the end.</p>\n<h2 id=\"applicative\">Applicative</h2>\n<p>When I give Haskell training, and I get to the <code>Functor</code>/<code>Applicative</code>/<code>Monad</code> section, most people are nervous about <code>Monad</code>s. In my experience, the really confusing part is really <code>Applicative</code>. Once you understand that, <code>Monad</code> is relatively speaking easy.</p>\n<p>The <code>Applicative</code> typeclass in Haskell has two methods. <code>pure</code> is equivalent to the <code>wrap</code> that I put into <code>Pointed</code>, so we can ignore it. The other method is <code><*></code>, known as "apply," or "splat", or "the tie fighter." I originally implemented <code>Applicative</code> with a method called <code>apply</code> that matches that operator, but found that it was better to go a different route.</p>\n<p>Instead, there's an alternate way to define an <code>Applicative</code> typeclass, based on a different function called <code>liftA2</code> (or, in Rust, <code>lift_a2</code>). Here's the idea. Suppose I have two functions:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">birth_year</span><span style=\"color:#657b83;\">() -> </span><span style=\"color:#859900;\">Option</span><span style=\"color:#657b83;\"><</span><span style=\"color:#268bd2;\">i32</span><span style=\"color:#657b83;\">> { </span><span style=\"color:#859900;\">... </span><span style=\"color:#657b83;\">}\n</span><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">current_year</span><span style=\"color:#657b83;\">() -> </span><span style=\"color:#859900;\">Option</span><span style=\"color:#657b83;\"><</span><span style=\"color:#268bd2;\">i32</span><span style=\"color:#657b83;\">> { </span><span style=\"color:#859900;\">... </span><span style=\"color:#657b83;\">}\n</span></code></pre>\n<p>I may not know the current year or the birth year, in which case I'll return <code>None</code>. But if I get a <code>Some</code> return for both of these function calls, then I can calculate the age. In normal Rust code, this may look like:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">age</span><span style=\"color:#657b83;\">() -> </span><span style=\"color:#859900;\">Option</span><span style=\"color:#657b83;\"><</span><span style=\"color:#268bd2;\">i32</span><span style=\"color:#657b83;\">> {\n </span><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> birth_year = </span><span style=\"color:#859900;\">birth_year</span><span style=\"color:#657b83;\">()</span><span style=\"color:#859900;\">?</span><span style=\"color:#657b83;\">;\n </span><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> current_year = </span><span style=\"color:#859900;\">current_year</span><span style=\"color:#657b83;\">()</span><span style=\"color:#859900;\">?</span><span style=\"color:#657b83;\">;\n </span><span style=\"color:#859900;\">Some</span><span style=\"color:#657b83;\">(current_year - birth_year)\n}\n</span></code></pre>\n<p>But that's leveraging <code>?</code> and early return. A primary purpose of <code>Applicative</code> is to address the same problem. So let's rewrite this without any early return, and instead use some pattern matching:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">age</span><span style=\"color:#657b83;\">() -> </span><span style=\"color:#859900;\">Option</span><span style=\"color:#657b83;\"><</span><span style=\"color:#268bd2;\">i32</span><span style=\"color:#657b83;\">> {\n </span><span style=\"color:#859900;\">match </span><span style=\"color:#657b83;\">(</span><span style=\"color:#859900;\">birth_year</span><span style=\"color:#657b83;\">(), </span><span style=\"color:#859900;\">current_year</span><span style=\"color:#657b83;\">()) {\n (</span><span style=\"color:#859900;\">Some</span><span style=\"color:#657b83;\">(birth_year), </span><span style=\"color:#859900;\">Some</span><span style=\"color:#657b83;\">(current_year)) </span><span style=\"color:#859900;\">=> Some</span><span style=\"color:#657b83;\">(current_year - birth_year),\n </span><span style=\"color:#859900;\">_ => None</span><span style=\"color:#657b83;\">,\n }\n}\n</span></code></pre>\n<p>This certainly works, but it's verbose. It also doesn't generalize to other cases, like a <code>Result</code>. And what about a really sophisticated case, like "I have a <code>Future</code> that will return the birth year, a <code>Future</code> that will return the current year, and I want to produce a <code>Future</code> that finds the difference." With async/await syntax, it's easy enough to do. But we can also do it with <code>Applicative</code>, using our <code>lift_a2</code> method.</p>\n<p>The point of <code>lift_a2</code> is: I've got two values wrapped up, perhaps both in an <code>Option</code>. I'd like to use a function to combine them together. Let's see what that looks like in Rust:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">trait </span><span style=\"color:#b58900;\">Applicative</span><span style=\"color:#657b83;\">: Pointed {\n </span><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">lift_a2</span><span style=\"color:#657b83;\"><F, B, C>(</span><span style=\"color:#268bd2;\">self</span><span style=\"color:#657b83;\">, </span><span style=\"color:#268bd2;\">b</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">Self::</span><span style=\"color:#657b83;\">Wrapped<B>, </span><span style=\"color:#268bd2;\">f</span><span style=\"color:#657b83;\">: F) -> </span><span style=\"color:#268bd2;\">Self::</span><span style=\"color:#657b83;\">Wrapped<C>\n </span><span style=\"color:#859900;\">where</span><span style=\"color:#657b83;\">\n F: FnMut(</span><span style=\"color:#268bd2;\">Self::</span><span style=\"color:#657b83;\">Unwrapped, B) -> C;\n}\n\nimpl<A> Applicative </span><span style=\"color:#859900;\">for Option</span><span style=\"color:#657b83;\"><A> {\n </span><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">lift_a2</span><span style=\"color:#657b83;\"><F, B, C>(</span><span style=\"color:#268bd2;\">self</span><span style=\"color:#657b83;\">, </span><span style=\"color:#268bd2;\">b</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">Self::</span><span style=\"color:#657b83;\">Wrapped<B>, </span><span style=\"color:#586e75;\">mut </span><span style=\"color:#268bd2;\">f</span><span style=\"color:#657b83;\">: F) -> </span><span style=\"color:#268bd2;\">Self::</span><span style=\"color:#657b83;\">Wrapped<C>\n </span><span style=\"color:#859900;\">where</span><span style=\"color:#657b83;\">\n F: FnMut(</span><span style=\"color:#268bd2;\">Self::</span><span style=\"color:#657b83;\">Unwrapped, B) -> C\n {\n </span><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> a = </span><span style=\"color:#d33682;\">self</span><span style=\"color:#859900;\">?</span><span style=\"color:#657b83;\">;\n </span><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> b = b</span><span style=\"color:#859900;\">?</span><span style=\"color:#657b83;\">;\n </span><span style=\"color:#859900;\">Some</span><span style=\"color:#657b83;\">(</span><span style=\"color:#859900;\">f</span><span style=\"color:#657b83;\">(a, b))\n }\n}\n</span></code></pre>\n<p>With this definition in place, we can now rewrite <code>age</code> as:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">age</span><span style=\"color:#657b83;\">() -> </span><span style=\"color:#859900;\">Option</span><span style=\"color:#657b83;\"><</span><span style=\"color:#268bd2;\">i32</span><span style=\"color:#657b83;\">> {\n </span><span style=\"color:#859900;\">current_year</span><span style=\"color:#657b83;\">().</span><span style=\"color:#859900;\">lift_a2</span><span style=\"color:#657b83;\">(</span><span style=\"color:#859900;\">birth_year</span><span style=\"color:#657b83;\">(), |</span><span style=\"color:#268bd2;\">cy</span><span style=\"color:#657b83;\">, </span><span style=\"color:#268bd2;\">by</span><span style=\"color:#657b83;\">| cy - by)\n}\n</span></code></pre>\n<p>Whether this is an improvement or not probably depends heavily on how much Haskell you've written in your life. Again, I'm not advocating changing Rust here, but it's certainly interesting.</p>\n<p>We could also do the same kind of thing with <code>Result</code>:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">birth_year</span><span style=\"color:#657b83;\">() -> </span><span style=\"color:#859900;\">Result</span><span style=\"color:#657b83;\"><</span><span style=\"color:#268bd2;\">i32</span><span style=\"color:#657b83;\">, </span><span style=\"color:#859900;\">String</span><span style=\"color:#657b83;\">> {\n </span><span style=\"color:#859900;\">Err</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">No birth year</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">.</span><span style=\"color:#859900;\">to_string</span><span style=\"color:#657b83;\">())\n}\n\n</span><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">current_year</span><span style=\"color:#657b83;\">() -> </span><span style=\"color:#859900;\">Result</span><span style=\"color:#657b83;\"><</span><span style=\"color:#268bd2;\">i32</span><span style=\"color:#657b83;\">, </span><span style=\"color:#859900;\">String</span><span style=\"color:#657b83;\">> {\n </span><span style=\"color:#859900;\">Err</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">No current year</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">.</span><span style=\"color:#859900;\">to_string</span><span style=\"color:#657b83;\">())\n}\n\n</span><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">age</span><span style=\"color:#657b83;\">() -> </span><span style=\"color:#859900;\">Result</span><span style=\"color:#657b83;\"><</span><span style=\"color:#268bd2;\">i32</span><span style=\"color:#657b83;\">, </span><span style=\"color:#859900;\">String</span><span style=\"color:#657b83;\">> {\n </span><span style=\"color:#859900;\">current_year</span><span style=\"color:#657b83;\">().</span><span style=\"color:#859900;\">lift_a2</span><span style=\"color:#657b83;\">(</span><span style=\"color:#859900;\">birth_year</span><span style=\"color:#657b83;\">(), |</span><span style=\"color:#268bd2;\">cy</span><span style=\"color:#657b83;\">, </span><span style=\"color:#268bd2;\">by</span><span style=\"color:#657b83;\">| cy - by)\n}\n</span></code></pre>\n<p>Which may beg the question: which of the two <code>Err</code> values do we take? Well, that depends on our implementation of <code>Applicative</code>, but typically we would prefer choosing the first:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">impl</span><span style=\"color:#657b83;\"><A, E> Applicative </span><span style=\"color:#859900;\">for </span><span style=\"color:#b58900;\">Result</span><span style=\"color:#657b83;\"><A, E> {\n </span><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">lift_a2</span><span style=\"color:#657b83;\"><F, B, C>(</span><span style=\"color:#268bd2;\">self</span><span style=\"color:#657b83;\">, </span><span style=\"color:#268bd2;\">b</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">Self::</span><span style=\"color:#657b83;\">Wrapped<B>, </span><span style=\"color:#586e75;\">mut </span><span style=\"color:#268bd2;\">f</span><span style=\"color:#657b83;\">: F) -> </span><span style=\"color:#268bd2;\">Self::</span><span style=\"color:#657b83;\">Wrapped<C>\n </span><span style=\"color:#859900;\">where</span><span style=\"color:#657b83;\">\n F: FnMut(</span><span style=\"color:#268bd2;\">Self::</span><span style=\"color:#657b83;\">Unwrapped, B) -> C\n {\n </span><span style=\"color:#859900;\">match </span><span style=\"color:#657b83;\">(</span><span style=\"color:#d33682;\">self</span><span style=\"color:#657b83;\">, b) {\n (</span><span style=\"color:#859900;\">Ok</span><span style=\"color:#657b83;\">(a), </span><span style=\"color:#859900;\">Ok</span><span style=\"color:#657b83;\">(b)) </span><span style=\"color:#859900;\">=> Ok</span><span style=\"color:#657b83;\">(</span><span style=\"color:#859900;\">f</span><span style=\"color:#657b83;\">(a, b)),\n (</span><span style=\"color:#859900;\">Err</span><span style=\"color:#657b83;\">(e), </span><span style=\"color:#859900;\">_</span><span style=\"color:#657b83;\">) </span><span style=\"color:#859900;\">=> Err</span><span style=\"color:#657b83;\">(e),\n (</span><span style=\"color:#859900;\">_</span><span style=\"color:#657b83;\">, </span><span style=\"color:#859900;\">Err</span><span style=\"color:#657b83;\">(e)) </span><span style=\"color:#859900;\">=> Err</span><span style=\"color:#657b83;\">(e),\n }\n }\n}\n</span></code></pre>\n<p>But what if we wanted both? Here's a case where <code>Applicative</code> gives us power that <code>?</code> doesn't.</p>\n<h2 id=\"validation\">Validation</h2>\n<p>The <code>Validation</code> type from Haskell represents the idea "I'm going to try lots of things, some of them may fail, and I want to collect together all of the error results." A typically example of this would be web form parsing. If a user enters an invalid email address, invalid phone number, <em>and</em> forgets to click the "I agree" box, you'd want to generate all three error messages. You don't want to generate just one.</p>\n<p>To start off our <code>Validation</code> implementation, we need to introduce one more Haskell-y typeclass, this time for representing the concept of "combining together multiple values." We <em>could</em> just hard-code <code>Vec</code> in here, but where's the fun in that? Instead, let's introduce the strangely-named <code>Semigroup</code> trait. This doesn't even require any special GAT code:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">trait </span><span style=\"color:#b58900;\">Semigroup </span><span style=\"color:#657b83;\">{\n </span><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">append</span><span style=\"color:#657b83;\">(</span><span style=\"color:#268bd2;\">self</span><span style=\"color:#657b83;\">, </span><span style=\"color:#268bd2;\">rhs</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">Self</span><span style=\"color:#657b83;\">) -> </span><span style=\"color:#268bd2;\">Self</span><span style=\"color:#657b83;\">;\n}\n\n</span><span style=\"color:#268bd2;\">impl </span><span style=\"color:#657b83;\">Semigroup </span><span style=\"color:#859900;\">for </span><span style=\"color:#b58900;\">String </span><span style=\"color:#657b83;\">{\n </span><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">append</span><span style=\"color:#657b83;\">(</span><span style=\"color:#586e75;\">mut </span><span style=\"color:#268bd2;\">self</span><span style=\"color:#657b83;\">, </span><span style=\"color:#268bd2;\">rhs</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">Self</span><span style=\"color:#657b83;\">) -> </span><span style=\"color:#268bd2;\">Self </span><span style=\"color:#657b83;\">{\n </span><span style=\"color:#d33682;\">self </span><span style=\"color:#657b83;\">+= </span><span style=\"color:#859900;\">&</span><span style=\"color:#657b83;\">rhs;\n </span><span style=\"color:#d33682;\">self\n </span><span style=\"color:#657b83;\">}\n}\n\n</span><span style=\"color:#268bd2;\">impl</span><span style=\"color:#657b83;\"><T> Semigroup </span><span style=\"color:#859900;\">for </span><span style=\"color:#b58900;\">Vec</span><span style=\"color:#657b83;\"><T> {\n </span><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">append</span><span style=\"color:#657b83;\">(</span><span style=\"color:#586e75;\">mut </span><span style=\"color:#268bd2;\">self</span><span style=\"color:#657b83;\">, </span><span style=\"color:#586e75;\">mut </span><span style=\"color:#268bd2;\">rhs</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">Self</span><span style=\"color:#657b83;\">) -> </span><span style=\"color:#268bd2;\">Self </span><span style=\"color:#657b83;\">{\n </span><span style=\"color:#859900;\">Vec</span><span style=\"color:#657b83;\">::append(</span><span style=\"color:#859900;\">&</span><span style=\"color:#586e75;\">mut </span><span style=\"color:#d33682;\">self</span><span style=\"color:#657b83;\">, </span><span style=\"color:#859900;\">&</span><span style=\"color:#586e75;\">mut</span><span style=\"color:#657b83;\"> rhs);\n </span><span style=\"color:#d33682;\">self\n </span><span style=\"color:#657b83;\">}\n}\n\n</span><span style=\"color:#268bd2;\">impl </span><span style=\"color:#657b83;\">Semigroup </span><span style=\"color:#859900;\">for</span><span style=\"color:#657b83;\"> () {\n </span><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">append</span><span style=\"color:#657b83;\">(</span><span style=\"color:#268bd2;\">self</span><span style=\"color:#657b83;\">, (): ()) -> () {}\n}\n</span></code></pre>\n<p>With that in place, we can now define a new <code>enum</code> called <code>Validation</code>:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#657b83;\">#[</span><span style=\"color:#268bd2;\">derive</span><span style=\"color:#657b83;\">(PartialEq, Debug)]\n</span><span style=\"color:#268bd2;\">enum </span><span style=\"color:#b58900;\">Validation</span><span style=\"color:#657b83;\"><A, E> {\n </span><span style=\"color:#859900;\">Ok</span><span style=\"color:#657b83;\">(A),\n </span><span style=\"color:#859900;\">Err</span><span style=\"color:#657b83;\">(E),\n}\n</span></code></pre>\n<p>The <code>Functor</code> and <code>Pointed</code> implementations are boring, let's skip straight to the meat with the <code>Applicative</code> implementation:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">impl</span><span style=\"color:#657b83;\"><A, E: Semigroup> Applicative </span><span style=\"color:#859900;\">for </span><span style=\"color:#b58900;\">Validation</span><span style=\"color:#657b83;\"><A, E> {\n </span><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">lift_a2</span><span style=\"color:#657b83;\"><F, B, C>(</span><span style=\"color:#268bd2;\">self</span><span style=\"color:#657b83;\">, </span><span style=\"color:#268bd2;\">b</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">Self::</span><span style=\"color:#657b83;\">Wrapped<B>, </span><span style=\"color:#586e75;\">mut </span><span style=\"color:#268bd2;\">f</span><span style=\"color:#657b83;\">: F) -> </span><span style=\"color:#268bd2;\">Self::</span><span style=\"color:#657b83;\">Wrapped<C>\n </span><span style=\"color:#859900;\">where</span><span style=\"color:#657b83;\">\n F: FnMut(</span><span style=\"color:#268bd2;\">Self::</span><span style=\"color:#657b83;\">Unwrapped, B) -> C\n {\n </span><span style=\"color:#859900;\">match </span><span style=\"color:#657b83;\">(</span><span style=\"color:#d33682;\">self</span><span style=\"color:#657b83;\">, b) {\n (Validation::Ok(a), Validation::Ok(b)) </span><span style=\"color:#859900;\">=> </span><span style=\"color:#657b83;\">Validation::Ok(</span><span style=\"color:#859900;\">f</span><span style=\"color:#657b83;\">(a, b)),\n (Validation::Err(e), Validation::Ok(</span><span style=\"color:#859900;\">_</span><span style=\"color:#657b83;\">)) </span><span style=\"color:#859900;\">=> </span><span style=\"color:#657b83;\">Validation::Err(e),\n (Validation::Ok(</span><span style=\"color:#859900;\">_</span><span style=\"color:#657b83;\">), Validation::Err(e)) </span><span style=\"color:#859900;\">=> </span><span style=\"color:#657b83;\">Validation::Err(e),\n (Validation::Err(e1), Validation::Err(e2)) </span><span style=\"color:#859900;\">=> </span><span style=\"color:#657b83;\">Validation::Err(e1.</span><span style=\"color:#859900;\">append</span><span style=\"color:#657b83;\">(e2)),\n }\n }\n}\n</span></code></pre>\n<p>Here, we're saying that the error type parameter must implement <code>Semigroup</code>. If both values are <code>Ok</code>, we apply the <code>f</code> function to them and wrap up the result in <code>Ok</code>. If only one of the values is <code>Err</code>, we return that error. But if <em>both</em> of them are error, we leverage the <code>append</code> method of <code>Semigroup</code> to combine them together. This is something you can't get with <code>?</code>-style error handling.</p>\n<h2 id=\"monad\">Monad</h2>\n<p>At last, the dreaded monad rears its head! But in reality, at least for Rustaceans, monad isn't much of a surprise. You're already used to it: it's the <code>and_then</code> method. Almost any chain of statements that end with <code>?</code> in Rust can be reimagined as monadic binds. In my opinion, the main reason monad has the allure of the unknowable was a series of particularly bad tutorials that cemented this idea in people's minds.</p>\n<p>Anyway, since we're just trying to match the existing method signature of <code>and_then</code> on <code>Option</code>, I'm not going to spend much time motivating "why monad." Instead, let's just look at the definition of the trait:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">trait </span><span style=\"color:#b58900;\">Monad</span><span style=\"color:#657b83;\"> : Applicative {\n </span><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">bind</span><span style=\"color:#657b83;\"><B, F>(</span><span style=\"color:#268bd2;\">self</span><span style=\"color:#657b83;\">, </span><span style=\"color:#268bd2;\">f</span><span style=\"color:#657b83;\">: F) -> </span><span style=\"color:#268bd2;\">Self::</span><span style=\"color:#657b83;\">Wrapped<B>\n </span><span style=\"color:#859900;\">where</span><span style=\"color:#657b83;\">\n F: FnMut(</span><span style=\"color:#268bd2;\">Self::</span><span style=\"color:#657b83;\">Unwrapped) -> </span><span style=\"color:#268bd2;\">Self::</span><span style=\"color:#657b83;\">Wrapped<B>;\n}\n\nimpl<A> Monad </span><span style=\"color:#859900;\">for Option</span><span style=\"color:#657b83;\"><A> {\n </span><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">bind</span><span style=\"color:#657b83;\"><B, F>(</span><span style=\"color:#268bd2;\">self</span><span style=\"color:#657b83;\">, </span><span style=\"color:#268bd2;\">f</span><span style=\"color:#657b83;\">: F) -> </span><span style=\"color:#859900;\">Option</span><span style=\"color:#657b83;\"><B>\n </span><span style=\"color:#859900;\">where</span><span style=\"color:#657b83;\">\n F: FnMut(A) -> </span><span style=\"color:#859900;\">Option</span><span style=\"color:#657b83;\"><B>,\n {\n </span><span style=\"color:#d33682;\">self</span><span style=\"color:#657b83;\">.</span><span style=\"color:#859900;\">and_then</span><span style=\"color:#657b83;\">(f)\n }\n}\n</span></code></pre>\n<p>And just like that, we've got monadic Rust. Time to ride off into the sunset.</p>\n<p>But wait, there's more!</p>\n<h2 id=\"monad-transformers\">Monad transformers</h2>\n<img src=\"/images/blog/transformers-rust.jpg\">\n<p>I'm overall not a huge fan of monad transformers. I think they are drastically overused in Haskell, and lead to huge amounts of complication. I instead advocate the <a href=\"https://www.fpcomplete.com/blog/2017/06/readert-design-pattern/\">ReaderT design pattern</a>. But again, this post is definitely not about best practices.</p>\n<p>Typically, each monad instance provides some kind of additional functionality. <code>Option</code> means "it might not produce a value." <code>Result</code> means "it might fail with an error." If we provided it, <code>Future</code> means "it won't produce a value immediately, but it will eventually." And as a final example, the <code>Reader</code> monad means "I have read-only access to some environmental data."</p>\n<p>But what if we want to have two pieces of functionality? There's no obvious way to combine a <code>Reader</code> and a <code>Result</code>. In Rust, we <em>do</em> combine together <code>Result</code> and <code>Future</code> via <code>async</code> functions and <code>?</code>, but that had to have carefully designed language support. Instead, the Haskell approach to this problem would be: just provide <code>do</code> notation (syntactic sugar for monads), and then layer up your monad transformers to add together all of the functionality.</p>\n<p>I've considered writing a blog post on this philosophical difference for a while. (If people are interested in such a post, please let me know.) But for now, let's simply explore what it looks like to provide a monad transformer in Rust. We'll implement it for the most boring of all monad transformers, <code>IdentityT</code>. This is the transformer that doesn't do anything at all. (And if you're wondering "why have it," consider why Rust has 1-tuples. Sometimes, you need something that fits a certain shape to make some generic code work nicely.)</p>\n<p>Since <code>IdentityT</code> doesn't do anything, it's comforting to see that its type reflects that perfectly:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">struct </span><span style=\"color:#b58900;\">IdentityT</span><span style=\"color:#657b83;\"><M>(M);\n</span></code></pre>\n<p>I'm calling the type parameter <code>M</code>, because it's going to itself be an implementation of <code>Monad</code>. That's the idea here: every monad transformer sits on top of a "base monad."</p>\n<p>Next, let's look at a <code>Functor</code> implementation. The idea is to unwrap the <code>IdentityT</code> layer, leverage the underlying <code>map</code> method, and then rewrap <code>IdentityT</code>.</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">impl</span><span style=\"color:#657b83;\"><M: Functor> Functor </span><span style=\"color:#859900;\">for </span><span style=\"color:#b58900;\">IdentityT</span><span style=\"color:#657b83;\"><M> {\n </span><span style=\"color:#268bd2;\">type </span><span style=\"color:#b58900;\">Unwrapped </span><span style=\"color:#657b83;\">= M::Unwrapped;\n </span><span style=\"color:#268bd2;\">type </span><span style=\"color:#b58900;\">Wrapped</span><span style=\"color:#657b83;\"><A> = IdentityT<</span><span style=\"color:#268bd2;\">M::</span><span style=\"color:#657b83;\">Wrapped<A>>;\n\n </span><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">map</span><span style=\"color:#657b83;\"><F, B>(</span><span style=\"color:#268bd2;\">self</span><span style=\"color:#657b83;\">, </span><span style=\"color:#268bd2;\">f</span><span style=\"color:#657b83;\">: F) -> </span><span style=\"color:#268bd2;\">Self::</span><span style=\"color:#657b83;\">Wrapped<B>\n </span><span style=\"color:#859900;\">where</span><span style=\"color:#657b83;\">\n F: FnMut(</span><span style=\"color:#268bd2;\">M::</span><span style=\"color:#657b83;\">Unwrapped) -> B\n {\n IdentityT(</span><span style=\"color:#d33682;\">self</span><span style=\"color:#657b83;\">.</span><span style=\"color:#6c71c4;\">0.</span><span style=\"color:#859900;\">map</span><span style=\"color:#657b83;\">(f))\n }\n}\n</span></code></pre>\n<p>For our associated types, we leverage the associated types of <code>M</code>. Inside <code>map</code>, we use <code>self.0</code> to get the underlying <code>M</code>, and wrap the result of the <code>map</code> method call with <code>IdentityT</code>. Cool!</p>\n<p>The <code>Pointed</code>, <code>Applicative</code>, and <code>Monad</code> implementations follow similar patterns, so I'll drop all of those in too:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">impl</span><span style=\"color:#657b83;\"><M: Pointed> Pointed </span><span style=\"color:#859900;\">for </span><span style=\"color:#b58900;\">IdentityT</span><span style=\"color:#657b83;\"><M> {\n </span><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">wrap</span><span style=\"color:#657b83;\"><T>(</span><span style=\"color:#268bd2;\">t</span><span style=\"color:#657b83;\">: T) -> IdentityT<</span><span style=\"color:#268bd2;\">M::</span><span style=\"color:#657b83;\">Wrapped<T>> {\n IdentityT(M::wrap(t))\n }\n}\n\n</span><span style=\"color:#268bd2;\">impl</span><span style=\"color:#657b83;\"><M: Applicative> Applicative </span><span style=\"color:#859900;\">for </span><span style=\"color:#b58900;\">IdentityT</span><span style=\"color:#657b83;\"><M> {\n </span><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">lift_a2</span><span style=\"color:#657b83;\"><F, B, C>(</span><span style=\"color:#268bd2;\">self</span><span style=\"color:#657b83;\">, </span><span style=\"color:#268bd2;\">b</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">Self::</span><span style=\"color:#657b83;\">Wrapped<B>, </span><span style=\"color:#268bd2;\">f</span><span style=\"color:#657b83;\">: F) -> </span><span style=\"color:#268bd2;\">Self::</span><span style=\"color:#657b83;\">Wrapped<C>\n </span><span style=\"color:#859900;\">where</span><span style=\"color:#657b83;\">\n F: FnMut(</span><span style=\"color:#268bd2;\">Self::</span><span style=\"color:#657b83;\">Unwrapped, B) -> C\n {\n IdentityT(</span><span style=\"color:#d33682;\">self</span><span style=\"color:#657b83;\">.</span><span style=\"color:#6c71c4;\">0.</span><span style=\"color:#859900;\">lift_a2</span><span style=\"color:#657b83;\">(b.</span><span style=\"color:#6c71c4;\">0</span><span style=\"color:#657b83;\">, f))\n }\n}\n\n</span><span style=\"color:#268bd2;\">impl</span><span style=\"color:#657b83;\"><M: Monad> Monad </span><span style=\"color:#859900;\">for </span><span style=\"color:#b58900;\">IdentityT</span><span style=\"color:#657b83;\"><M> {\n </span><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">bind</span><span style=\"color:#657b83;\"><B, F>(</span><span style=\"color:#268bd2;\">self</span><span style=\"color:#657b83;\">, </span><span style=\"color:#586e75;\">mut </span><span style=\"color:#268bd2;\">f</span><span style=\"color:#657b83;\">: F) -> </span><span style=\"color:#268bd2;\">Self::</span><span style=\"color:#657b83;\">Wrapped<B>\n </span><span style=\"color:#859900;\">where</span><span style=\"color:#657b83;\">\n F: FnMut(</span><span style=\"color:#268bd2;\">Self::</span><span style=\"color:#657b83;\">Unwrapped) -> </span><span style=\"color:#268bd2;\">Self::</span><span style=\"color:#657b83;\">Wrapped<B>\n {\n IdentityT(</span><span style=\"color:#d33682;\">self</span><span style=\"color:#657b83;\">.</span><span style=\"color:#6c71c4;\">0.</span><span style=\"color:#859900;\">bind</span><span style=\"color:#657b83;\">(|</span><span style=\"color:#268bd2;\">x</span><span style=\"color:#657b83;\">| </span><span style=\"color:#859900;\">f</span><span style=\"color:#657b83;\">(x).</span><span style=\"color:#6c71c4;\">0</span><span style=\"color:#657b83;\">))\n }\n}\n</span></code></pre>\n<p>And finally, we'll define one new trait: <code>MonadTrans</code>. <code>MonadTrans</code> captures the idea of "layering up" a base monad into the transformed monad. In Haskell, you'll often see code like <code>lift (readFile "foo.txt")</code>, where <code>readFile</code> works in the base monad, and we're sitting in a layer on top of that.</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">trait </span><span style=\"color:#b58900;\">MonadTrans </span><span style=\"color:#657b83;\">{\n </span><span style=\"color:#268bd2;\">type </span><span style=\"color:#b58900;\">Base</span><span style=\"color:#657b83;\">: Monad;\n\n </span><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">lift</span><span style=\"color:#657b83;\">(</span><span style=\"color:#268bd2;\">base</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">Self::</span><span style=\"color:#657b83;\">Base) -> </span><span style=\"color:#268bd2;\">Self</span><span style=\"color:#657b83;\">;\n}\n\n</span><span style=\"color:#268bd2;\">impl</span><span style=\"color:#657b83;\"><M: Monad> MonadTrans </span><span style=\"color:#859900;\">for </span><span style=\"color:#b58900;\">IdentityT</span><span style=\"color:#657b83;\"><M> {\n </span><span style=\"color:#268bd2;\">type </span><span style=\"color:#b58900;\">Base </span><span style=\"color:#657b83;\">= M;\n\n </span><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">lift</span><span style=\"color:#657b83;\">(</span><span style=\"color:#268bd2;\">base</span><span style=\"color:#657b83;\">: M) -> </span><span style=\"color:#268bd2;\">Self </span><span style=\"color:#657b83;\">{\n IdentityT(base)\n }\n}\n</span></code></pre>\n<p>So is this useful? Not terribly on its own. We could arguably create an ecosystem of <code>ReaderT</code>, <code>WriterT</code>, <code>ContT</code>, <code>ConduitT</code>, and more, and start building up sophisticated systems. But I'm strongly of the opinion that we don't need that stuff in Rust, at least not yet. I'm happy to go this far in my implementation to explore the wonders of GATs, but let's not go crazy and try to make something useful just because we can.</p>\n<h2 id=\"join\">join</h2>\n<p>Alright, now the fun begins. We've seen GATs in practice. And it seems like Rust is keeping pace with Haskell pretty well. That's about to end.</p>\n<p>There's another method that goes along with <code>Monad</code>s in Haskell, called <code>join</code>. It's equivalent in power to the <code>bind</code> method we've already seen, but works differently. <code>join</code> "flattens" two layers of monads in Haskell. And a side note: there's already <a href=\"https://doc.rust-lang.org/stable/std/option/enum.Option.html#impl-10\">a method called <code>flatten</code></a> in Rust that does just this for <code>Option</code> and <code>Result</code>.</p>\n<p>The catch with <code>join</code>: the monads have to be the same. In other words, <code>join (Just (Just 5)) == Just 5</code>, but <code>join (Just (Right 6))</code> is a type error, since <code>Just</code> is a <code>Maybe</code> data constructor, and <code>Right</code> is an <code>Either</code> data constructor.</p>\n<p>Now we're in a bit of a quandary. In Haskell, where we have higher kinded types, it's easy to say "<code>Maybe</code> must be the same as <code>Maybe</code>":</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#b58900;\">join </span><span style=\"color:#859900;\">:: </span><span style=\"color:#268bd2;\">Monad m </span><span style=\"color:#859900;\">=> </span><span style=\"color:#268bd2;\">m</span><span style=\"color:#657b83;\"> (</span><span style=\"color:#268bd2;\">m a</span><span style=\"color:#657b83;\">) </span><span style=\"color:#859900;\">-> </span><span style=\"color:#268bd2;\">m a\n</span><span style=\"color:#657b83;\">join m </span><span style=\"color:#859900;\">=</span><span style=\"color:#657b83;\"> bind m (</span><span style=\"color:#859900;\">\\</span><span style=\"color:#657b83;\">x </span><span style=\"color:#859900;\">-></span><span style=\"color:#657b83;\"> x)\n</span></code></pre>\n<p>But I couldn't figure out a way to express the same idea with GATs in Rust and get the syntax accepted by the compiler. This is the closest I came:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">join</span><span style=\"color:#657b83;\"><MOuter, MInner, A>(</span><span style=\"color:#268bd2;\">outer</span><span style=\"color:#657b83;\">: MOuter) -> </span><span style=\"color:#268bd2;\">MOuter::</span><span style=\"color:#657b83;\">Wrapped<A>\n</span><span style=\"color:#859900;\">where</span><span style=\"color:#657b83;\">\n MOuter: Monad<Unwrapped = MInner>,\n MInner: Monad<Unwrapped = A, Wrapped = </span><span style=\"color:#268bd2;\">MOuter::</span><span style=\"color:#657b83;\">Wrapped<A>>,\n{\n outer.</span><span style=\"color:#859900;\">bind</span><span style=\"color:#657b83;\">(|</span><span style=\"color:#268bd2;\">inner</span><span style=\"color:#657b83;\">| inner)\n}\n\n#[</span><span style=\"color:#268bd2;\">test</span><span style=\"color:#657b83;\">]\n</span><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">test_join</span><span style=\"color:#657b83;\">() {\n </span><span style=\"color:#859900;\">assert_eq!</span><span style=\"color:#657b83;\">(</span><span style=\"color:#859900;\">join</span><span style=\"color:#657b83;\">(</span><span style=\"color:#859900;\">Some</span><span style=\"color:#657b83;\">(</span><span style=\"color:#859900;\">Some</span><span style=\"color:#657b83;\">(</span><span style=\"color:#b58900;\">true</span><span style=\"color:#657b83;\">))), </span><span style=\"color:#859900;\">Some</span><span style=\"color:#657b83;\">(</span><span style=\"color:#b58900;\">true</span><span style=\"color:#657b83;\">));\n}\n</span></code></pre>\n<p>Unfortunately, this broke the compiler:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#657b83;\">error: internal compiler error: compiler\\rustc_middle\\src\\ty\\subst.rs:529:17: type parameter `B/#1` (B/1) out of range when substituting, substs=[MInner]\n\nthread 'rustc' panicked at 'Box<Any>', /rustc/b7ebc6b0c1ba3c27ebb17c0b496ece778ef11e18\\compiler\\rustc_errors\\src\\lib.rs:904:9\nnote: run with `RUST_BACKTRACE=1` environment variable to display a backtrace\n\nnote: the compiler unexpectedly panicked. this is a bug.\n\nnote: we would appreciate a bug report: https://github.com/rust-lang/rust/issues/new?labels=C-bug%2C+I-ICE%2C+T-compiler&template=ice.md\n\nnote: rustc 1.50.0-nightly (b7ebc6b0c 2020-11-30) running on x86_64-pc-windows-msvc\n\nnote: compiler flags: -C embed-bitcode=no -C debuginfo=2 -C incremental --crate-type bin\n\nnote: some of the compiler flags provided by cargo are hidden\n</span></code></pre>\n<p>I think it's fair to say I was pushing the compiler to the limit here. In any event, I opened up <a href=\"https://github.com/rust-lang/rust/issues/79636\">a GitHub issue</a> for this.</p>\n<h2 id=\"mapm-traverse\">mapM/traverse</h2>\n<p>Already, we were stymied by <code>join</code>. How about another popular functional idiom: <code>traverse</code>. As I <a href=\"https://www.fpcomplete.com/blog/collect-rust-traverse-haskell-scala/\">previously mentioned</a>, <code>traverse</code> is incredibly popular in Scala, and pretty common in Haskell. It functions very much like a <code>map</code>, except the result of each step through the <code>map</code> is wrapped in some <code>Applicative</code>, and the <code>Applicative</code> values are combined into an overall data structure.</p>\n<p>Sound confusing? Fair enough. As a simpler example: if you have a <code>Vec<A></code> value, and a function from <code>A</code> to <code>Option<B></code>, <code>traverse</code> can put these together into an <code>Option<Vec<B>></code>. Or using the <code>Validation</code> type we had above, you could combine <code>Vec<A></code> and <code>Fn(A) -> Validation<B, Vec<MyErr>></code> into a <code>Validation<Vec<B>, Vec<MyErr>></code>, returning either all of the successfully generated <code>B</code> values, or all of the errors that occurred along the way.</p>\n<p>Anyway, I ended up with this as a starting type signature for our function:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">traverse</span><span style=\"color:#657b83;\"><F, M, A, B, I>(</span><span style=\"color:#268bd2;\">iter</span><span style=\"color:#657b83;\">: I, </span><span style=\"color:#268bd2;\">f</span><span style=\"color:#657b83;\">: F) -> </span><span style=\"color:#268bd2;\">M::</span><span style=\"color:#657b83;\">Wrapped<</span><span style=\"color:#859900;\">Vec</span><span style=\"color:#657b83;\"><B>>\n</span></code></pre>\n<p>Then we have the following trait bounds:</p>\n<ul>\n<li><code>I: IntoIterator<Item = A></code>: <code>I</code> is an iterator of <code>A</code> values. To simplify, you can think of it as <code>Vec<A></code>.j</li>\n<li><code>M: Applicative<Unwrapped = B></code>: <code>M</code> is some implementation of <code>Applicative</code> which unwraps to a <code>B</code>. In our example: this would be <code>Validation<B, Vec<MyErr>></code>.</li>\n<li><code>F: FnMut(A) -> M</code>: <code>F</code> is a function that takes the <code>A</code> values from the iterator and produces <code>M</code> values.</li>\n<li><code>M::Wrapped<Vec<B>>: Applicative<Unwrapped = Vec<B>></code>: wrapping up the result <code>Vec<B></code> in <code>M</code>'s wrapping produces a value which is also an <code>Applicative</code></li>\n</ul>\n<p>This last bullet shows one of the pain points I mentioned above. Since we don't really know from the <code>Wrapped</code> associated type itself much at all, we only get the <code>Functor</code> bound "for free". We need to explicitly say that it's also <code>Applicative</code>, and that unwrapping it again will get you back a <code>Vec<B></code>.</p>\n<p>In any event, I wasn't clever enough to figure out a way to make all of this compile. This was the final version of the code I came up with:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">traverse</span><span style=\"color:#657b83;\"><F, M, A, B, I>(</span><span style=\"color:#268bd2;\">iter</span><span style=\"color:#657b83;\">: I, </span><span style=\"color:#268bd2;\">f</span><span style=\"color:#657b83;\">: F) -> </span><span style=\"color:#268bd2;\">M::</span><span style=\"color:#657b83;\">Wrapped<</span><span style=\"color:#859900;\">Vec</span><span style=\"color:#657b83;\"><B>>\n</span><span style=\"color:#859900;\">where</span><span style=\"color:#657b83;\">\n F: FnMut(A) -> M,\n M: Applicative<Unwrapped = B>,\n I: </span><span style=\"color:#859900;\">IntoIterator</span><span style=\"color:#657b83;\"><Item = A>,\n </span><span style=\"color:#268bd2;\">M::</span><span style=\"color:#657b83;\">Wrapped<</span><span style=\"color:#859900;\">Vec</span><span style=\"color:#657b83;\"><B>>: Applicative<Unwrapped = </span><span style=\"color:#859900;\">Vec</span><span style=\"color:#657b83;\"><B>>,\n{\n </span><span style=\"color:#268bd2;\">let </span><span style=\"color:#586e75;\">mut</span><span style=\"color:#657b83;\"> iter = iter.</span><span style=\"color:#859900;\">into_iter</span><span style=\"color:#657b83;\">().</span><span style=\"color:#859900;\">map</span><span style=\"color:#657b83;\">(f);\n\n </span><span style=\"color:#268bd2;\">let </span><span style=\"color:#586e75;\">mut</span><span style=\"color:#657b83;\"> result: M::Wrapped<</span><span style=\"color:#859900;\">Vec</span><span style=\"color:#657b83;\"><B>> = </span><span style=\"color:#859900;\">match</span><span style=\"color:#657b83;\"> iter.</span><span style=\"color:#859900;\">next</span><span style=\"color:#657b83;\">() {\n </span><span style=\"color:#859900;\">Some</span><span style=\"color:#657b83;\">(b) </span><span style=\"color:#859900;\">=></span><span style=\"color:#657b83;\"> b.</span><span style=\"color:#859900;\">map</span><span style=\"color:#657b83;\">(|</span><span style=\"color:#268bd2;\">x</span><span style=\"color:#657b83;\">| </span><span style=\"color:#859900;\">vec!</span><span style=\"color:#657b83;\">[x]),\n </span><span style=\"color:#859900;\">None => return </span><span style=\"color:#657b83;\">M::wrap(</span><span style=\"color:#859900;\">Vec</span><span style=\"color:#657b83;\">::new()),\n };\n\n </span><span style=\"color:#859900;\">for</span><span style=\"color:#657b83;\"> m </span><span style=\"color:#859900;\">in</span><span style=\"color:#657b83;\"> iter {\n result = result.</span><span style=\"color:#859900;\">lift_a2</span><span style=\"color:#657b83;\">(m, |</span><span style=\"color:#268bd2;\">vec</span><span style=\"color:#657b83;\">, </span><span style=\"color:#268bd2;\">b</span><span style=\"color:#657b83;\">| {\n vec.</span><span style=\"color:#859900;\">push</span><span style=\"color:#657b83;\">(b);\n vec\n });\n }\n\n result\n}\n</span></code></pre>\n<p>But this fails with the error messages:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#657b83;\">error[E0308]: mismatched types\n --> src\\main.rs:448:33\n |\n433 | fn traverse<F, M, A, B, I>(iter: I, f: F) -> M::Wrapped<Vec<B>>\n | - this type parameter\n...\n448 | result = result.lift_a2(m, |vec, b| {\n | ^ expected associated type, found type parameter `M`\n |\n = note: expected associated type `<<M as Functor>::Wrapped<Vec<B>> as Functor>::Wrapped<_>`\n found type parameter `M`\n = note: you might be missing a type parameter or trait bound\n\nerror[E0308]: mismatched types\n --> src\\main.rs:448:18\n |\n433 | fn traverse<F, M, A, B, I>(iter: I, f: F) -> M::Wrapped<Vec<B>>\n | - this type parameter\n...\n448 | result = result.lift_a2(m, |vec, b| {\n | __________________^\n449 | | vec.push(b);\n450 | | vec\n451 | | });\n | |__________^ expected type parameter `M`, found associated type\n |\n = note: expected associated type `<M as Functor>::Wrapped<Vec<B>>`\n found associated type `<<M as Functor>::Wrapped<Vec<B>> as Functor>::Wrapped<Vec<B>>`\nhelp: consider further restricting this bound\n |\n436 | M: Applicative<Unwrapped = B> + Functor<Wrapped = M>,\n | ^^^^^^^^^^^^^^^^^^^^^^\n</span></code></pre>\n<p>Maybe this is a limitation in GATs. Maybe I'm just not clever enough to figure it out. But I thought this was a good point to call it quits. If anyone knows a trick to make this work, let me know!</p>\n<h2 id=\"should-we-have-hkts-in-rust\">Should we have HKTs in Rust?</h2>\n<p>This was a fun adventure. GATs look like a nice extension to the trait system in Rust. I look forward to the feature stabilizing and landing. And it's certainly fun to play with all of this.</p>\n<p>But Rust is not Haskell. The ergonomics of GATs, in my opinion, will never compete with higher kinded types on Haskell's home turf. And I'm not at all convinced that it should. Rust is a wonderful language as is. I'm happy to write Rust style in a Rust codebase, and save my Haskell coding for my Haskell codebases.</p>\n<p>I hope others enjoyed this adventure as much as I have. A really ugly version of my code is available <a href=\"https://gist.github.com/snoyberg/91ae892199bc8a6687d3798343a9ee54\">as a Gist</a>. You'll need to use a recent nightly Rust build, but otherwise it has no dependencies.</p>\n<p>If you liked this post, you may be interested in some other Haskell/Rust hybrid posts:</p>\n<ul>\n<li><a href=\"https://www.fpcomplete.com/blog/2017/07/iterators-streams-rust-haskell/\">Iterators and Streams in Rust and Haskell</a></li>\n<li><a href=\"https://www.fpcomplete.com/blog/2018/07/streaming-utf8-haskell-rust/\">Streaming UTF-8 in Haskell and Rust</a></li>\n<li><a href=\"https://www.fpcomplete.com/blog/2018/10/is-rust-functional/\">Is Rust functional?</a></li>\n<li><a href=\"https://www.fpcomplete.com/blog/async-exceptions-haskell-rust/\">Async Exceptions in Haskell, and Rust</a></li>\n</ul>\n<p>FP Complete offers training, consulting, and review services in both Haskell and Rust. Want to hear more? <a href=\"https://www.fpcomplete.com/contact-us/\">Contact us to speak with one of our engineers about how we can help.</a></p>\n",
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"content": "<p>This blog post will use mostly Rust and Haskell code snippets to demonstrate its points. But I don't believe the core point is language-specific at all.</p>\n<p>Here's a bit of Rust code to read the contents of <code>input.txt</code> and print it to <code>stdout</code>. What's wrong with it?</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">main</span><span style=\"color:#657b83;\">() {\n </span><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> s = std::fs::read_to_string(</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">input.txt</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">).</span><span style=\"color:#859900;\">unwrap</span><span style=\"color:#657b83;\">();\n </span><span style=\"color:#859900;\">println!</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#cb4b16;\">{}</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, s);\n}\n</span></code></pre>\n<p>If you're Rust-fluent, that <code>.unwrap()</code> may stick out to you like a sore thumb. You know it means "convert any error that occurred into a panic." And panics are a Bad Thing. It's not correct error handling. Instead, something like this is "better":</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">main</span><span style=\"color:#657b83;\">() {\n </span><span style=\"color:#859900;\">match </span><span style=\"color:#657b83;\">std::fs::read_to_string(</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">input.txt</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">) {\n </span><span style=\"color:#859900;\">Ok</span><span style=\"color:#657b83;\">(s) </span><span style=\"color:#859900;\">=> println!</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#cb4b16;\">{}</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, s),\n </span><span style=\"color:#859900;\">Err</span><span style=\"color:#657b83;\">(e) </span><span style=\"color:#859900;\">=> eprintln!</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">Unable to read from input.txt: </span><span style=\"color:#cb4b16;\">{:?}</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, e),\n }\n}\n</span></code></pre>\n<p>The presence of <code>enum</code>s in Rust makes it really easy to ensure you properly handle all failure cases fully. The code above will not panic. If an I/O error occurs, such as file not found, permissions denied, or a hardware failure, it will print an error message to <code>stderr</code>. But this <em>still</em> isn't good error handling, for two reasons:</p>\n<ol>\n<li>The exit code of the program doesn't indicate an error occurred. We'd need to use something like <a href=\"https://doc.rust-lang.org/stable/std/process/fn.abort.html\"><code>abort</code></a> to fix that, which isn't too hard. But it's something else to remember.</li>\n<li>This is <em>very</em> verbose! We've got a trivial little program here, and we're obscuring the actual behavior of the program with all of this line noise around matching different <code>enum</code> variants.</li>\n</ol>\n<p>Fortunately, the Rust language is benevolent, and it makes it possible to do things <em>even better</em> than before. The <code>?</code> operator will try to do something, and automatically short-circuit if an error occurs. We now get to avoid those pesky panics without cluttering our code. And we get the proper exit code to boot!</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">main</span><span style=\"color:#657b83;\">() -> </span><span style=\"color:#859900;\">Result</span><span style=\"color:#657b83;\"><(), std::io::Error> {\n </span><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> s = std::fs::read_to_string(</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">input.txt</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">)</span><span style=\"color:#859900;\">?</span><span style=\"color:#657b83;\">;\n </span><span style=\"color:#859900;\">println!</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#cb4b16;\">{}</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, s);\n </span><span style=\"color:#859900;\">Ok</span><span style=\"color:#657b83;\">(())\n}\n</span></code></pre>\n<p>All is good in the world, we can stop this post here and go home. The greatest marvel of error handling has arrived!</p>\n<h2 id=\"look-again\">Look again</h2>\n<p>So it turns out I forgot to create my <code>input.txt</code> file. Let's see the beautiful error message generated by my program:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#657b83;\">Error: Os { code: 2, kind: NotFound, message: "The system cannot find the file specified." }\n</span></code></pre>\n<p>Huh... that's thoroughly unhelpful. In my 5-line program, it's trivial enough to figure out which file doesn't exist. But imagine a 5,000 line program. Or if the code in question is in a dependency. Or if you're a member of the ops team, have never written a line of Rust in your life, don't have access to the codebase, the production server is down at 2am, and you see this error message in your logs.</p>\n<h2 id=\"runtime-exceptions-to-the-rescue\">Runtime exceptions to the rescue?</h2>\n<p>Well, <em>obviously</em> this is just because Rust uses error returns instead of Good Ol' Runtime Exceptions. Obviously something like Haskell solves this problem better, right? Well, sort of. With this program, and no <code>input.txt</code>:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#657b83;\">main </span><span style=\"color:#859900;\">= do</span><span style=\"color:#657b83;\">\n s </span><span style=\"color:#859900;\"><-</span><span style=\"color:#657b83;\"> readFile </span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">input.txt</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">\n putStrLn s\n</span></code></pre>\n<p>I do in fact get a much nicer error message:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#657b83;\">input.txt: openFile: does not exist (No such file or directory)\n</span></code></pre>\n<p>I didn't even need to include any error handling logic in the code; it's all implicit! But in reality, the clarity of this error message has little to do with exception handling semantics. It has to do with the construction of this specific error message. It contains enough information to help debug this.</p>\n<p>But there are plenty of counterexamples in Haskell. Calling <code>head</code> on an empty list provides a line number these days, but you used to just get an error that "oops, tried to <code>head</code> an empty list, somewhere, in one of your libraries. Good luck!" Some low-level network functionality still gives vague error messages.</p>\n<p>And even the glorious <code>does not exist</code> message above is only marginally useful. And that's because of...</p>\n<h2 id=\"context\">Context!</h2>\n<p>In a trivial 2-line program, the reality is that "file not found" without any additional information is perfectly reasonable. That's because I know <em>exactly</em> the context in which the error occurred. It either occurred on line 1, or line 2. By contrast, in a 500k SLOC codebase, knowing that <code>input.txt</code> doesn't exist is probably not nearly enough to debug things.</p>\n<ul>\n<li>What content is <code>input.txt</code> supposed to have?</li>\n<li>What part of the code was trying to read it?</li>\n<li>What was I going to do with the contents of the file?</li>\n</ul>\n<p>Similarly, knowing that I can't connect to IP address 255.813.20.1 may be sufficient in a small network test. But in a reasonably complicated program, I'd <em>much</em> rather get the context that I'm trying to make an HTTPS request to example.com proxied through a server with IP address 255.813.20.1, which was specified via the <code>HTTP_PROXY</code> environment variable. That last bit of information may shortcircuit days of debugging to point out "doh, I had a typo in my Kubernetes manifest file!"</p>\n<p>Stack traces are often a huge help here. They tell you a lot of useful context. And both Rust and Haskell are particularly weak at providing this context in their error representations. But it's still not a panacea. The ugly reality is that...</p>\n<h2 id=\"there-s-an-inherent-trade-off\">There's an inherent trade-off</h2>\n<p>Like so many other things, error handling ultimately is a trade-off. When we're writing our initial code, <strong>we don't want to think about errors</strong>. We code to the happy path. How productive would you be if you had to derail every line of code with thought processes around the myriad ways your code could fail?</p>\n<p>But then we're debugging a production issue, and <strong>we definitely want to think about errors</strong>. We curse our lazy selves for not handling an error case that <em>obviously</em> could have arisen. "Why did I decide to abort the process when the TCP connection failed? I should have retried! I should have logged the address I tried to connect to!"</p>\n<p>Then we flood our code with log messages, and are frustrated when we can't see the important bits.</p>\n<p>Finding the right balance is an art. And typically it's an art that we don't spend enough time thinking about. There are some well-established tools for this, like runtime-configurable log levels. That's a huge step in the right direction.</p>\n<p>Rust is such a great example of this. Explicit <code>match</code>ing on <code>Result</code> values really forces you to think through all of the different error cases and how to report them correctly. Complex custom <code>enum</code> error types allow you to define all of the different values you'd want reported. But all of this adds huge line noise compared to <code>?</code>. So <code>?</code> wins the day.</p>\n<h2 id=\"the-method-is-secondary\">The method is secondary</h2>\n<p>The Rust community accepts that panics are bad. The Haskell community constantly argues about whether runtime exceptions are a good or bad thing. Java is either loved or hated for checked exceptions. Golang is either lauded or mocked for <code>if err != nil</code>.</p>\n<p>I'm not at all arguing that those discussions are irrelevant. There are significant trade-offs to these various approaches. They affect performance, trackability of errors, and more.</p>\n<p>What I'm arguing here is that we spend a disproportionate time on how we report and recover from errors, and far less on discussing what a good error actually contains.</p>\n<h2 id=\"my-ideal\">My ideal</h2>\n<p>These are evolving thoughts for me. So take them with a grain of salt. And I'm very interested to hear differing opinions.</p>\n<p>I've long held that in Haskell, we should use runtime exceptions. This has been interpreted by many as my <em>advocacy</em> of runtime exceptions. Instead, I would advocate: use the language's native mechanism. I don't pine for exceptions when writing Rust. Quite the opposite in fact. I overall prefer explicit error handling. But it's not worth fighting the battle against runtime exceptions when they are already ubiquitous.</p>\n<p>I think Rust and Haskell are both close to the sweet spot in error handling. There's relatively little verbosity around adding this handling. If you leverage libraries like <a href=\"https://crates.io/crates/anyhow\"><code>anyhow</code></a> in Rust, there's even less.</p>\n<p>My biggest concern with a library like <code>anyhow</code> is how easy it becomes to do the wrong thing. Taking our broken example from above. It's trivial to "upgrade" it to use <code>anyhow</code>:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">main</span><span style=\"color:#657b83;\">() -> anyhow::</span><span style=\"color:#859900;\">Result</span><span style=\"color:#657b83;\"><()> {\n </span><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> s = std::fs::read_to_string(</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">input.txt</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">)</span><span style=\"color:#859900;\">?</span><span style=\"color:#657b83;\">;\n </span><span style=\"color:#859900;\">println!</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#cb4b16;\">{}</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, s);\n </span><span style=\"color:#859900;\">Ok</span><span style=\"color:#657b83;\">(())\n}\n</span></code></pre>\n<p>However, this still produces the same useless error message we started with. Instead, we need to be a bit more explicit with a <code>context</code> method call to get a nicer message:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#859900;\">use </span><span style=\"color:#657b83;\">anyhow::Context;\n\n</span><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">main</span><span style=\"color:#657b83;\">() -> anyhow::</span><span style=\"color:#859900;\">Result</span><span style=\"color:#657b83;\"><()> {\n </span><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> s = std::fs::read_to_string(</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">input.txt</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">)\n .</span><span style=\"color:#859900;\">context</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">Failed to read input.txt</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">)</span><span style=\"color:#859900;\">?</span><span style=\"color:#657b83;\">;\n </span><span style=\"color:#859900;\">println!</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#cb4b16;\">{}</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, s);\n </span><span style=\"color:#859900;\">Ok</span><span style=\"color:#657b83;\">(())\n}\n</span></code></pre>\n<p>Now we get the much more helpful error message:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#657b83;\">Error: Failed to read input.txt\n\nCaused by:\n The system cannot find the file specified. (os error 2)\n</span></code></pre>\n<p>This is a good balance of concision and helpfulness. The downside is that lack of <em>enforcement</em>. Nothing forced me to add the <code>.context</code> call. I worry that in a large codebase, or under time pressure, people like me will end up forgetting to add the helpful context.</p>\n<p>Could we design a modified <code>anyhow</code> that <em>forces</em> a <code>context</code> call? Certainly. But:</p>\n<ol>\n<li>It will lose out on the current simple ergonomics.</li>\n<li>No tool can force the "right" level of context, that requires human insight and thought. And those are quantities in short supply, and not usually interested in error messages.</li>\n</ol>\n<h2 id=\"advice\">Advice</h2>\n<p>I don't have an answer here. I would advise people to start by recognizing that good error handling is <em>difficult</em>. We like to think of it as a trivial but tedious task. It isn't. Doing this correctly requires real thought and design. We're too quick to sweep it under the rug as the unimportant parts of our code.</p>\n<p>I'll continue with my general advice of using your language's preferred mechanisms for error handling. In Rust, that means using <code>Result</code> and avoiding panics. In Haskell, it means some mixture of explicit <code>Either</code> return values and runtime exceptions (the exact mixture very much up for debate). In Java, it's mostly checked exceptions, though there's plenty of added unchecked exceptions to gum up the works too.</p>\n<p>But consider spending a bit more time on thinking through not just <em>how</em> to report/raise/throw an error/exception, but what exactly you're reporting/raising/throwing. Think of the poor ops guy drinking his 7th cup of coffee at 4am trying to figure out what part of the codebase needs <code>input.txt</code>, or why in the world the program is trying to connect to an invalid IP address.</p>\n",
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"content": "<p>Most of the web services I've written in Rust have used <code>actix-web</code>. Recently, I needed to write something that will provide some reverse proxy functionality. I'm more familiar with the hyper-powered HTTP client libraries (<code>reqwest</code> in particular). I decided this would be a good time to experiment again with hyper on the server side as well. The theory was that having matching <code>Request</code> and <code>Response</code> types between the client and server would work nicely. And it certainly did.</p>\n<p>In the process, I ended up with an interesting example of battling ownership through closures and async blocks. This is a topic I typically mention in my Rust training sessions as the hardest thing I had to learn when learning Rust. So I figure a blog post demonstrating one of these crazy cases would be worthwhile.</p>\n<p>Side note: If you're interested in learning more about Rust, we'll be offering a <a href=\"https://www.fpcomplete.com/training/\">free Rust training course</a> in December. Sign up for more information.</p>\n<h2 id=\"cargo-toml\">Cargo.toml</h2>\n<p>If you want to play along, you should start off with a <code>cargo new</code>. I'm using the following <code>[dependencies]</code> in my <code>Cargo.toml</code></p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#657b83;\">[</span><span style=\"color:#b58900;\">dependencies</span><span style=\"color:#657b83;\">]\n</span><span style=\"color:#268bd2;\">hyper </span><span style=\"color:#657b83;\">= </span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">0.13</span><span style=\"color:#839496;\">"\n</span><span style=\"color:#268bd2;\">tokio </span><span style=\"color:#657b83;\">= { </span><span style=\"color:#268bd2;\">version </span><span style=\"color:#657b83;\">= </span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">0.2</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, </span><span style=\"color:#268bd2;\">features </span><span style=\"color:#657b83;\">= [</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">full</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">] }\n</span><span style=\"color:#268bd2;\">log </span><span style=\"color:#657b83;\">= </span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">0.4.11</span><span style=\"color:#839496;\">"\n</span><span style=\"color:#268bd2;\">env_logger </span><span style=\"color:#657b83;\">= </span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">0.8.1</span><span style=\"color:#839496;\">"\n</span><span style=\"color:#268bd2;\">hyper-tls </span><span style=\"color:#657b83;\">= </span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">0.4.3</span><span style=\"color:#839496;\">"\n</span></code></pre>\n<p>I'm also compiling with Rust version 1.47.0. If you'd like, you can add <code>1.47.0</code> to your <code>rust-toolchain</code>. And finally, my full <code>Cargo.lock</code> is <a href=\"https://gist.github.com/snoyberg/550a96c3888a2563f20afcec2c652801\">available as a Gist</a>.</p>\n<h2 id=\"basic-web-service\">Basic web service</h2>\n<p>To get started with a hyper-powered web service, we can use the example straight from the <a href=\"https://hyper.rs/\">hyper homepage</a>:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#859900;\">use </span><span style=\"color:#657b83;\">std::{convert::Infallible, net::SocketAddr};\n</span><span style=\"color:#859900;\">use </span><span style=\"color:#657b83;\">hyper::{Body, Request, Response, Server};\n</span><span style=\"color:#859900;\">use </span><span style=\"color:#657b83;\">hyper::service::{make_service_fn, service_fn};\n\nasync </span><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">handle</span><span style=\"color:#657b83;\">(</span><span style=\"color:#859900;\">_</span><span style=\"color:#657b83;\">: Request<Body>) -> </span><span style=\"color:#859900;\">Result</span><span style=\"color:#657b83;\"><Response<Body>, Infallible> {\n </span><span style=\"color:#859900;\">Ok</span><span style=\"color:#657b83;\">(Response::new(</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">Hello, World!</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">.</span><span style=\"color:#859900;\">into</span><span style=\"color:#657b83;\">()))\n}\n\n#[</span><span style=\"color:#268bd2;\">tokio</span><span style=\"color:#657b83;\">::</span><span style=\"color:#268bd2;\">main</span><span style=\"color:#657b83;\">]\nasync </span><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">main</span><span style=\"color:#657b83;\">() {\n </span><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> addr = SocketAddr::from(([</span><span style=\"color:#6c71c4;\">127</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">0</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">0</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">1</span><span style=\"color:#657b83;\">], </span><span style=\"color:#6c71c4;\">3000</span><span style=\"color:#657b83;\">));\n\n </span><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> make_svc = </span><span style=\"color:#859900;\">make_service_fn</span><span style=\"color:#657b83;\">(|</span><span style=\"color:#268bd2;\">_conn</span><span style=\"color:#657b83;\">| async {\n </span><span style=\"color:#859900;\">Ok</span><span style=\"color:#657b83;\">::<</span><span style=\"color:#859900;\">_</span><span style=\"color:#657b83;\">, Infallible>(</span><span style=\"color:#859900;\">service_fn</span><span style=\"color:#657b83;\">(handle))\n });\n\n </span><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> server = Server::bind(</span><span style=\"color:#859900;\">&</span><span style=\"color:#657b83;\">addr).</span><span style=\"color:#859900;\">serve</span><span style=\"color:#657b83;\">(make_svc);\n\n </span><span style=\"color:#859900;\">if </span><span style=\"color:#268bd2;\">let </span><span style=\"color:#859900;\">Err</span><span style=\"color:#657b83;\">(e) = server.await {\n </span><span style=\"color:#859900;\">eprintln!</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">server error: </span><span style=\"color:#cb4b16;\">{}</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, e);\n }\n}\n</span></code></pre>\n<p>It's worth explaining this a little bit, since at least in my opinion the distinction between <code>make_service_fn</code> and <code>service_fn</code> wasn't clear. There are two different things we're trying to create here:</p>\n<ul>\n<li>A <code>MakeService</code>, which takes a <code>&AddrStream</code> and gives back a <code>Service</code></li>\n<li>A <code>Service</code>, which takes a <code>Request</code> and gives back a <code>Response</code></li>\n</ul>\n<p>This glosses over a number of details, such as:</p>\n<ul>\n<li>Error handling</li>\n<li>Everything is async (<code>Future</code>s are everywhere)</li>\n<li>Everything is expressed in terms of general purpose <code>trait</code>s</li>\n</ul>\n<p>To help us with that "glossing", hyper provides two convenience functions for creating <code>MakeService</code> and <code>Service</code> values, <code>make_service_fn</code> and <code>service_fn</code>. Each of these will convert a closure into their respective types. Then the <code>MakeService</code> closure can return a <code>Service</code> value, and the <code>MakeService</code> value can be provided to <code>hyper::server::Builder::serve</code>. Let's get even more concrete from the code above:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#657b83;\">async </span><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">handle</span><span style=\"color:#657b83;\">(</span><span style=\"color:#859900;\">_</span><span style=\"color:#657b83;\">: Request<Body>) -> </span><span style=\"color:#859900;\">Result</span><span style=\"color:#657b83;\"><Response<Body>, Infallible> {</span><span style=\"color:#859900;\">...</span><span style=\"color:#657b83;\">}\n</span><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> make_svc = </span><span style=\"color:#859900;\">make_service_fn</span><span style=\"color:#657b83;\">(|</span><span style=\"color:#268bd2;\">_conn</span><span style=\"color:#657b83;\">| async {\n </span><span style=\"color:#859900;\">Ok</span><span style=\"color:#657b83;\">::<</span><span style=\"color:#859900;\">_</span><span style=\"color:#657b83;\">, Infallible>(</span><span style=\"color:#859900;\">service_fn</span><span style=\"color:#657b83;\">(handle))\n});\n</span></code></pre>\n<p>The <code>handle</code> function takes a <code>Request<Body></code> and returns a <code>Future<Output=Result<Response<Body, Infallible>>></code>. The <code>Infallible</code> is a nice way of saying "no errors can possibly occur here." The type signatures at play require that we use a <code>Result</code>, but morally <code>Result<T, Infallible></code> is equivalent to <code>T</code>.</p>\n<p><code>service_fn</code> converts this <code>handle</code> value into a <code>Service</code> value. This new value implements all of the appropriate traits to satisfy the requirements of <code>make_service_fn</code> and <code>serve</code>. We wrap up that new <code>Service</code> in its own <code>Result<_, Infallible></code>, ignore the input <code>&AddrStream</code> value, and pass all of this to <code>make_service_fn</code>. <code>make_svc</code> is now a value that can be passed to <code>serve</code>, and we have "Hello, world!"</p>\n<p>And if all of this seems a bit complicated for a "Hello world," you may understand why there are lots of frameworks built on top of hyper to make it easier to work with. Anyway, onwards!</p>\n<h2 id=\"initial-reverse-proxy\">Initial reverse proxy</h2>\n<p>Next up, we want to modify our <code>handle</code> function to perform a reverse proxy instead of returning the "Hello, World!" text. For this example, we're going to hard-code <code>https://www.fpcomplete.com</code> as the destination site for this reverse proxy. To make this happen, we'll need to:</p>\n<ul>\n<li>Construct a <code>Request</code> value, based on the incoming <code>Request</code>'s request headers and path, but targeting the <code>www.fpcomplete.com</code> server</li>\n<li>Construct a <code>Client</code> value from hyper with TLS support</li>\n<li>Perform the request</li>\n<li>Return the <code>Response</code> as the response from <code>handle</code></li>\n<li>Introduce error handling</li>\n</ul>\n<p>I'm also going to move over to the <code>env-logger</code> and <code>log</code> crates for producing output. I did this when working on the code myself, and switching to <code>RUST_LOG=debug</code> was a great way to debug things. (When I was working on this, I forgot I needed to create a special <code>Client</code> with TLS support.)</p>\n<p>So from the top! We now have the following <code>use</code> statements:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#859900;\">use </span><span style=\"color:#657b83;\">hyper::service::{make_service_fn, service_fn};\n</span><span style=\"color:#859900;\">use </span><span style=\"color:#657b83;\">hyper::{Body, Client, Request, Response, Server};\n</span><span style=\"color:#859900;\">use </span><span style=\"color:#657b83;\">hyper_tls::HttpsConnector;\n</span><span style=\"color:#859900;\">use </span><span style=\"color:#657b83;\">std::net::SocketAddr;\n</span></code></pre>\n<p>We next have three constants. The <code>SCHEME</code> and <code>HOST</code> are pretty self-explanatory: the hardcoded destination.</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">const </span><span style=\"color:#cb4b16;\">SCHEME</span><span style=\"color:#657b83;\">: </span><span style=\"color:#859900;\">&</span><span style=\"color:#268bd2;\">str </span><span style=\"color:#657b83;\">= </span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">https</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">;\n</span><span style=\"color:#268bd2;\">const </span><span style=\"color:#cb4b16;\">HOST</span><span style=\"color:#657b83;\">: </span><span style=\"color:#859900;\">&</span><span style=\"color:#268bd2;\">str </span><span style=\"color:#657b83;\">= </span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">www.fpcomplete.com</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">;\n</span></code></pre>\n<p>Next we have some HTTP request headers that should <em>not</em> be forwarded onto the destination server. This blacklist approach to HTTP headers in reverse proxies works well enough. It's probably a better idea in general to follow a whitelist approach. In any event, these six headers have the potential to change behavior at the transport layer, and therefore cannot be passed on from the client:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#93a1a1;\">/// HTTP headers to strip, a whitelist is probably a better idea\n</span><span style=\"color:#268bd2;\">const </span><span style=\"color:#cb4b16;\">STRIPPED</span><span style=\"color:#657b83;\">: [</span><span style=\"color:#859900;\">&</span><span style=\"color:#268bd2;\">str</span><span style=\"color:#657b83;\">; </span><span style=\"color:#6c71c4;\">6</span><span style=\"color:#657b83;\">] = [\n </span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">content-length</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">,\n </span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">transfer-encoding</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">,\n </span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">accept-encoding</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">,\n </span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">content-encoding</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">,\n </span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">host</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">,\n </span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">connection</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">,\n];\n</span></code></pre>\n<p>And next we have a fairly boilerplate error type definition. We can generate a <code>hyper::Error</code> when performing the HTTP request to the destination server, and a <code>hyper::http::Error</code> when constructing the new <code>Request</code>. Arguably we should simply panic if the latter error occurs, since it indicates programmer error. But I've decided to treat it as its own error variant. So here's some boilerplate!</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#657b83;\">#[</span><span style=\"color:#268bd2;\">derive</span><span style=\"color:#657b83;\">(Debug)]\n</span><span style=\"color:#268bd2;\">enum </span><span style=\"color:#b58900;\">ReverseProxyError </span><span style=\"color:#657b83;\">{\n Hyper(hyper::Error),\n HyperHttp(hyper::http::Error),\n}\n\n</span><span style=\"color:#268bd2;\">impl </span><span style=\"color:#859900;\">From</span><span style=\"color:#657b83;\"><hyper::Error> </span><span style=\"color:#859900;\">for </span><span style=\"color:#b58900;\">ReverseProxyError </span><span style=\"color:#657b83;\">{\n </span><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">from</span><span style=\"color:#657b83;\">(</span><span style=\"color:#268bd2;\">e</span><span style=\"color:#657b83;\">: hyper::Error) -> </span><span style=\"color:#268bd2;\">Self </span><span style=\"color:#657b83;\">{\n ReverseProxyError::Hyper(e)\n }\n}\n\n</span><span style=\"color:#268bd2;\">impl </span><span style=\"color:#859900;\">From</span><span style=\"color:#657b83;\"><hyper::http::Error> </span><span style=\"color:#859900;\">for </span><span style=\"color:#b58900;\">ReverseProxyError </span><span style=\"color:#657b83;\">{\n </span><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">from</span><span style=\"color:#657b83;\">(</span><span style=\"color:#268bd2;\">e</span><span style=\"color:#657b83;\">: hyper::http::Error) -> </span><span style=\"color:#268bd2;\">Self </span><span style=\"color:#657b83;\">{\n ReverseProxyError::HyperHttp(e)\n }\n}\n\n</span><span style=\"color:#268bd2;\">impl </span><span style=\"color:#657b83;\">std::fmt::Display </span><span style=\"color:#859900;\">for </span><span style=\"color:#b58900;\">ReverseProxyError </span><span style=\"color:#657b83;\">{\n </span><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">fmt</span><span style=\"color:#657b83;\">(</span><span style=\"color:#859900;\">&</span><span style=\"color:#268bd2;\">self</span><span style=\"color:#657b83;\">, </span><span style=\"color:#268bd2;\">fmt</span><span style=\"color:#657b83;\">: </span><span style=\"color:#859900;\">&</span><span style=\"color:#586e75;\">mut </span><span style=\"color:#657b83;\">std::fmt::Formatter) -> std::fmt::Result {\n </span><span style=\"color:#859900;\">write!</span><span style=\"color:#657b83;\">(fmt, </span><span style=\"color:#839496;\">"</span><span style=\"color:#cb4b16;\">{:?}</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, </span><span style=\"color:#d33682;\">self</span><span style=\"color:#657b83;\">)\n }\n}\n\n</span><span style=\"color:#268bd2;\">impl </span><span style=\"color:#657b83;\">std::error::Error </span><span style=\"color:#859900;\">for </span><span style=\"color:#b58900;\">ReverseProxyError </span><span style=\"color:#657b83;\">{}\n</span></code></pre>\n<p>With all of this in place, we can finally start writing our <code>handle</code> function:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#657b83;\">async </span><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">handle</span><span style=\"color:#657b83;\">(</span><span style=\"color:#586e75;\">mut </span><span style=\"color:#268bd2;\">req</span><span style=\"color:#657b83;\">: Request<Body>) -> </span><span style=\"color:#859900;\">Result</span><span style=\"color:#657b83;\"><Response<Body>, ReverseProxyError> {\n}\n</span></code></pre>\n<p>We're going to mutate the incoming <code>Request</code> to have our new destination, and then pass it along to the destination server. This is where the beauty of using hyper for client <em>and</em> server comes into play: no need to futz around with changing body or header representations. The first thing we do is strip out any of the <code>STRIPPED</code> request headers:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> h = req.</span><span style=\"color:#859900;\">headers_mut</span><span style=\"color:#657b83;\">();\n</span><span style=\"color:#859900;\">for</span><span style=\"color:#657b83;\"> key </span><span style=\"color:#859900;\">in &</span><span style=\"color:#cb4b16;\">STRIPPED </span><span style=\"color:#657b83;\">{\n h.</span><span style=\"color:#859900;\">remove</span><span style=\"color:#657b83;\">(*key);\n}\n</span></code></pre>\n<p>Next, we're going to construct the new request URI by combining:</p>\n<ul>\n<li>The hard-coded scheme (<code>https</code>)</li>\n<li>The hard-coded authority (<code>www.fpcomplete.com</code>)</li>\n<li>The path and query from the incoming request</li>\n</ul>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">let </span><span style=\"color:#586e75;\">mut</span><span style=\"color:#657b83;\"> builder = hyper::Uri::builder()\n .</span><span style=\"color:#859900;\">scheme</span><span style=\"color:#657b83;\">(</span><span style=\"color:#cb4b16;\">SCHEME</span><span style=\"color:#657b83;\">)\n .</span><span style=\"color:#859900;\">authority</span><span style=\"color:#657b83;\">(</span><span style=\"color:#cb4b16;\">HOST</span><span style=\"color:#657b83;\">);\n</span><span style=\"color:#859900;\">if </span><span style=\"color:#268bd2;\">let </span><span style=\"color:#859900;\">Some</span><span style=\"color:#657b83;\">(pq) = req.</span><span style=\"color:#859900;\">uri</span><span style=\"color:#657b83;\">().</span><span style=\"color:#859900;\">path_and_query</span><span style=\"color:#657b83;\">() {\n builder = builder.</span><span style=\"color:#859900;\">path_and_query</span><span style=\"color:#657b83;\">(pq.</span><span style=\"color:#859900;\">clone</span><span style=\"color:#657b83;\">());\n}\n*req.</span><span style=\"color:#859900;\">uri_mut</span><span style=\"color:#657b83;\">() = builder.</span><span style=\"color:#859900;\">build</span><span style=\"color:#657b83;\">()</span><span style=\"color:#859900;\">?</span><span style=\"color:#657b83;\">;\n</span></code></pre>\n<p>Panicking if <code>req.uri().path_and_query()</code> is <code>None</code> would be appropriate here, but as is my wont, I'm avoiding panics if possible. Next, for good measure, let's add in a little bit of debug output:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#657b83;\">log::debug</span><span style=\"color:#859900;\">!</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">request == {:?}</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, req);\n</span></code></pre>\n<p>Now we can construct our <code>Client</code> value to perform the HTTPS request:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> https = HttpsConnector::new();\n</span><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> client = Client::builder().</span><span style=\"color:#859900;\">build</span><span style=\"color:#657b83;\">(https);\n</span></code></pre>\n<p>And finally, let's perform the request, log the response, and return the response:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> response = client.</span><span style=\"color:#859900;\">request</span><span style=\"color:#657b83;\">(req).await</span><span style=\"color:#859900;\">?</span><span style=\"color:#657b83;\">;\nlog::debug</span><span style=\"color:#859900;\">!</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">response == {:?}</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, response);\n</span><span style=\"color:#859900;\">Ok</span><span style=\"color:#657b83;\">(response)\n</span></code></pre>\n<p>Our <code>main</code> function looks pretty similar to what we had before. I've added in initialization of <code>env-logger</code> with a default to <code>info</code> level output, and modified the program to <code>abort</code> if the server produces any errors:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#657b83;\">#[</span><span style=\"color:#268bd2;\">tokio</span><span style=\"color:#657b83;\">::</span><span style=\"color:#268bd2;\">main</span><span style=\"color:#657b83;\">]\nasync </span><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">main</span><span style=\"color:#657b83;\">() {\n env_logger::Builder::from_env(env_logger::Env::default().</span><span style=\"color:#859900;\">default_filter_or</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">info</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">)).</span><span style=\"color:#859900;\">init</span><span style=\"color:#657b83;\">();\n </span><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> addr = SocketAddr::from(([</span><span style=\"color:#6c71c4;\">0</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">0</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">0</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">0</span><span style=\"color:#657b83;\">], </span><span style=\"color:#6c71c4;\">3000</span><span style=\"color:#657b83;\">));\n\n </span><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> make_svc = </span><span style=\"color:#859900;\">make_service_fn</span><span style=\"color:#657b83;\">(|</span><span style=\"color:#268bd2;\">_conn</span><span style=\"color:#657b83;\">| async {\n </span><span style=\"color:#859900;\">Ok</span><span style=\"color:#657b83;\">::<</span><span style=\"color:#859900;\">_</span><span style=\"color:#657b83;\">, ReverseProxyError>(</span><span style=\"color:#859900;\">service_fn</span><span style=\"color:#657b83;\">(handle))\n });\n\n </span><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> server = Server::bind(</span><span style=\"color:#859900;\">&</span><span style=\"color:#657b83;\">addr).</span><span style=\"color:#859900;\">serve</span><span style=\"color:#657b83;\">(make_svc);\n log::info</span><span style=\"color:#859900;\">!</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">Server started, bound on {}</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, addr);\n\n </span><span style=\"color:#859900;\">if </span><span style=\"color:#268bd2;\">let </span><span style=\"color:#859900;\">Err</span><span style=\"color:#657b83;\">(e) = server.await {\n log::error</span><span style=\"color:#859900;\">!</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">server error: {}</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, e);\n std::process::abort();\n }\n}\n</span></code></pre>\n<p>The full code is <a href=\"https://gist.github.com/snoyberg/ab29c50671858e82ed5f6a88f8170449\">available as a Gist</a>. This program works as expected, and if I <code>cargo run</code> it and connect to <code>http://localhost:3000</code>, I see the FP Complete homepage. Yay!</p>\n<h2 id=\"wasteful-client\">Wasteful Client</h2>\n<p>The problem with this program is that it constructs a brand new <code>Client</code> value on every incoming request. That's expensive. Instead, we would like to produce the <code>Client</code> once, in <code>main</code>, and reuse it for each request. And herein lies the ownership puzzle. While we're at this, let's move away from using <code>const</code>s for the scheme and host, and instead bundle together the client, scheme, and host into a new <code>struct</code>:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">struct </span><span style=\"color:#b58900;\">ReverseProxy </span><span style=\"color:#657b83;\">{\n </span><span style=\"color:#268bd2;\">scheme</span><span style=\"color:#657b83;\">: String,\n </span><span style=\"color:#268bd2;\">host</span><span style=\"color:#657b83;\">: String,\n </span><span style=\"color:#268bd2;\">client</span><span style=\"color:#657b83;\">: Client<HttpsConnector<hyper::client::HttpConnector>>,\n}\n</span></code></pre>\n<p>Next, we'll want to change <code>handle</code> from a standalone function to a method on <code>ReverseProxy</code>. (We could equivalently pass in a reference to <code>ReverseProxy</code> for <code>handle</code>, but this feels more idiomatic):</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">impl </span><span style=\"color:#b58900;\">ReverseProxy </span><span style=\"color:#657b83;\">{\n async </span><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">handle</span><span style=\"color:#657b83;\">(</span><span style=\"color:#859900;\">&</span><span style=\"color:#268bd2;\">self</span><span style=\"color:#657b83;\">, </span><span style=\"color:#586e75;\">mut </span><span style=\"color:#268bd2;\">req</span><span style=\"color:#657b83;\">: Request<Body>) -> </span><span style=\"color:#859900;\">Result</span><span style=\"color:#657b83;\"><Response<Body>, ReverseProxyError> {\n </span><span style=\"color:#859900;\">...\n </span><span style=\"color:#657b83;\">}\n}\n</span></code></pre>\n<p>Then, within <code>handle</code>, we can replace <code>SCHEME</code> and <code>HOST</code> with <code>&*self.scheme</code> and <code>&*self.host</code>. You may be wondering "why <code>&*</code> and not <code>&</code>." Without <code>&*</code>, you'll get an error message:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#657b83;\">error[E0277]: the trait bound `hyper::http::uri::Scheme: std::convert::From<&std::string::String>` is not satisfied\n --> src\\main.rs:59:14\n |\n59 | .scheme(&self.scheme)\n | ^^^^^^ the trait `std::convert::From<&std::string::String>` is not implemented for `hyper::http::uri::Scheme`\n</span></code></pre>\n<p>This is one of those examples where the magic of deref coercion seems to fall apart. Personally, I prefer using <code>self.scheme.as_str()</code> instead of <code>&*self.scheme</code> to be more explicit, but <code>&*self.scheme</code> is likely more idiomatic.</p>\n<p>Anyway, the final change within <code>handle</code> is to remove the <code>let https = ...;</code> and <code>let client = ...;</code> statements, and instead construct our response with:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> response = </span><span style=\"color:#d33682;\">self</span><span style=\"color:#657b83;\">.client.</span><span style=\"color:#859900;\">request</span><span style=\"color:#657b83;\">(req).await</span><span style=\"color:#859900;\">?</span><span style=\"color:#657b83;\">;\n</span></code></pre>\n<p>With that, our <code>handle</code> method is done, and we can focus our efforts on the true puzzle: the <code>main</code> function itself.</p>\n<h2 id=\"the-easy-part\">The easy part</h2>\n<p>The easy part of this is great: construct a <code>ReverseProxy</code> value, and provide the <code>make_svc</code> to <code>serve</code>:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#657b83;\">#[</span><span style=\"color:#268bd2;\">tokio</span><span style=\"color:#657b83;\">::</span><span style=\"color:#268bd2;\">main</span><span style=\"color:#657b83;\">]\nasync </span><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">main</span><span style=\"color:#657b83;\">() {\n env_logger::Builder::from_env(env_logger::Env::default().</span><span style=\"color:#859900;\">default_filter_or</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">info</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">)).</span><span style=\"color:#859900;\">init</span><span style=\"color:#657b83;\">();\n </span><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> addr = SocketAddr::from(([</span><span style=\"color:#6c71c4;\">0</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">0</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">0</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">0</span><span style=\"color:#657b83;\">], </span><span style=\"color:#6c71c4;\">3000</span><span style=\"color:#657b83;\">));\n\n </span><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> https = HttpsConnector::new();\n </span><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> client = Client::builder().</span><span style=\"color:#859900;\">build</span><span style=\"color:#657b83;\">(https);\n\n </span><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> rp = ReverseProxy {\n client,\n scheme: </span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">https</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">.</span><span style=\"color:#859900;\">to_owned</span><span style=\"color:#657b83;\">(),\n host: </span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">www.fpcomplete.com</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">.</span><span style=\"color:#859900;\">to_owned</span><span style=\"color:#657b83;\">(),\n };\n\n </span><span style=\"color:#93a1a1;\">// here be dragons\n\n </span><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> server = Server::bind(</span><span style=\"color:#859900;\">&</span><span style=\"color:#657b83;\">addr).</span><span style=\"color:#859900;\">serve</span><span style=\"color:#657b83;\">(make_svc);\n log::info</span><span style=\"color:#859900;\">!</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">Server started, bound on {}</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, addr);\n\n </span><span style=\"color:#859900;\">if </span><span style=\"color:#268bd2;\">let </span><span style=\"color:#859900;\">Err</span><span style=\"color:#657b83;\">(e) = server.await {\n log::error</span><span style=\"color:#859900;\">!</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">server error: {}</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, e);\n std::process::abort();\n }\n}\n</span></code></pre>\n<p>That middle part is where the difficulty lies. Previously, this code looked like:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> make_svc = </span><span style=\"color:#859900;\">make_service_fn</span><span style=\"color:#657b83;\">(|</span><span style=\"color:#268bd2;\">_conn</span><span style=\"color:#657b83;\">| async {\n </span><span style=\"color:#859900;\">Ok</span><span style=\"color:#657b83;\">::<</span><span style=\"color:#859900;\">_</span><span style=\"color:#657b83;\">, ReverseProxyError>(</span><span style=\"color:#859900;\">service_fn</span><span style=\"color:#657b83;\">(handle))\n});\n</span></code></pre>\n<p>We no longer have a <code>handle</code> function. Working around that little enigma doesn't seem so bad initially. We'll create a closure as the argument to <code>service_fn</code>:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> make_svc = </span><span style=\"color:#859900;\">make_service_fn</span><span style=\"color:#657b83;\">(|</span><span style=\"color:#268bd2;\">_conn</span><span style=\"color:#657b83;\">| async {\n </span><span style=\"color:#859900;\">Ok</span><span style=\"color:#657b83;\">::<</span><span style=\"color:#859900;\">_</span><span style=\"color:#657b83;\">, ReverseProxyError>(</span><span style=\"color:#859900;\">service_fn</span><span style=\"color:#657b83;\">(|</span><span style=\"color:#268bd2;\">req</span><span style=\"color:#657b83;\">| {\n rp.</span><span style=\"color:#859900;\">handle</span><span style=\"color:#657b83;\">(req)\n }))\n});\n</span></code></pre>\n<p>While that looks appealing, it fails lifetimes completely:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#657b83;\">error[E0597]: `rp` does not live long enough\n --> src\\main.rs:90:13\n |\n88 | let make_svc = make_service_fn(|_conn| async {\n | ____________________________________-------_-\n | | |\n | | value captured here\n89 | | Ok::<_, ReverseProxyError>(service_fn(|req| {\n90 | | rp.handle(req)\n | | ^^ borrowed value does not live long enough\n91 | | }))\n92 | | });\n | |_____- returning this value requires that `rp` is borrowed for `'static`\n...\n101 | }\n | - `rp` dropped here while still borrowed\n</span></code></pre>\n<p>Nothing in the lifetimes of these values tells us that the <code>ReverseProxy</code> value will outlive the service. We cannot simply borrow a reference to <code>ReverseProxy</code> inside our closure. Instead, we're going to need to move ownership of the <code>ReverseProxy</code> to the closure.</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> make_svc = </span><span style=\"color:#859900;\">make_service_fn</span><span style=\"color:#657b83;\">(|</span><span style=\"color:#268bd2;\">_conn</span><span style=\"color:#657b83;\">| async {\n </span><span style=\"color:#859900;\">Ok</span><span style=\"color:#657b83;\">::<</span><span style=\"color:#859900;\">_</span><span style=\"color:#657b83;\">, ReverseProxyError>(</span><span style=\"color:#859900;\">service_fn</span><span style=\"color:#657b83;\">(</span><span style=\"color:#586e75;\">move </span><span style=\"color:#859900;\">|</span><span style=\"color:#657b83;\">req</span><span style=\"color:#859900;\">| </span><span style=\"color:#657b83;\">{\n rp.</span><span style=\"color:#859900;\">handle</span><span style=\"color:#657b83;\">(req)\n }))\n});\n</span></code></pre>\n<p>Note the addition of <code>move</code> in front of the closure. Unfortunately, this doesn't work, and gives us a confusing error message:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#657b83;\">error[E0495]: cannot infer an appropriate lifetime for autoref due to conflicting requirements\n --> src\\main.rs:90:16\n |\n90 | rp.handle(req)\n | ^^^^^^\n |\nnote: first, the lifetime cannot outlive the lifetime `'_` as defined on the body at 89:47...\n --> src\\main.rs:89:47\n |\n89 | Ok::<_, ReverseProxyError>(service_fn(move |req| {\n | ^^^^^^^^^^\nnote: ...so that closure can access `rp`\n --> src\\main.rs:90:13\n |\n90 | rp.handle(req)\n | ^^\n = note: but, the lifetime must be valid for the static lifetime...\nnote: ...so that the type `hyper::proto::h2::server::H2Stream<impl std::future::Future, hyper::Body>` will meet its required lifetime bounds\n --> src\\main.rs:94:38\n |\n94 | let server = Server::bind(&addr).serve(make_svc);\n | ^^^^^\n\nerror: aborting due to previous error\n</span></code></pre>\n<p>Instead of trying to parse that, let's take a step back, reassess, and then try again.</p>\n<h2 id=\"so-many-layers\">So many layers!</h2>\n<p>Remember way back to the beginning of this post. I went into some details around the process of having a <code>MakeService</code>, which would be run for each new incoming connection, and a <code>Service</code>, which would be run for each new request on an existing connection. The way we've written things so far, the first time we handle a request, that request handler will consume the <code>ReverseProxy</code>. That means that we would have a use-after-move for each subsequent request on that connection. We'd <em>also</em> have a use-after-move for each subsequent connection we receive.</p>\n<p>We want to share our <code>ReverseProxy</code> across multiple different <code>MakeService</code> and <code>Service</code> instantiations. Since this will occur across multiple system threads, the most straightforward way to handle this is to wrap our <code>ReverseProxy</code> in an <code>Arc</code>:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> rp = std::sync::Arc::new(ReverseProxy {\n client,\n scheme: </span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">https</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">.</span><span style=\"color:#859900;\">to_owned</span><span style=\"color:#657b83;\">(),\n host: </span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">www.fpcomplete.com</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">.</span><span style=\"color:#859900;\">to_owned</span><span style=\"color:#657b83;\">(),\n});\n</span></code></pre>\n<p>Now we're going to need to play around with <code>clone</code>ing this <code>Arc</code> at appropriate times. In particular, we'll need to clone twice: once inside the <code>make_service_fn</code> closure, and once inside the <code>service_fn</code> closure. This will ensure that we never move the <code>ReverseProxy</code> value out of the closure's environment, and that our closure can remain a <code>FnMut</code> instead of an <code>FnOnce</code>.</p>\n<p>And, in order to make <em>that</em> happen, we'll need to convince the compiler through appropriate usages of <code>move</code> to move ownership of the <code>ReverseProxy</code>, instead of borrowing a reference to a value with a different lifetime. This is where the fun begins! Let's go through a series of modifications until we get to our final mind-bender.</p>\n<h2 id=\"adding-move\">Adding move</h2>\n<p>To recap, we'll start with this code:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> rp = std::sync::Arc::new(ReverseProxy {\n client,\n scheme: </span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">https</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">.</span><span style=\"color:#859900;\">to_owned</span><span style=\"color:#657b83;\">(),\n host: </span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">www.fpcomplete.com</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">.</span><span style=\"color:#859900;\">to_owned</span><span style=\"color:#657b83;\">(),\n});\n\n</span><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> make_svc = </span><span style=\"color:#859900;\">make_service_fn</span><span style=\"color:#657b83;\">(|</span><span style=\"color:#268bd2;\">_conn</span><span style=\"color:#657b83;\">| async {\n </span><span style=\"color:#859900;\">Ok</span><span style=\"color:#657b83;\">::<</span><span style=\"color:#859900;\">_</span><span style=\"color:#657b83;\">, ReverseProxyError>(</span><span style=\"color:#859900;\">service_fn</span><span style=\"color:#657b83;\">(|</span><span style=\"color:#268bd2;\">req</span><span style=\"color:#657b83;\">| {\n rp.</span><span style=\"color:#859900;\">handle</span><span style=\"color:#657b83;\">(req)\n }))\n});\n</span></code></pre>\n<p>The first thing I tried was adding an <code>rp.clone()</code> inside the first <code>async</code> block:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> make_svc = </span><span style=\"color:#859900;\">make_service_fn</span><span style=\"color:#657b83;\">(|</span><span style=\"color:#268bd2;\">_conn</span><span style=\"color:#657b83;\">| async {\n </span><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> rp = rp.</span><span style=\"color:#859900;\">clone</span><span style=\"color:#657b83;\">();\n </span><span style=\"color:#859900;\">Ok</span><span style=\"color:#657b83;\">::<</span><span style=\"color:#859900;\">_</span><span style=\"color:#657b83;\">, ReverseProxyError>(</span><span style=\"color:#859900;\">service_fn</span><span style=\"color:#657b83;\">(|</span><span style=\"color:#268bd2;\">req</span><span style=\"color:#657b83;\">| {\n rp.</span><span style=\"color:#859900;\">handle</span><span style=\"color:#657b83;\">(req)\n }))\n});\n</span></code></pre>\n<p>This doesn't work, presumably because I need to stick some <code>move</code>s on the initial closure and <code>async</code> block like so:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> make_svc = </span><span style=\"color:#859900;\">make_service_fn</span><span style=\"color:#657b83;\">(</span><span style=\"color:#586e75;\">move </span><span style=\"color:#859900;\">|</span><span style=\"color:#657b83;\">_conn</span><span style=\"color:#859900;\">|</span><span style=\"color:#657b83;\"> async </span><span style=\"color:#586e75;\">move </span><span style=\"color:#657b83;\">{\n </span><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> rp = rp.</span><span style=\"color:#859900;\">clone</span><span style=\"color:#657b83;\">();\n </span><span style=\"color:#859900;\">Ok</span><span style=\"color:#657b83;\">::<</span><span style=\"color:#859900;\">_</span><span style=\"color:#657b83;\">, ReverseProxyError>(</span><span style=\"color:#859900;\">service_fn</span><span style=\"color:#657b83;\">(|</span><span style=\"color:#268bd2;\">req</span><span style=\"color:#657b83;\">| {\n rp.</span><span style=\"color:#859900;\">handle</span><span style=\"color:#657b83;\">(req)\n }))\n});\n</span></code></pre>\n<p>This unfortunately still doesn't work, and gives me the error message:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#657b83;\">error[E0507]: cannot move out of `rp`, a captured variable in an `FnMut` closure\n --> src\\main.rs:88:60\n |\n82 | let rp = std::sync::Arc::new(ReverseProxy {\n | -- captured outer variable\n...\n88 | let make_svc = make_service_fn(move |_conn| async move {\n | ____________________________________________________________^\n89 | | let rp = rp.clone();\n | | --\n | | |\n | | move occurs because `rp` has type `std::sync::Arc<ReverseProxy>`, which does not implement the `Copy` trait\n | | move occurs due to use in generator\n90 | | Ok::<_, ReverseProxyError>(service_fn(|req| {\n91 | | rp.handle(req)\n92 | | }))\n93 | | });\n | |_____^ move out of `rp` occurs here\n</span></code></pre>\n<p>It took me a while to grok what was happening. And in fact, I'm not 100% certain I grok it yet. But I believe what is happening is:</p>\n<ul>\n<li>The closure grabs ownership of <code>rp</code> (good)</li>\n<li>The <code>async</code> block grabs ownership of <code>rp</code>, which seemed good, but isn't</li>\n<li>Inside the <code>async</code> block, we make a clone of <code>rp</code></li>\n<li>When the <code>async</code> block is dropped, its ownership of the original <code>rp</code> is dropped</li>\n<li>Since the <code>rp</code> was moved out of the closure, the closure is now an <code>FnOnce</code> and cannot be called a second time</li>\n</ul>\n<p>That's no good! It turns out the trick to fixing this isn't so difficult. Don't grab ownership in the <code>async</code> block. Instead, clone the <code>rp</code> in the closure, before the <code>async</code> block:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> make_svc = </span><span style=\"color:#859900;\">make_service_fn</span><span style=\"color:#657b83;\">(</span><span style=\"color:#586e75;\">move </span><span style=\"color:#859900;\">|</span><span style=\"color:#657b83;\">_conn</span><span style=\"color:#859900;\">| </span><span style=\"color:#657b83;\">{\n </span><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> rp = rp.</span><span style=\"color:#859900;\">clone</span><span style=\"color:#657b83;\">();\n async </span><span style=\"color:#586e75;\">move </span><span style=\"color:#657b83;\">{\n </span><span style=\"color:#859900;\">Ok</span><span style=\"color:#657b83;\">::<</span><span style=\"color:#859900;\">_</span><span style=\"color:#657b83;\">, ReverseProxyError>(</span><span style=\"color:#859900;\">service_fn</span><span style=\"color:#657b83;\">(|</span><span style=\"color:#268bd2;\">req</span><span style=\"color:#657b83;\">| {\n rp.</span><span style=\"color:#859900;\">handle</span><span style=\"color:#657b83;\">(req)\n }))\n }\n});\n</span></code></pre>\n<p>Woohoo! One <code>clone</code> down. This code still doesn't compile, but we're closer. The next change to make is simple: stick a <code>move</code> on the inner closure:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> make_svc = </span><span style=\"color:#859900;\">make_service_fn</span><span style=\"color:#657b83;\">(</span><span style=\"color:#586e75;\">move </span><span style=\"color:#859900;\">|</span><span style=\"color:#657b83;\">_conn</span><span style=\"color:#859900;\">| </span><span style=\"color:#657b83;\">{\n </span><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> rp = rp.</span><span style=\"color:#859900;\">clone</span><span style=\"color:#657b83;\">();\n async </span><span style=\"color:#586e75;\">move </span><span style=\"color:#657b83;\">{\n </span><span style=\"color:#859900;\">Ok</span><span style=\"color:#657b83;\">::<</span><span style=\"color:#859900;\">_</span><span style=\"color:#657b83;\">, ReverseProxyError>(</span><span style=\"color:#859900;\">service_fn</span><span style=\"color:#657b83;\">(</span><span style=\"color:#586e75;\">move </span><span style=\"color:#859900;\">|</span><span style=\"color:#657b83;\">req</span><span style=\"color:#859900;\">| </span><span style=\"color:#657b83;\">{\n rp.</span><span style=\"color:#859900;\">handle</span><span style=\"color:#657b83;\">(req)\n }))\n }\n});\n</span></code></pre>\n<p>This also fails, but going back to our description before, it's easy to see why. We still need a second <code>clone</code>, to make sure we aren't moving the <code>ReverseProxy</code> value out of the closure. Making that change is easy, but unfortunately doesn't fully solve our problem. This code:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> make_svc = </span><span style=\"color:#859900;\">make_service_fn</span><span style=\"color:#657b83;\">(</span><span style=\"color:#586e75;\">move </span><span style=\"color:#859900;\">|</span><span style=\"color:#657b83;\">_conn</span><span style=\"color:#859900;\">| </span><span style=\"color:#657b83;\">{\n </span><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> rp = rp.</span><span style=\"color:#859900;\">clone</span><span style=\"color:#657b83;\">();\n async </span><span style=\"color:#586e75;\">move </span><span style=\"color:#657b83;\">{\n </span><span style=\"color:#859900;\">Ok</span><span style=\"color:#657b83;\">::<</span><span style=\"color:#859900;\">_</span><span style=\"color:#657b83;\">, ReverseProxyError>(</span><span style=\"color:#859900;\">service_fn</span><span style=\"color:#657b83;\">(</span><span style=\"color:#586e75;\">move </span><span style=\"color:#859900;\">|</span><span style=\"color:#657b83;\">req</span><span style=\"color:#859900;\">| </span><span style=\"color:#657b83;\">{\n </span><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> rp = rp.</span><span style=\"color:#859900;\">clone</span><span style=\"color:#657b83;\">();\n rp.</span><span style=\"color:#859900;\">handle</span><span style=\"color:#657b83;\">(req)\n }))\n }\n});\n</span></code></pre>\n<p>Still gives us the error message:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#657b83;\">error[E0515]: cannot return value referencing local variable `rp`\n --> src\\main.rs:93:17\n |\n93 | rp.handle(req)\n | --^^^^^^^^^^^^\n | |\n | returns a value referencing data owned by the current function\n | `rp` is borrowed here\n</span></code></pre>\n<p>What's going on here?</p>\n<h2 id=\"did-your-future-borrow-my-reference\">Did your Future borrow my reference?</h2>\n<p>Again, referring to the introduction, I mentioned that the <code>service_fn</code> parameter had to return a <code>Future<Output...></code>. This is an example of the <code>impl Trait</code> approach. I've previously <a href=\"https://www.fpcomplete.com/rust/ownership-and-impl-trait/\">blogged about ownership and impl trait</a>. There are some pain points around this combination. And we've hit one of them.</p>\n<p>The return type of our <code>handle</code> method doesn't indicate what underlying type is implementing <code>Future</code>. That underlying implementation <em>may</em> choose to hold onto references passed into the <code>handle</code> method. That would include references to <code>&self</code>. And that means if we return that <code>Future</code> outside of our closure, a reference may outlive the value.</p>\n<p>I can think of two ways to solve this problem, though there are probably more. The first one I'll show you isn't the one I prefer, but is the one that likely gets the idea across more clearly. Our <code>handle</code> method is taking a reference to <code>ReverseProxy</code>. But if it didn't take a reference, and instead received the <code>ReverseProxy</code> by move, there would be no references to accidentally end up in the <code>Future</code>.</p>\n<p>Cloning the <code>ReverseProxy</code> itself is expensive. Fortunately, we have another option: pass in the <code>Arc<ReverseProxy></code>!</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">impl </span><span style=\"color:#b58900;\">ReverseProxy </span><span style=\"color:#657b83;\">{\n async </span><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">handle</span><span style=\"color:#657b83;\">(</span><span style=\"color:#268bd2;\">self</span><span style=\"color:#657b83;\">: std::sync::Arc<</span><span style=\"color:#268bd2;\">Self</span><span style=\"color:#657b83;\">>, </span><span style=\"color:#586e75;\">mut </span><span style=\"color:#268bd2;\">req</span><span style=\"color:#657b83;\">: Request<Body>) -> </span><span style=\"color:#859900;\">Result</span><span style=\"color:#657b83;\"><Response<Body>, ReverseProxyError> {\n </span><span style=\"color:#859900;\">...\n </span><span style=\"color:#657b83;\">}\n}\n</span></code></pre>\n<p>Without changing any code inside the <code>handle</code> method or the <code>main</code> function, this compiles and behaves correctly. But like I said: I don't like it very much. This is limiting the generality of our <code>handle</code> method. It feels like putting the complexity in the wrong place. (Maybe you'll disagree and say that this is the better solution. That's fine, I'd be really interested to hear people's thoughts.)</p>\n<p>Instead, another possibility is to introduce an <code>async move</code> inside <code>main</code>. This will take ownership of the <code>Arc<ReverseProxy></code>, and ensure that it lives as long as the <code>Future</code> generated by that <code>async move</code> block itself. This solution looks like this:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> make_svc = </span><span style=\"color:#859900;\">make_service_fn</span><span style=\"color:#657b83;\">(</span><span style=\"color:#586e75;\">move </span><span style=\"color:#859900;\">|</span><span style=\"color:#657b83;\">_conn</span><span style=\"color:#859900;\">| </span><span style=\"color:#657b83;\">{\n </span><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> rp = rp.</span><span style=\"color:#859900;\">clone</span><span style=\"color:#657b83;\">();\n async </span><span style=\"color:#586e75;\">move </span><span style=\"color:#657b83;\">{\n </span><span style=\"color:#859900;\">Ok</span><span style=\"color:#657b83;\">::<</span><span style=\"color:#859900;\">_</span><span style=\"color:#657b83;\">, ReverseProxyError>(</span><span style=\"color:#859900;\">service_fn</span><span style=\"color:#657b83;\">(</span><span style=\"color:#586e75;\">move </span><span style=\"color:#859900;\">|</span><span style=\"color:#657b83;\">req</span><span style=\"color:#859900;\">| </span><span style=\"color:#657b83;\">{\n </span><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> rp = rp.</span><span style=\"color:#859900;\">clone</span><span style=\"color:#657b83;\">();\n async </span><span style=\"color:#586e75;\">move </span><span style=\"color:#657b83;\">{ rp.</span><span style=\"color:#859900;\">handle</span><span style=\"color:#657b83;\">(req).await }\n }))\n }\n});\n</span></code></pre>\n<p>We need to call <code>.await</code> inside the <code>async</code> block to ensure we don't return a future-of-a-future. But with that change, everything works. I'm not terribly thrilled with this. It feels like an ugly hack. I don't have any recommendations, but I hope there are improvements to the <code>impl Trait</code> ownership story in the future.</p>\n<h2 id=\"one-final-improvement\">One final improvement</h2>\n<p>One final tweak. We put <code>async move</code> after the first <code>rp.clone()</code> originally. This helped make the error messages more tractable. But it turns out that that <code>move</code> isn't doing anything useful. The <code>move</code> on the inner <code>closure</code> already forces a move of the cloned <code>rp</code>. So we can simplify our code by removing just one <code>move</code>:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> make_svc = </span><span style=\"color:#859900;\">make_service_fn</span><span style=\"color:#657b83;\">(</span><span style=\"color:#586e75;\">move </span><span style=\"color:#859900;\">|</span><span style=\"color:#657b83;\">_conn</span><span style=\"color:#859900;\">| </span><span style=\"color:#657b83;\">{\n </span><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> rp = rp.</span><span style=\"color:#859900;\">clone</span><span style=\"color:#657b83;\">();\n async {\n </span><span style=\"color:#859900;\">Ok</span><span style=\"color:#657b83;\">::<</span><span style=\"color:#859900;\">_</span><span style=\"color:#657b83;\">, ReverseProxyError>(</span><span style=\"color:#859900;\">service_fn</span><span style=\"color:#657b83;\">(</span><span style=\"color:#586e75;\">move </span><span style=\"color:#859900;\">|</span><span style=\"color:#657b83;\">req</span><span style=\"color:#859900;\">| </span><span style=\"color:#657b83;\">{\n </span><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> rp = rp.</span><span style=\"color:#859900;\">clone</span><span style=\"color:#657b83;\">();\n async </span><span style=\"color:#586e75;\">move </span><span style=\"color:#657b83;\">{ rp.</span><span style=\"color:#859900;\">handle</span><span style=\"color:#657b83;\">(req).await }\n }))\n }\n});\n</span></code></pre>\n<p>This final version of the code is <a href=\"https://gist.github.com/snoyberg/54df3cc7fa1ee1fa77cbde6c75f3df0c\">available as a Gist too</a>.</p>\n<h2 id=\"conclusion\">Conclusion</h2>\n<p>I hope this was a fun trip down ownership lane. If this seemed overly complicated, keep in mind a few things:</p>\n<ul>\n<li>It's strongly recommended to use a higher level web framework when writing server side code in Rust</li>\n<li>We ended up implementing something pretty sophisticated in about 100 lines of code</li>\n<li>Hopefully the story around ownership and <code>impl Trait</code> will improve over time</li>\n</ul>\n<p>If you enjoyed this, you may want to check out our <a href=\"https://www.fpcomplete.com/rust/crash-course/\">Rust Crash Course</a>, or sign up for our <a href=\"https://www.fpcomplete.com/training/\">free December Rust training course</a>.</p>\n<ul>\n<li><a href=\"https://www.fpcomplete.com/rust/ownership-and-impl-trait/\">Ownership and impl Trait</a></li>\n<li><a href=\"https://www.fpcomplete.com/blog/collect-rust-traverse-haskell-scala/\">Collect in Rust, traverse in Haskell and Scala</a></li>\n<li><a href=\"https://www.fpcomplete.com/blog/where-rust-fits-in-your-organization/\">Where Rust fits in your organization</a></li>\n<li><a href=\"https://www.fpcomplete.com/blog/avoiding-duplicating-strings-rust/\">Avoiding duplicating strings in Rust</a></li>\n</ul>\n",
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"content": "<p>A few years back, we <a href=\"https://www.fpcomplete.com/blog/2018/07/deploying-rust-with-docker-and-kubernetes/\">published a blog post</a> about deploying a Rust application using Docker and Kubernetes. That application was a Telegram bot. We're going to do something similar today, but with a few meaningful differences:</p>\n<ol>\n<li>We're going to be deploying a web app. Don't get too excited: this will be an incredibly simply piece of code, basically copy-pasted from the <a href=\"https://actix.rs/docs/application/\">actix-web documentation</a>.</li>\n<li>We're going to build the deployment image on Github Actions</li>\n<li>And we're going to be building this using Windows Containers instead of Linux. (Sorry for burying the lead.)</li>\n</ol>\n<p>We put this together for testing purposes when rolling out Windows support in our <a href=\"https://www.fpcomplete.com/products/kube360/\">managed Kubernetes product, Kube360®</a> here at FP Complete. I wanted to put this post together to demonstrate a few things:</p>\n<ul>\n<li>How pleasant and familiar Windows Containers workflows were versus the more familiar Linux approaches</li>\n<li>Github Actions work seamlessly for building Windows Containers</li>\n<li>With the correct configuration, Kubernetes is a great platform for deploying Windows Containers</li>\n<li>And, of course, how wonderful the Rust toolchain is on Windows</li>\n</ul>\n<p>Alright, let's dive in! And if any of those topics sound interesting, and you'd like to learn more about FP Complete offerings, please <a href=\"https://www.fpcomplete.com/contact-us/\">contact us for more information on our offerings</a>.</p>\n<h2 id=\"prereqs\">Prereqs</h2>\n<p>Quick sidenote before we dive in. Windows Containers only run on Windows machines. Not even all Windows machines will support Windows Containers. You'll need Windows 10 Pro or a similar license, and have Docker installed on that machine. You'll also need to ensure that Docker is set to use Windows instead of Linux containers.</p>\n<p>If you have all of that set up, you'll be able to follow along with most of the steps below. If not, you won't be able to build or run the Docker images on your local machine.</p>\n<p>Also, for running the application on Kubernetes, you'll need a Kubernetes cluster with Windows nodes. I'll be using the FP Complete Kube360 test cluster on Azure in this blog post, though we've previously tested in on both AWS and on-prem clusters too.</p>\n<h2 id=\"the-rust-application\">The Rust application</h2>\n<p>The source code for this application will be, by far, the most uninteresting part of this post. As mentioned, it's basically a copy-paste of an example straight from the actix-web documentation featuring mutable state. It turns out this was a great way to test out basic Kubernetes functionality like health checks, replicas, and autohealing.</p>\n<p>We're going to build this using the latest stable Rust version as of writing this post, so create a <code>rust-toolchain</code> file with the contents:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#657b83;\">1.47.0\n</span></code></pre>\n<p>Our <code>Cargo.toml</code> file will be pretty vanilla, just adding in the dependency on <code>actix-web</code>:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#657b83;\">[</span><span style=\"color:#b58900;\">package</span><span style=\"color:#657b83;\">]\n</span><span style=\"color:#268bd2;\">name </span><span style=\"color:#657b83;\">= </span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">windows-docker-web</span><span style=\"color:#839496;\">"\n</span><span style=\"color:#268bd2;\">version </span><span style=\"color:#657b83;\">= </span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">0.1.0</span><span style=\"color:#839496;\">"\n</span><span style=\"color:#268bd2;\">authors </span><span style=\"color:#657b83;\">= [</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">Michael Snoyman <msnoyman@fpcomplete.com></span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">]\n</span><span style=\"color:#268bd2;\">edition </span><span style=\"color:#657b83;\">= </span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">2018</span><span style=\"color:#839496;\">"\n\n</span><span style=\"color:#657b83;\">[</span><span style=\"color:#b58900;\">dependencies</span><span style=\"color:#657b83;\">]\n</span><span style=\"color:#268bd2;\">actix-web </span><span style=\"color:#657b83;\">= </span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">3.1</span><span style=\"color:#839496;\">"\n</span></code></pre>\n<p>If you want to see the <code>Cargo.lock</code> file I compiled with, it's <a href=\"https://github.com/fpco/windows-docker-web/blob/f8a3192e63f2e699cc67716488a633f5e0893446/Cargo.lock\">available in the source repo</a>.</p>\n<p>And finally, the actual code in <code>src/main.rs</code>:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#859900;\">use </span><span style=\"color:#657b83;\">actix_web::{get, web, App, HttpServer};\n</span><span style=\"color:#859900;\">use </span><span style=\"color:#657b83;\">std::sync::Mutex;\n\n</span><span style=\"color:#268bd2;\">struct </span><span style=\"color:#b58900;\">AppState </span><span style=\"color:#657b83;\">{\n </span><span style=\"color:#268bd2;\">counter</span><span style=\"color:#657b83;\">: Mutex<</span><span style=\"color:#268bd2;\">i32</span><span style=\"color:#657b83;\">>,\n}\n\n#[</span><span style=\"color:#268bd2;\">get</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">/</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">)]\nasync </span><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">index</span><span style=\"color:#657b83;\">(</span><span style=\"color:#268bd2;\">data</span><span style=\"color:#657b83;\">: web::Data<AppState>) -> String {\n </span><span style=\"color:#268bd2;\">let </span><span style=\"color:#586e75;\">mut</span><span style=\"color:#657b83;\"> counter = data.counter.</span><span style=\"color:#859900;\">lock</span><span style=\"color:#657b83;\">().</span><span style=\"color:#859900;\">unwrap</span><span style=\"color:#657b83;\">();\n *counter += </span><span style=\"color:#6c71c4;\">1</span><span style=\"color:#657b83;\">;\n </span><span style=\"color:#859900;\">format!</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">Counter is at </span><span style=\"color:#cb4b16;\">{}</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, counter)\n}\n\n#[</span><span style=\"color:#268bd2;\">actix_web</span><span style=\"color:#657b83;\">::</span><span style=\"color:#268bd2;\">main</span><span style=\"color:#657b83;\">]\nasync </span><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">main</span><span style=\"color:#657b83;\">() -> std::io::</span><span style=\"color:#859900;\">Result</span><span style=\"color:#657b83;\"><()> {\n </span><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> host = </span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">0.0.0.0:8080</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">;\n </span><span style=\"color:#859900;\">println!</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">Trying to listen on </span><span style=\"color:#cb4b16;\">{}</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, host);\n </span><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> app_state = web::Data::new(AppState {\n counter: Mutex::new(</span><span style=\"color:#6c71c4;\">0</span><span style=\"color:#657b83;\">),\n });\n HttpServer::new(</span><span style=\"color:#586e75;\">move </span><span style=\"color:#859900;\">|| </span><span style=\"color:#657b83;\">App::new().</span><span style=\"color:#859900;\">app_data</span><span style=\"color:#657b83;\">(app_state.</span><span style=\"color:#859900;\">clone</span><span style=\"color:#657b83;\">()).</span><span style=\"color:#859900;\">service</span><span style=\"color:#657b83;\">(index))\n .</span><span style=\"color:#859900;\">bind</span><span style=\"color:#657b83;\">(host)</span><span style=\"color:#859900;\">?\n </span><span style=\"color:#657b83;\">.</span><span style=\"color:#859900;\">run</span><span style=\"color:#657b83;\">()\n .await\n}\n</span></code></pre>\n<p>This code creates an application state (a mutex of an <code>i32</code>), defines a single <code>GET</code> handler that increments that variable and prints the current value, and then hosts this on <code>0.0.0.0:8080</code>. Not too shabby.</p>\n<p>If you're following along with the code, now would be a good time to <code>cargo run</code> and make sure you're able to load up the site on your <code>localhost:8080</code>.</p>\n<h2 id=\"dockerfile\">Dockerfile</h2>\n<p>If this is your first foray into Windows Containers, you may be surprised to hear me say "Dockerfile." Windows Container images can be built with the same kind of Dockerfiles you're used to from the Linux world. This even supports more advanced features, such as multistage Dockerfiles, which we're going to take advantage of here.</p>\n<p>There are a number of different base images provided by Microsoft for Windows Containers. We're going to be using Windows Server Core. It provides enough capabilities for installing Rust dependencies (which we'll see shortly), without including too much unneeded extras. Nanoserver is a much lighterweight image, but it doesn't play nicely with the Microsoft Visual C++ runtime we're using for the <code>-msvc</code> Rust target.</p>\n<p><strong>NOTE</strong> I've elected to use the <code>-msvc</code> target here instead of <code>-gnu</code> for two reasons. Firstly, it's closer to the actual use cases we need to support in Kube360, and therefore made a better test case. Also, as the default target for Rust on Windows, it seemed appropriate. It should be possible to set up a more minimal nanoserver-based image based on the <code>-gnu</code> target, if someone's interested in a "fun" side project.</p>\n<p>The <a href=\"https://github.com/fpco/windows-docker-web/blob/f8a3192e63f2e699cc67716488a633f5e0893446/Dockerfile\">complete Dockerfile is available on Github</a>, but let's step through it more carefully. As mentioned, we'll be performing a multistage build. We'll start with the build image, which will install the Rust build toolchain and compile our application. We start off by using the Windows Server Core base image and switching the shell back to the standard <code>cmd.exe</code>:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#657b83;\">FROM mcr.microsoft.com/windows/servercore:1809 as build\n\n# Restore the default Windows shell for correct batch processing.\nSHELL ["cmd", "/S", "/C"]\n</span></code></pre>\n<p>Next we're going to install the Visual Studio buildtools necessary for building Rust code:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#657b83;\"># Download the Build Tools bootstrapper.\nADD https://aka.ms/vs/16/release/vs_buildtools.exe /vs_buildtools.exe\n\n# Install Build Tools with the Microsoft.VisualStudio.Workload.AzureBuildTools workload,\n# excluding workloads and components with known issues.\nRUN vs_buildtools.exe --quiet --wait --norestart --nocache \\\n --installPath C:\\BuildTools \\\n --add Microsoft.Component.MSBuild \\\n --add Microsoft.VisualStudio.Component.Windows10SDK.18362 \\\n --add Microsoft.VisualStudio.Component.VC.Tools.x86.x64\t\\\n || IF "%ERRORLEVEL%"=="3010" EXIT 0\n</span></code></pre>\n<p>And then we'll modify the entrypoint to include the environment modifications necessary to use those buildtools:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#657b83;\"># Define the entry point for the docker container.\n# This entry point starts the developer command prompt and launches the PowerShell shell.\nENTRYPOINT ["C:\\\\BuildTools\\\\Common7\\\\Tools\\\\VsDevCmd.bat", "&&", "powershell.exe", "-NoLogo", "-ExecutionPolicy", "Bypass"]\n</span></code></pre>\n<p>Next up is installing <code>rustup</code>, which is fortunately pretty easy:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#657b83;\">RUN curl -fSLo rustup-init.exe https://win.rustup.rs/x86_64\nRUN start /w rustup-init.exe -y -v && echo "Error level is %ERRORLEVEL%"\nRUN del rustup-init.exe\n\nRUN setx /M PATH "C:\\Users\\ContainerAdministrator\\.cargo\\bin;%PATH%"\n</span></code></pre>\n<p>Then we copy over the relevant source files and kick off a build, storing the generated executable in <code>c:\\output</code>:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#657b83;\">COPY Cargo.toml /project/Cargo.toml\nCOPY Cargo.lock /project/Cargo.lock\nCOPY rust-toolchain /project/rust-toolchain\nCOPY src/ /project/src\nRUN cargo install --path /project --root /output\n</span></code></pre>\n<p>And with that, we're done with our build! Time to jump over to our runtime image. We don't need the Visual Studio buildtools in this image, but we do need the Visual C++ runtime:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#657b83;\">FROM mcr.microsoft.com/windows/servercore:1809\n\nADD https://download.microsoft.com/download/6/A/A/6AA4EDFF-645B-48C5-81CC-ED5963AEAD48/vc_redist.x64.exe /vc_redist.x64.exe\nRUN c:\\vc_redist.x64.exe /install /quiet /norestart\n</span></code></pre>\n<p>With that in place, we can copy over our executable from the build image and set it as the default <code>CMD</code> in the image:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#657b83;\">COPY --from=build c:/output/bin/windows-docker-web.exe /\n\nCMD ["/windows-docker-web.exe"]\n</span></code></pre>\n<p>And just like that, we've got a real life Windows Container. If you'd like to, you can test it out yourself by running:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#657b83;\">> docker run --rm -p 8080:8080 fpco/windows-docker-web:f8a3192e63f2e699cc67716488a633f5e0893446\n</span></code></pre>\n<p>If you connect to port 8080, you should see our painfully simple app. Hurrah!</p>\n<h2 id=\"building-with-github-actions\">Building with Github Actions</h2>\n<p>One of the nice things about using a multistage Dockerfile for performing the build is that our CI scripts become very simple. Instead of needing to set up an environment with correct build tools or any other configuration, our script:</p>\n<ul>\n<li>Logs into the Docker Hub registry</li>\n<li>Performs a <code>docker build</code></li>\n<li>Pushes to the Docker Hub registry</li>\n</ul>\n<p>The downside is that there is no build caching at play with this setup. There are multiple methods to mitigate this problem, such as creating helper build images that pre-bake the dependencies. Or you can perform the builds on the host on CI and only use the Dockerfile for generating the runtime image. Those are interesting tweaks to try out another time. </p>\n<p>Taking on the simple multistage approach though, we have the following in our <code>.github/workflows/container.yml</code> file:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">name</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">Build a Windows container\n\n</span><span style=\"color:#b58900;\">on</span><span style=\"color:#657b83;\">:\n </span><span style=\"color:#268bd2;\">push</span><span style=\"color:#657b83;\">:\n </span><span style=\"color:#268bd2;\">branches</span><span style=\"color:#657b83;\">: [</span><span style=\"color:#2aa198;\">master</span><span style=\"color:#657b83;\">]\n\n</span><span style=\"color:#268bd2;\">jobs</span><span style=\"color:#657b83;\">:\n </span><span style=\"color:#268bd2;\">build</span><span style=\"color:#657b83;\">:\n </span><span style=\"color:#268bd2;\">runs-on</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">windows-latest\n\n steps</span><span style=\"color:#657b83;\">:\n - </span><span style=\"color:#268bd2;\">uses</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">actions/checkout@v1\n\n </span><span style=\"color:#657b83;\">- </span><span style=\"color:#268bd2;\">name</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">Build and push\n shell</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">bash\n run</span><span style=\"color:#657b83;\">: </span><span style=\"color:#859900;\">|\n</span><span style=\"color:#2aa198;\"> echo "${{ secrets.DOCKER_HUB_TOKEN }}" | docker login --username fpcojenkins --password-stdin\n IMAGE_ID=fpco/windows-docker-web:$GITHUB_SHA\n docker build -t $IMAGE_ID .\n docker push $IMAGE_ID\n</span></code></pre>\n<p>I like following the convention of tagging my images with the Git SHA of the commit. Other people prefer different tagging schemes, it's all up to you.</p>\n<h2 id=\"manifest-files\">Manifest files</h2>\n<p>Now that we have a working Windows Container image, the next step is to deploy it to our Kube360 cluster. Generally, we use ArgoCD and Kustomize for managing app deployments within Kube360, which lets us keep a very nice Gitops workflow. Instead, for this blog post, I'll show you the raw manifest files. It will also let us play with the <code>k3</code> command line tool, which also happens to be written in Rust.</p>\n<p>First we'll have a Deployment manifest to manage the pods running the application itself. Since this is a simple Rust application, we can put very low resource limits on this. We're going to disable the Istio sidebar, since it's not compatible with Windows. We're going to ask Kubernetes to use the Windows machines to host these pods. And we're going to set up some basic health checks. All told, this is what our manifest file looks like:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">apiVersion</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">apps/v1\nkind</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">Deployment\nmetadata</span><span style=\"color:#657b83;\">:\n </span><span style=\"color:#268bd2;\">name</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">windows-docker-web\n labels</span><span style=\"color:#657b83;\">:\n </span><span style=\"color:#268bd2;\">app.kubernetes.io/component</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">webserver\nspec</span><span style=\"color:#657b83;\">:\n </span><span style=\"color:#268bd2;\">replicas</span><span style=\"color:#657b83;\">: </span><span style=\"color:#6c71c4;\">1\n </span><span style=\"color:#268bd2;\">minReadySeconds</span><span style=\"color:#657b83;\">: </span><span style=\"color:#6c71c4;\">5\n </span><span style=\"color:#268bd2;\">selector</span><span style=\"color:#657b83;\">:\n </span><span style=\"color:#268bd2;\">matchLabels</span><span style=\"color:#657b83;\">:\n </span><span style=\"color:#268bd2;\">app.kubernetes.io/component</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">webserver\n template</span><span style=\"color:#657b83;\">:\n </span><span style=\"color:#268bd2;\">metadata</span><span style=\"color:#657b83;\">:\n </span><span style=\"color:#268bd2;\">labels</span><span style=\"color:#657b83;\">:\n </span><span style=\"color:#268bd2;\">app.kubernetes.io/component</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">webserver\n annotations</span><span style=\"color:#657b83;\">:\n </span><span style=\"color:#268bd2;\">sidecar.istio.io/inject</span><span style=\"color:#657b83;\">: </span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">false</span><span style=\"color:#839496;\">"\n </span><span style=\"color:#268bd2;\">spec</span><span style=\"color:#657b83;\">:\n </span><span style=\"color:#268bd2;\">runtimeClassName</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">windows-2019\n containers</span><span style=\"color:#657b83;\">:\n - </span><span style=\"color:#268bd2;\">name</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">windows-docker-web\n image</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">fpco/windows-docker-web:f8a3192e63f2e699cc67716488a633f5e0893446\n ports</span><span style=\"color:#657b83;\">:\n - </span><span style=\"color:#268bd2;\">name</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">http\n containerPort</span><span style=\"color:#657b83;\">: </span><span style=\"color:#6c71c4;\">8080\n </span><span style=\"color:#268bd2;\">readinessProbe</span><span style=\"color:#657b83;\">:\n </span><span style=\"color:#268bd2;\">httpGet</span><span style=\"color:#657b83;\">:\n </span><span style=\"color:#268bd2;\">path</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">/\n port</span><span style=\"color:#657b83;\">: </span><span style=\"color:#6c71c4;\">8080\n </span><span style=\"color:#268bd2;\">initialDelaySeconds</span><span style=\"color:#657b83;\">: </span><span style=\"color:#6c71c4;\">10\n </span><span style=\"color:#268bd2;\">periodSeconds</span><span style=\"color:#657b83;\">: </span><span style=\"color:#6c71c4;\">10\n </span><span style=\"color:#268bd2;\">livenessProbe</span><span style=\"color:#657b83;\">:\n </span><span style=\"color:#268bd2;\">httpGet</span><span style=\"color:#657b83;\">:\n </span><span style=\"color:#268bd2;\">path</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">/\n port</span><span style=\"color:#657b83;\">: </span><span style=\"color:#6c71c4;\">8080\n </span><span style=\"color:#268bd2;\">initialDelaySeconds</span><span style=\"color:#657b83;\">: </span><span style=\"color:#6c71c4;\">10\n </span><span style=\"color:#268bd2;\">periodSeconds</span><span style=\"color:#657b83;\">: </span><span style=\"color:#6c71c4;\">10\n </span><span style=\"color:#268bd2;\">resources</span><span style=\"color:#657b83;\">:\n </span><span style=\"color:#268bd2;\">requests</span><span style=\"color:#657b83;\">:\n </span><span style=\"color:#268bd2;\">memory</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">128Mi\n cpu</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">100m\n limits</span><span style=\"color:#657b83;\">:\n </span><span style=\"color:#268bd2;\">memory</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">128Mi\n cpu</span><span style=\"color:#657b83;\">: </span><span style=\"color:#2aa198;\">100m\n</span></code></pre>\n<p>Awesome, that's the most complicated by far of the three manifests. Next we'll put a fairly stock-standard Service in front of that deployment:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">apiVersion</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">v1\nkind</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">Service\nmetadata</span><span style=\"color:#657b83;\">:\n </span><span style=\"color:#268bd2;\">name</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">windows-docker-web\n labels</span><span style=\"color:#657b83;\">:\n </span><span style=\"color:#268bd2;\">app.kubernetes.io/component</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">webserver\nspec</span><span style=\"color:#657b83;\">:\n </span><span style=\"color:#268bd2;\">ports</span><span style=\"color:#657b83;\">:\n - </span><span style=\"color:#268bd2;\">name</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">http\n port</span><span style=\"color:#657b83;\">: </span><span style=\"color:#6c71c4;\">80\n </span><span style=\"color:#268bd2;\">targetPort</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">http\n type</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">ClusterIP\n selector</span><span style=\"color:#657b83;\">:\n </span><span style=\"color:#268bd2;\">app.kubernetes.io/component</span><span style=\"color:#657b83;\">: </span><span style=\"color:#2aa198;\">webserver\n</span></code></pre>\n<p>This exposes a services on port 80, and targets the <code>http</code> port (port 8080) inside the deployment. Finally, we have our Ingress. Kube360 uses external DNS to automatically set DNS records, and cert-manager to automatically grab TLS certificates. Our manifest looks like this:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">apiVersion</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">networking.k8s.io/v1beta1\nkind</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">Ingress\nmetadata</span><span style=\"color:#657b83;\">:\n </span><span style=\"color:#268bd2;\">annotations</span><span style=\"color:#657b83;\">:\n </span><span style=\"color:#268bd2;\">cert-manager.io/cluster-issuer</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">letsencrypt-ingress-prod\n kubernetes.io/ingress.class</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">nginx\n nginx.ingress.kubernetes.io/force-ssl-redirect</span><span style=\"color:#657b83;\">: </span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">true</span><span style=\"color:#839496;\">"\n </span><span style=\"color:#268bd2;\">name</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">windows-docker-web\nspec</span><span style=\"color:#657b83;\">:\n </span><span style=\"color:#268bd2;\">rules</span><span style=\"color:#657b83;\">:\n - </span><span style=\"color:#268bd2;\">host</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">windows-docker-web.az.fpcomplete.com\n http</span><span style=\"color:#657b83;\">:\n </span><span style=\"color:#268bd2;\">paths</span><span style=\"color:#657b83;\">:\n - </span><span style=\"color:#268bd2;\">backend</span><span style=\"color:#657b83;\">:\n </span><span style=\"color:#268bd2;\">serviceName</span><span style=\"color:#657b83;\">: </span><span style=\"color:#268bd2;\">windows-docker-web\n servicePort</span><span style=\"color:#657b83;\">: </span><span style=\"color:#6c71c4;\">80\n </span><span style=\"color:#268bd2;\">tls</span><span style=\"color:#657b83;\">:\n - </span><span style=\"color:#268bd2;\">hosts</span><span style=\"color:#657b83;\">:\n - </span><span style=\"color:#268bd2;\">windows-docker-web.az.fpcomplete.com\n secretName</span><span style=\"color:#657b83;\">: </span><span style=\"color:#2aa198;\">windows-docker-web-tls\n</span></code></pre>\n<p>Now that we have our application inside a Docker image, and we have our manifest files to instruct Kubernetes on how to run it, we just need to deploy these manifests and we'll be done.</p>\n<h2 id=\"launch\">Launch</h2>\n<p>With our manifests in place, we can finally deploy them. You can use <code>kubectl</code> directly to do this. Since I'm deploying to Kube360, I'm going to use the <code>k3</code> command line tool, which automates the process of logging in, getting temporary Kubernetes credentials, and providing those to the <code>kubectl</code> command via an environment variable. These steps could be run on Windows, Mac, or Linux. But since we've done the rest of this post on Windows, I'll use my Windows machine for this too.</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#657b83;\">> k3 init test.az.fpcomplete.com\n> k3 kubectl apply -f deployment.yaml\nWeb browser opened to https://test.az.fpcomplete.com/k3-confirm?nonce=c1f764d8852f4ff2a2738fb0a2078e68\nPlease follow the login steps there (if needed).\nThen return to this terminal.\nPolling the server. Please standby.\nChecking ...\nThanks, got the token response. Verifying token is valid\nRetrieving a kubeconfig for use with k3 kubectl\nKubeconfig retrieved. You are now ready to run kubectl commands with `k3 kubectl ...`\ndeployment.apps/windows-docker-web created\n> k3 kubectl apply -f ingress.yaml\ningress.networking.k8s.io/windows-docker-web created\n> k3 kubectl apply -f service.yaml\nservice/windows-docker-web created\n</span></code></pre>\n<p>I told <code>k3</code> to use the <code>test.az.fpcomplete.com</code> cluster. On the first <code>k3 kubectl</code> call, it detected that I did not have valid credentials for the cluster, and opened up my browser to a page that allowed me to log in. One of the design goals in Kube360 is to strongly leverage existing identity providers, such as Azure AD, Google Directory, Okta, Microsoft 365, and others. This is not only more secure than copy-pasting <code>kubeconfig</code> files with permanent credentials around, but more user friendly. As you can see, the process above was pretty automated.</p>\n<p>It's easy enough to check that the pods are actually running and healthy:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#657b83;\">> k3 kubectl get pods\nNAME READY STATUS RESTARTS AGE\nwindows-docker-web-5687668cdf-8tmn2 1/1 Running 0 3m2s\n</span></code></pre>\n<p>Initially, the ingress controller looked like this while it was getting TLS certificates:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#657b83;\">> k3 kubectl get ingress\nNAME CLASS HOSTS ADDRESS PORTS AGE\ncm-acme-http-solver-zlq6j <none> windows-docker-web.az.fpcomplete.com 80 0s\nwindows-docker-web <none> windows-docker-web.az.fpcomplete.com 80, 443 3s\n</span></code></pre>\n<p>And after cert-manager gets the TLS certificate, it will switch over to:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#657b83;\">> k3 kubectl get ingress\nNAME CLASS HOSTS ADDRESS PORTS AGE\nwindows-docker-web <none> windows-docker-web.az.fpcomplete.com 52.151.225.139 80, 443 90s\n</span></code></pre>\n<p>And finally, our site is live! Hurrah, a Rust web application compiled for Windows and running on Kubernetes inside Azure.</p>\n<p><strong>NOTE</strong> Depending on when you read this post, the web app may or may not still be live, so don't be surprised if you don't get a response if you try to connect to that host.</p>\n<h2 id=\"conclusion\">Conclusion</h2>\n<p>This post was a bit light on actual Rust code, but heavy on a lot of Windows scripting. As I think many Rustaceans already know, the dev experience for Rust on Windows is top notch. What may not have been obvious is how pleasant the Docker experience is on Windows. There are definitely some pain points, like the large images involved and needing to install the VC runtime. But overall, with a bit of cargo-culting, it's not too bad. And finally, having a cluster with Windows support ready via Kube360 makes deployment a breeze.</p>\n<p>If anyone has follow up questions about anything here, please <a href=\"https://twitter.com/snoyberg\">reach out to me on Twitter</a> or <a href=\"https://www.fpcomplete.com/contact-us/\">contact our team at FP Complete</a>. In addition to our <a href=\"https://www.fpcomplete.com/products/kube360/\">Kube360 product offering</a>, FP Complete provides many related services, including:</p>\n<ul>\n<li><a href=\"https://www.fpcomplete.com/devops/\">DevOps consulting</a></li>\n<li><a href=\"https://www.fpcomplete.com/rust/\">Rust consulting and training</a></li>\n<li><a href=\"https://www.fpcomplete.com/services/\">General training and consulting services</a></li>\n<li><a href=\"https://www.fpcomplete.com/haskell/\">Haskell consulting and training</a></li>\n</ul>\n<p>If you liked this post, please check out some related posts:</p>\n<ul>\n<li><a href=\"https://www.fpcomplete.com/blog/2018/07/deploying-rust-with-docker-and-kubernetes/\">Deploying Rust with Docker and Kubernetes</a></li>\n<li><a href=\"https://www.fpcomplete.com/blog/rust-for-devops-tooling/\">Using Rust for DevOps tooling</a></li>\n<li><a href=\"https://www.fpcomplete.com/rust/crash-course/\">The Rust Crash Course eBook</a></li>\n<li><a href=\"https://www.fpcomplete.com/blog/devops-for-developers/\">DevOps for (Skeptical) Developers</a></li>\n<li><a href=\"https://www.fpcomplete.com/blog/understanding-cloud-auth/\">Understanding cloud auth</a></li>\n</ul>\n",
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"content": "<p>There's a running joke in the functional programming community. Any Scala program can be written by combining the <code>traverse</code> function the correct number of times. This blog post is dedicated to that joke.</p>\n<p>In Rust, the <code>Iterator</code> trait defines a stream of values of a specific type. Many common types provide an <code>Iterator</code> interface. And the built in <code>for</code> loop construct works directly with the <code>Iterator</code> trait. Using that, we can easily do something like "print all the numbers in a <code>Vec</code>":</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">main</span><span style=\"color:#657b83;\">() {\n </span><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> myvec: </span><span style=\"color:#859900;\">Vec</span><span style=\"color:#657b83;\"><</span><span style=\"color:#268bd2;\">i32</span><span style=\"color:#657b83;\">> = </span><span style=\"color:#859900;\">vec!</span><span style=\"color:#657b83;\">[</span><span style=\"color:#6c71c4;\">1</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">2</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">3</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">4</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">5</span><span style=\"color:#657b83;\">];\n\n </span><span style=\"color:#859900;\">for</span><span style=\"color:#657b83;\"> num </span><span style=\"color:#859900;\">in</span><span style=\"color:#657b83;\"> myvec {\n </span><span style=\"color:#859900;\">println!</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#cb4b16;\">{}</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, num);\n }\n}\n</span></code></pre>\n<p>Let's say we want to do something a bit different: double every value in the <code>Vec</code>. The most idiomatic and performant way to do that in Rust is with mutable references, e.g.:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">main</span><span style=\"color:#657b83;\">() {\n </span><span style=\"color:#268bd2;\">let </span><span style=\"color:#586e75;\">mut</span><span style=\"color:#657b83;\"> myvec: </span><span style=\"color:#859900;\">Vec</span><span style=\"color:#657b83;\"><</span><span style=\"color:#268bd2;\">i32</span><span style=\"color:#657b83;\">> = </span><span style=\"color:#859900;\">vec!</span><span style=\"color:#657b83;\">[</span><span style=\"color:#6c71c4;\">1</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">2</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">3</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">4</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">5</span><span style=\"color:#657b83;\">];\n\n </span><span style=\"color:#859900;\">for</span><span style=\"color:#657b83;\"> num </span><span style=\"color:#859900;\">in &</span><span style=\"color:#586e75;\">mut</span><span style=\"color:#657b83;\"> myvec {\n *num *= </span><span style=\"color:#6c71c4;\">2</span><span style=\"color:#657b83;\">;\n }\n\n </span><span style=\"color:#859900;\">println!</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#cb4b16;\">{:?}</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, myvec);\n}\n</span></code></pre>\n<p>Since we're dedicating this post to functional programmers, it's worth noting: this looks decidedly not-functional. "Take a collection and apply a function over each value" is well understood in FP circles—and increasingly in non-FP circles—as a <code>map</code>. Or using more category-theoretic nomenclature, it's a <code>Functor</code>. Fortunately, Rust provides a <code>map</code> method for <code>Iterator</code>s. Unfortunately, unlike Scala or Haskell, <code>map</code> doesn't work on data types like <code>Vec</code>. Let's compare, using Haskell:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#b58900;\">list </span><span style=\"color:#859900;\">::</span><span style=\"color:#657b83;\"> [</span><span style=\"color:#268bd2;\">Int</span><span style=\"color:#657b83;\">]\nlist </span><span style=\"color:#859900;\">=</span><span style=\"color:#657b83;\"> [</span><span style=\"color:#6c71c4;\">1</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">2</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">3</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">4</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">5</span><span style=\"color:#657b83;\">]\n\n</span><span style=\"color:#b58900;\">main </span><span style=\"color:#859900;\">:: </span><span style=\"color:#268bd2;\">IO </span><span style=\"color:#859900;\">()\n</span><span style=\"color:#657b83;\">main </span><span style=\"color:#859900;\">= do\n let</span><span style=\"color:#657b83;\"> newList </span><span style=\"color:#859900;\">::</span><span style=\"color:#657b83;\"> [</span><span style=\"color:#cb4b16;\">Int</span><span style=\"color:#657b83;\">]\n newList </span><span style=\"color:#859900;\">=</span><span style=\"color:#657b83;\"> map (* </span><span style=\"color:#6c71c4;\">2</span><span style=\"color:#657b83;\">) list\n print newList\n</span></code></pre>\n<p>The <code>map</code> function from the <code>Functor</code> typeclass works directly on a list. It produces a new list with the function applied to each value. Let's try to do the most equivalent thing in Rust:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">main</span><span style=\"color:#657b83;\">() {\n </span><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> myvec: </span><span style=\"color:#859900;\">Vec</span><span style=\"color:#657b83;\"><</span><span style=\"color:#268bd2;\">i32</span><span style=\"color:#657b83;\">> = </span><span style=\"color:#859900;\">vec!</span><span style=\"color:#657b83;\">[</span><span style=\"color:#6c71c4;\">1</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">2</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">3</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">4</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">5</span><span style=\"color:#657b83;\">];\n\n </span><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> new_vec: </span><span style=\"color:#859900;\">Vec</span><span style=\"color:#657b83;\"><</span><span style=\"color:#268bd2;\">i32</span><span style=\"color:#657b83;\">> = myvec.</span><span style=\"color:#859900;\">map</span><span style=\"color:#657b83;\">(|</span><span style=\"color:#268bd2;\">x</span><span style=\"color:#657b83;\">| x * </span><span style=\"color:#6c71c4;\">2</span><span style=\"color:#657b83;\">);\n\n </span><span style=\"color:#859900;\">println!</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#cb4b16;\">{:?}</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, new_vec);\n}\n</span></code></pre>\n<p>This fails with the error message:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#657b83;\">no method named `map` found for struct `std::vec::Vec<i32>` in the current scope\n</span></code></pre>\n<p>That's because, in Rust, <code>map</code> applies to the <code>Iterator</code> itself, not the underlying data structures. In order to use <code>map</code> on a <code>Vec</code>, we have to:</p>\n<ol>\n<li>Convert the <code>Vec</code> into an <code>Iterator</code></li>\n<li>Perform the <code>map</code> on the <code>Iterator</code></li>\n<li>Convert the <code>Iterator</code> back into a <code>Vec</code></li>\n</ol>\n<p>(1) can be performed using the <code>IntoIterator</code> trait, which provides a method named <code>into_iter</code>. And for (3), we could write our own <code>for</code> loop that fills up a <code>Vec</code>. But the right way is to use the <code>FromIterator</code> trait. And the easiest way to do that is with the <code>collect</code> method on <code>Iterator</code>.</p>\n<h2 id=\"using-fromiterator-and-collect\">Using FromIterator and collect</h2>\n<p>Let's write a program that properly uses <code>map</code>:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">main</span><span style=\"color:#657b83;\">() {\n </span><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> myvec: </span><span style=\"color:#859900;\">Vec</span><span style=\"color:#657b83;\"><</span><span style=\"color:#268bd2;\">i32</span><span style=\"color:#657b83;\">> = </span><span style=\"color:#859900;\">vec!</span><span style=\"color:#657b83;\">[</span><span style=\"color:#6c71c4;\">1</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">2</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">3</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">4</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">5</span><span style=\"color:#657b83;\">];\n\n </span><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> new_vec: </span><span style=\"color:#859900;\">Vec</span><span style=\"color:#657b83;\"><</span><span style=\"color:#268bd2;\">i32</span><span style=\"color:#657b83;\">> = myvec\n .</span><span style=\"color:#859900;\">into_iter</span><span style=\"color:#657b83;\">()\n .</span><span style=\"color:#859900;\">map</span><span style=\"color:#657b83;\">(|</span><span style=\"color:#268bd2;\">x</span><span style=\"color:#657b83;\">| x * </span><span style=\"color:#6c71c4;\">2</span><span style=\"color:#657b83;\">)\n .</span><span style=\"color:#859900;\">collect</span><span style=\"color:#657b83;\">();\n\n </span><span style=\"color:#859900;\">println!</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#cb4b16;\">{:?}</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, new_vec);\n}\n</span></code></pre>\n<p>Fairly straightforward, and our 3 steps turn into three chained method calls. Unfortunately, in practice, using <code>collect</code> is often not quite as straightforward as this. That's due to type inference. To see what I mean, let's take all of the type annotations out of the program above:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">main</span><span style=\"color:#657b83;\">() {\n </span><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> myvec = </span><span style=\"color:#859900;\">vec!</span><span style=\"color:#657b83;\">[</span><span style=\"color:#6c71c4;\">1</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">2</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">3</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">4</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">5</span><span style=\"color:#657b83;\">];\n\n </span><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> new_vec = myvec\n .</span><span style=\"color:#859900;\">into_iter</span><span style=\"color:#657b83;\">()\n .</span><span style=\"color:#859900;\">map</span><span style=\"color:#657b83;\">(|</span><span style=\"color:#268bd2;\">x</span><span style=\"color:#657b83;\">| x * </span><span style=\"color:#6c71c4;\">2</span><span style=\"color:#657b83;\">)\n .</span><span style=\"color:#859900;\">collect</span><span style=\"color:#657b83;\">();\n\n </span><span style=\"color:#859900;\">println!</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#cb4b16;\">{:?}</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, new_vec);\n}\n</span></code></pre>\n<p>This gives us the very friendly message:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#657b83;\">error[E0282]: type annotations needed\n --> src\\main.rs:4:9\n |\n4 | let new_vec = myvec\n | ^^^^^^^ consider giving `new_vec` a type\n</span></code></pre>\n<p>The issue here is that we don't know which implementation of <code>FromIterator</code> we should be using. This is a problem that didn't exist in the pure FP world with <code>map</code> and <code>Functor</code>. In that world, <code>Functor</code>'s <code>map</code> is always "shape preserving." When you <code>map</code> over a list in Haskell, the result will always be a list.</p>\n<p>That's not the case with the <code>IntoIterator</code>/<code>FromIterator</code> combination. <code>IntoIterator</code> destroys the original data structure, fully consuming it and producing an <code>Iterator</code>. Similarly, <code>FromIterator</code> produces a brand new data structure out of thin air, without any reference to the original data structure. Therefore, an explicit type annotation saying what the output type should be is necessary. In our program above, we did this by annotating <code>new_vec</code>. Another way is to use "turbofish" to annotate which <code>collect</code> to use:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">main</span><span style=\"color:#657b83;\">() {\n </span><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> myvec = </span><span style=\"color:#859900;\">vec!</span><span style=\"color:#657b83;\">[</span><span style=\"color:#6c71c4;\">1</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">2</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">3</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">4</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">5</span><span style=\"color:#657b83;\">];\n\n </span><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> new_vec = myvec\n .</span><span style=\"color:#859900;\">into_iter</span><span style=\"color:#657b83;\">()\n .</span><span style=\"color:#859900;\">map</span><span style=\"color:#657b83;\">(|</span><span style=\"color:#268bd2;\">x</span><span style=\"color:#657b83;\">| x * </span><span style=\"color:#6c71c4;\">2</span><span style=\"color:#657b83;\">)\n .collect::<</span><span style=\"color:#859900;\">Vec</span><span style=\"color:#657b83;\"><</span><span style=\"color:#859900;\">_</span><span style=\"color:#657b83;\">>>();\n\n </span><span style=\"color:#859900;\">println!</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#cb4b16;\">{:?}</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, new_vec);\n}\n</span></code></pre>\n<p>Note that we only needed indicate that we were collecting into a <code>Vec</code>. Rust's normal type inference was able to figure out:</p>\n<ul>\n<li>Which numeric type to use for the values</li>\n<li>That the original <code>myvec</code> should be a <code>Vec</code>, since it was produced by the <code>vec!</code> macro</li>\n</ul>\n<h2 id=\"side-effects-and-traverse\">Side effects and traverse</h2>\n<p>Alright, I want to announce to the world that I'll be doubling these values. It's easy to modify our <code>map</code>-using code in Rust to do this:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">main</span><span style=\"color:#657b83;\">() {\n </span><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> myvec = </span><span style=\"color:#859900;\">vec!</span><span style=\"color:#657b83;\">[</span><span style=\"color:#6c71c4;\">1</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">2</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">3</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">4</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">5</span><span style=\"color:#657b83;\">];\n\n </span><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> new_vec = myvec\n .</span><span style=\"color:#859900;\">into_iter</span><span style=\"color:#657b83;\">()\n .</span><span style=\"color:#859900;\">map</span><span style=\"color:#657b83;\">(|</span><span style=\"color:#268bd2;\">x</span><span style=\"color:#657b83;\">| {\n </span><span style=\"color:#859900;\">println!</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">About to double </span><span style=\"color:#cb4b16;\">{}</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, x);\n x * </span><span style=\"color:#6c71c4;\">2\n </span><span style=\"color:#657b83;\">})\n .collect::<</span><span style=\"color:#859900;\">Vec</span><span style=\"color:#657b83;\"><</span><span style=\"color:#859900;\">_</span><span style=\"color:#657b83;\">>>();\n\n </span><span style=\"color:#859900;\">println!</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#cb4b16;\">{:?}</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, new_vec);\n}\n</span></code></pre>\n<p>But Haskellers will warn you that this isn't quite so simple. <code>map</code> in Haskell is a pure function, meaning it doesn't allow for any side-effects (like printing to the screen). You can see this in action fairly easily:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#b58900;\">list </span><span style=\"color:#859900;\">::</span><span style=\"color:#657b83;\"> [</span><span style=\"color:#268bd2;\">Int</span><span style=\"color:#657b83;\">]\nlist </span><span style=\"color:#859900;\">=</span><span style=\"color:#657b83;\"> [</span><span style=\"color:#6c71c4;\">1</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">2</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">3</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">4</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">5</span><span style=\"color:#657b83;\">]\n\n</span><span style=\"color:#b58900;\">main </span><span style=\"color:#859900;\">:: </span><span style=\"color:#268bd2;\">IO </span><span style=\"color:#859900;\">()\n</span><span style=\"color:#657b83;\">main </span><span style=\"color:#859900;\">= do\n let</span><span style=\"color:#657b83;\"> newList </span><span style=\"color:#859900;\">::</span><span style=\"color:#657b83;\"> [</span><span style=\"color:#cb4b16;\">Int</span><span style=\"color:#657b83;\">]\n newList </span><span style=\"color:#859900;\">=</span><span style=\"color:#657b83;\">\n map\n (</span><span style=\"color:#859900;\">\\</span><span style=\"color:#657b83;\">x </span><span style=\"color:#859900;\">-> do</span><span style=\"color:#657b83;\">\n putStrLn (</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">About to double </span><span style=\"color:#839496;\">" </span><span style=\"color:#859900;\">++</span><span style=\"color:#657b83;\"> show x)\n pure (x * </span><span style=\"color:#6c71c4;\">2</span><span style=\"color:#657b83;\">))\n list\n print newList\n</span></code></pre>\n<p>This code won't compile, due to the mismatch between an <code>Int</code> (a pure number) and an <code>IO Int</code> (an action with side effects which produces an <code>Int</code>):</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#657b83;\">Couldn't match type 'IO Int' with 'Int'\nExpected type: [Int]\nActual type: [IO Int]\n</span></code></pre>\n<p>Instead, we need to use <code>map</code>'s more powerful cousin, <code>traverse</code> (a.k.a. <code>mapM</code>, or "monadic map"). <code>traverse</code> allows us to perform a series of actions, and produce a new list with all of their results. This looks like:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#b58900;\">list </span><span style=\"color:#859900;\">::</span><span style=\"color:#657b83;\"> [</span><span style=\"color:#268bd2;\">Int</span><span style=\"color:#657b83;\">]\nlist </span><span style=\"color:#859900;\">=</span><span style=\"color:#657b83;\"> [</span><span style=\"color:#6c71c4;\">1</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">2</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">3</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">4</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">5</span><span style=\"color:#657b83;\">]\n\n</span><span style=\"color:#b58900;\">main </span><span style=\"color:#859900;\">:: </span><span style=\"color:#268bd2;\">IO </span><span style=\"color:#859900;\">()\n</span><span style=\"color:#657b83;\">main </span><span style=\"color:#859900;\">= do</span><span style=\"color:#657b83;\">\n newList </span><span style=\"color:#859900;\"><-</span><span style=\"color:#657b83;\">\n traverse\n (</span><span style=\"color:#859900;\">\\</span><span style=\"color:#657b83;\">x </span><span style=\"color:#859900;\">-> do</span><span style=\"color:#657b83;\">\n putStrLn (</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">About to double </span><span style=\"color:#839496;\">" </span><span style=\"color:#859900;\">++</span><span style=\"color:#657b83;\"> show x)\n pure (x * </span><span style=\"color:#6c71c4;\">2</span><span style=\"color:#657b83;\">))\n list\n print newList\n</span></code></pre>\n<p>So why the difference between Haskell and Rust here? That's because Rust is not a pure language. Any function can perform side effects, like printing to the screen. Haskell, on the other hand, doesn't allow this, and therefore we need special helper functions like <code>traverse</code> to account for the potential side effects.</p>\n<p>I won't get into the philosophical differences between the two languages. Suffice it to say that both approaches have merit, and both have advantages and disadvantages. Let's see where the Rust approach "breaks down", and how <code>FromIterator</code> steps up to the plate.</p>\n<h2 id=\"handling-failure\">Handling failure</h2>\n<p>In the example above with Haskell, we used side effects via the <code>IO</code> type. However, <code>traverse</code> isn't limited to working with <code>IO</code>. It can work with <em>many</em> different types, anything which is considered <code>Applicative</code>. And this covers many different common needs, including error handling. For example, we can change our program to not allow doubling "big" numbers greater than 5:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#b58900;\">list </span><span style=\"color:#859900;\">::</span><span style=\"color:#657b83;\"> [</span><span style=\"color:#268bd2;\">Int</span><span style=\"color:#657b83;\">]\nlist </span><span style=\"color:#859900;\">=</span><span style=\"color:#657b83;\"> [</span><span style=\"color:#6c71c4;\">1</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">2</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">3</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">4</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">5</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">6</span><span style=\"color:#657b83;\">]\n\n</span><span style=\"color:#b58900;\">main </span><span style=\"color:#859900;\">:: </span><span style=\"color:#268bd2;\">IO </span><span style=\"color:#859900;\">()\n</span><span style=\"color:#657b83;\">main </span><span style=\"color:#859900;\">= do\n let</span><span style=\"color:#657b83;\"> newList </span><span style=\"color:#859900;\">=</span><span style=\"color:#657b83;\">\n traverse\n (</span><span style=\"color:#859900;\">\\</span><span style=\"color:#657b83;\">x </span><span style=\"color:#859900;\">->\n if</span><span style=\"color:#657b83;\"> x </span><span style=\"color:#859900;\">> </span><span style=\"color:#6c71c4;\">5\n </span><span style=\"color:#859900;\">then </span><span style=\"color:#cb4b16;\">Left </span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">Not allowed to double big numbers</span><span style=\"color:#839496;\">"\n </span><span style=\"color:#859900;\">else </span><span style=\"color:#cb4b16;\">Right</span><span style=\"color:#657b83;\"> (x * </span><span style=\"color:#6c71c4;\">2</span><span style=\"color:#657b83;\">))\n list\n </span><span style=\"color:#859900;\">case</span><span style=\"color:#657b83;\"> newList </span><span style=\"color:#859900;\">of\n </span><span style=\"color:#cb4b16;\">Left</span><span style=\"color:#657b83;\"> err </span><span style=\"color:#859900;\">-></span><span style=\"color:#657b83;\"> putStrLn err\n </span><span style=\"color:#cb4b16;\">Right</span><span style=\"color:#657b83;\"> newList' </span><span style=\"color:#859900;\">-></span><span style=\"color:#657b83;\"> print newList\n</span></code></pre>\n<p><code>Either</code> is a sum type, like an <code>enum</code> in Rust. It's equivalent to <code>Result</code> in Rust, but with different names. Instead of <code>Ok</code> and <code>Err</code>, we have <code>Right</code> (used by convention for success) and <code>Left</code> (used by convention for failure). The <code>Applicative</code> instance for it will stop processing when it encounters the first <code>Left</code>. So our program above will ultimately produce the output <code>Not allowed to double big numbers</code>. You can put as many values after the <code>6</code> in <code>list</code> as you want, and it will produce the same output. In fact, it will never even inspect those numbers.</p>\n<p>Coming back to Rust, let's first simply collect all of our <code>Result</code>s together into a single <code>Vec</code> to make sure the basics work:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">main</span><span style=\"color:#657b83;\">() {\n </span><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> myvec = </span><span style=\"color:#859900;\">vec!</span><span style=\"color:#657b83;\">[</span><span style=\"color:#6c71c4;\">1</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">2</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">3</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">4</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">5</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">6</span><span style=\"color:#657b83;\">];\n\n </span><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> new_vec: </span><span style=\"color:#859900;\">Vec</span><span style=\"color:#657b83;\"><</span><span style=\"color:#859900;\">Result</span><span style=\"color:#657b83;\"><</span><span style=\"color:#268bd2;\">i32</span><span style=\"color:#657b83;\">, </span><span style=\"color:#859900;\">&</span><span style=\"color:#268bd2;\">str</span><span style=\"color:#657b83;\">>> = myvec\n .</span><span style=\"color:#859900;\">into_iter</span><span style=\"color:#657b83;\">()\n .</span><span style=\"color:#859900;\">map</span><span style=\"color:#657b83;\">(|</span><span style=\"color:#268bd2;\">x</span><span style=\"color:#657b83;\">| {\n </span><span style=\"color:#859900;\">if</span><span style=\"color:#657b83;\"> x > </span><span style=\"color:#6c71c4;\">5 </span><span style=\"color:#657b83;\">{\n </span><span style=\"color:#859900;\">Err</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">Not allowed to double big numbers</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">)\n } </span><span style=\"color:#859900;\">else </span><span style=\"color:#657b83;\">{\n </span><span style=\"color:#859900;\">Ok</span><span style=\"color:#657b83;\">(x * </span><span style=\"color:#6c71c4;\">2</span><span style=\"color:#657b83;\">)\n }\n })\n .</span><span style=\"color:#859900;\">collect</span><span style=\"color:#657b83;\">();\n\n </span><span style=\"color:#859900;\">println!</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#cb4b16;\">{:?}</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, new_vec);\n}\n</span></code></pre>\n<p>That makes sense. We've already seen that <code>.collect()</code> can take all the values in an <code>Iterator</code>'s stream and stick them into a <code>Vec</code>. And the <code>map</code> method is now generating <code>Result<i32, &str></code> values, so everything lines up.</p>\n<p>But this isn't the behavior we want. We want two changes:</p>\n<ul>\n<li><code>new_vec</code> should result in a <code>Result<Vec<i32>, &str></code>. In other words, it should result in either a single <code>Err</code> value, or a vector of successful results. Right now, it has a vector of success-or-failure values.</li>\n<li>We should immediately stop processing the original <code>Vec</code> once we see a value that's too big.</li>\n</ul>\n<p>To make it a bit more clear, it's easy enough to implement this with a <code>for</code> loop:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">main</span><span style=\"color:#657b83;\">() {\n </span><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> myvec = </span><span style=\"color:#859900;\">vec!</span><span style=\"color:#657b83;\">[</span><span style=\"color:#6c71c4;\">1</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">2</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">3</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">4</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">5</span><span style=\"color:#657b83;\">];\n\n </span><span style=\"color:#268bd2;\">let </span><span style=\"color:#586e75;\">mut</span><span style=\"color:#657b83;\"> new_vec = </span><span style=\"color:#859900;\">Vec</span><span style=\"color:#657b83;\">::new();\n\n </span><span style=\"color:#859900;\">for</span><span style=\"color:#657b83;\"> x </span><span style=\"color:#859900;\">in</span><span style=\"color:#657b83;\"> myvec {\n </span><span style=\"color:#859900;\">if</span><span style=\"color:#657b83;\"> x > </span><span style=\"color:#6c71c4;\">5 </span><span style=\"color:#657b83;\">{\n </span><span style=\"color:#859900;\">println!</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">Not allowed to double big numbers</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">);\n </span><span style=\"color:#859900;\">return</span><span style=\"color:#657b83;\">;\n } </span><span style=\"color:#859900;\">else </span><span style=\"color:#657b83;\">{\n new_vec.</span><span style=\"color:#859900;\">push</span><span style=\"color:#657b83;\">(x);\n }\n }\n\n </span><span style=\"color:#859900;\">println!</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#cb4b16;\">{:?}</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, new_vec);\n}\n</span></code></pre>\n<p>But now we've lost out on our <code>map</code> entirely, and we're dropping down to using explicit loops, mutation, and short-circuiting (via <code>return</code>). In other words, this code doesn't feel nearly as elegant to me.</p>\n<p>It turns out that our original code was almost perfect. Let's see a bit of magic, and then explain how it happend. Our previous version of the code used <code>map</code> and resulted in a <code>Vec<Result<i32, &str>></code>. And we wanted <code>Result<Vec<i32>, &str></code>. What happens if we simply change the type to what we want?</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">main</span><span style=\"color:#657b83;\">() {\n </span><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> myvec = </span><span style=\"color:#859900;\">vec!</span><span style=\"color:#657b83;\">[</span><span style=\"color:#6c71c4;\">1</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">2</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">3</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">4</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">5</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">6</span><span style=\"color:#657b83;\">];\n\n </span><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> new_vec: </span><span style=\"color:#859900;\">Result</span><span style=\"color:#657b83;\"><</span><span style=\"color:#859900;\">Vec</span><span style=\"color:#657b83;\"><</span><span style=\"color:#268bd2;\">i32</span><span style=\"color:#657b83;\">>, </span><span style=\"color:#859900;\">&</span><span style=\"color:#268bd2;\">str</span><span style=\"color:#657b83;\">> = myvec\n .</span><span style=\"color:#859900;\">into_iter</span><span style=\"color:#657b83;\">()\n .</span><span style=\"color:#859900;\">map</span><span style=\"color:#657b83;\">(|</span><span style=\"color:#268bd2;\">x</span><span style=\"color:#657b83;\">| {\n </span><span style=\"color:#859900;\">if</span><span style=\"color:#657b83;\"> x > </span><span style=\"color:#6c71c4;\">5 </span><span style=\"color:#657b83;\">{\n </span><span style=\"color:#859900;\">Err</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">Not allowed to double big numbers</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">)\n } </span><span style=\"color:#859900;\">else </span><span style=\"color:#657b83;\">{\n </span><span style=\"color:#859900;\">Ok</span><span style=\"color:#657b83;\">(x * </span><span style=\"color:#6c71c4;\">2</span><span style=\"color:#657b83;\">)\n }\n })\n .</span><span style=\"color:#859900;\">collect</span><span style=\"color:#657b83;\">();\n\n </span><span style=\"color:#859900;\">match</span><span style=\"color:#657b83;\"> new_vec {\n </span><span style=\"color:#859900;\">Ok</span><span style=\"color:#657b83;\">(new_vec) </span><span style=\"color:#859900;\">=> println!</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#cb4b16;\">{:?}</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, new_vec),\n </span><span style=\"color:#859900;\">Err</span><span style=\"color:#657b83;\">(e) </span><span style=\"color:#859900;\">=> println!</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#cb4b16;\">{}</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, e),\n }\n}\n</span></code></pre>\n<p>Thanks to the power of <code>FromIterator</code>, this simply works! To understand why, let's see some <a href=\"https://doc.rust-lang.org/stable/std/iter/trait.FromIterator.html#method.from_iter-15\">documentation on <code>FromIterator</code></a>:</p>\n<blockquote>\n<p>Takes each element in the <code>Iterator</code>: if it is an <code>Err</code>, no further elements are taken, and the <code>Err</code> is returned. Should no <code>Err</code> occur, a container with the values of each <code>Result</code> is returned.</p>\n</blockquote>\n<p>And suddenly it seems that Rust has implemented <code>traverse</code> all along! This extra flexibility in the <code>FromIterator</code> setup allows us to regain the short-circuiting error-handling behavior that FP people are familiar with in <code>traverse</code>.</p>\n<p>In contrast to <code>traverse</code>, we're still dealing with two different traits (<code>IntoIterator</code> and <code>FromIterator</code>), and there's nothing preventing these from being different types. Therefore, some kind of type annotation is still necessary. On the one hand, that could be seen as a downside of Rust's approach. On the other hand, it allows us to be more flexible in what types we generate, which we'll look at in the next section.</p>\n<p>And finally, it turns out we can use turbofish to rescue us yet again. For example:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">main</span><span style=\"color:#657b83;\">() {\n </span><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> myvec = </span><span style=\"color:#859900;\">vec!</span><span style=\"color:#657b83;\">[</span><span style=\"color:#6c71c4;\">1</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">2</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">3</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">4</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">5</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">6</span><span style=\"color:#657b83;\">];\n\n </span><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> new_vec = myvec\n .</span><span style=\"color:#859900;\">into_iter</span><span style=\"color:#657b83;\">()\n .</span><span style=\"color:#859900;\">map</span><span style=\"color:#657b83;\">(|</span><span style=\"color:#268bd2;\">x</span><span style=\"color:#657b83;\">| {\n </span><span style=\"color:#859900;\">if</span><span style=\"color:#657b83;\"> x > </span><span style=\"color:#6c71c4;\">5 </span><span style=\"color:#657b83;\">{\n </span><span style=\"color:#859900;\">Err</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">Not allowed to double big numbers</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">)\n } </span><span style=\"color:#859900;\">else </span><span style=\"color:#657b83;\">{\n </span><span style=\"color:#859900;\">Ok</span><span style=\"color:#657b83;\">(x * </span><span style=\"color:#6c71c4;\">2</span><span style=\"color:#657b83;\">)\n }\n })\n .collect::<</span><span style=\"color:#859900;\">Result</span><span style=\"color:#657b83;\"><</span><span style=\"color:#859900;\">Vec</span><span style=\"color:#657b83;\"><</span><span style=\"color:#859900;\">_</span><span style=\"color:#657b83;\">>, </span><span style=\"color:#859900;\">_</span><span style=\"color:#657b83;\">>>();\n\n </span><span style=\"color:#859900;\">match</span><span style=\"color:#657b83;\"> new_vec {\n </span><span style=\"color:#859900;\">Ok</span><span style=\"color:#657b83;\">(new_vec) </span><span style=\"color:#859900;\">=> println!</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#cb4b16;\">{:?}</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, new_vec),\n </span><span style=\"color:#859900;\">Err</span><span style=\"color:#657b83;\">(e) </span><span style=\"color:#859900;\">=> println!</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#cb4b16;\">{}</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, e),\n }\n}\n</span></code></pre><h2 id=\"different-fromiterator-impls\">Different FromIterator impls</h2>\n<p>So far, we've only seen two implementations of <code>FromIterator</code>: <code>Vec</code> and <code>Result</code>. There are many more available. One of my favorite is <code>HashMap</code>, which lets you collect a sequence of key/value pairs into a mapping.</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#859900;\">use </span><span style=\"color:#657b83;\">std::collections::HashMap;\n\n</span><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">main</span><span style=\"color:#657b83;\">() {\n </span><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> people = </span><span style=\"color:#859900;\">vec!</span><span style=\"color:#657b83;\">[\n (</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">Alice</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">30</span><span style=\"color:#657b83;\">),\n (</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">Bob</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">35</span><span style=\"color:#657b83;\">),\n (</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">Charlies</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">25</span><span style=\"color:#657b83;\">),\n ].</span><span style=\"color:#859900;\">into_iter</span><span style=\"color:#657b83;\">().collect::<HashMap<</span><span style=\"color:#859900;\">_</span><span style=\"color:#657b83;\">, </span><span style=\"color:#859900;\">_</span><span style=\"color:#657b83;\">>>();\n\n </span><span style=\"color:#859900;\">println!</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">Alice is: </span><span style=\"color:#cb4b16;\">{:?}</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, people.</span><span style=\"color:#859900;\">get</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">Alice</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">));\n}\n</span></code></pre>\n<p>And due to how the <code>FromIterator</code> impl for <code>Result</code> works, you can layer these two together to collect a stream of <code>Result</code>s of pairs into a <code>Result<HashMap<_, _>, _></code>:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#859900;\">use </span><span style=\"color:#657b83;\">std::collections::HashMap;\n\n</span><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">main</span><span style=\"color:#657b83;\">() {\n </span><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> people = </span><span style=\"color:#859900;\">vec!</span><span style=\"color:#657b83;\">[\n </span><span style=\"color:#859900;\">Ok</span><span style=\"color:#657b83;\">((</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">Alice</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">30</span><span style=\"color:#657b83;\">)),\n </span><span style=\"color:#859900;\">Ok</span><span style=\"color:#657b83;\">((</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">Bob</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">35</span><span style=\"color:#657b83;\">)),\n </span><span style=\"color:#859900;\">Err</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">Uh-oh, this didn't work!</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">),\n </span><span style=\"color:#859900;\">Ok</span><span style=\"color:#657b83;\">((</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">Charlies</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">25</span><span style=\"color:#657b83;\">)),\n ].</span><span style=\"color:#859900;\">into_iter</span><span style=\"color:#657b83;\">().collect::<</span><span style=\"color:#859900;\">Result</span><span style=\"color:#657b83;\"><HashMap<</span><span style=\"color:#859900;\">_</span><span style=\"color:#657b83;\">, </span><span style=\"color:#859900;\">_</span><span style=\"color:#657b83;\">>, </span><span style=\"color:#859900;\">&</span><span style=\"color:#268bd2;\">str</span><span style=\"color:#657b83;\">>>();\n\n </span><span style=\"color:#859900;\">match</span><span style=\"color:#657b83;\"> people {\n </span><span style=\"color:#859900;\">Err</span><span style=\"color:#657b83;\">(e) </span><span style=\"color:#859900;\">=> println!</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">Error occurred: </span><span style=\"color:#cb4b16;\">{}</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, e),\n </span><span style=\"color:#859900;\">Ok</span><span style=\"color:#657b83;\">(people) </span><span style=\"color:#859900;\">=> </span><span style=\"color:#657b83;\">{\n </span><span style=\"color:#859900;\">println!</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">Alice is: </span><span style=\"color:#cb4b16;\">{:?}</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, people.</span><span style=\"color:#859900;\">get</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">Alice</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">));\n }\n }\n}\n</span></code></pre><h2 id=\"validation\">Validation</h2>\n<p>In the Haskell world, we have two different concepts of error collection:</p>\n<ul>\n<li><code>Either</code>, which says "stop on the first error"</li>\n<li><code>Validation</code>, which says "collect all of the errors together"</li>\n</ul>\n<p><code>Validation</code> can be very useful for things like parsing web forms. You don't want to generate just the first failure, but collect all of the failures together for producing a more user-friendly experience. For fun, I decided to implement this in Rust as well:</p>\n<blockquote class=\"twitter-tweet\"><p lang=\"en\" dir=\"ltr\">I'm tempted to write a Validation "Applicative" in Rust with a FromIterator impl to collect multiple Err values. I have no real need for this, but it still seems fun.</p>— Michael Snoyman (@snoyberg) <a href=\"https://twitter.com/snoyberg/status/1311639149171159041?ref_src=twsrc%5Etfw\">October 1, 2020</a></blockquote> <script async src=\"https://platform.twitter.com/widgets.js\" charset=\"utf-8\"></script>\n<p>And as you'll see from the rest of that thread, a lot of the motivation for this blog post came from the Twitter replies.</p>\n<p>The implementation in Rust is fairly straightforward, and pretty easy to understand. I've <a href=\"https://github.com/snoyberg/validation-rs\">made it available on Github</a>. If there's any interest in seeing this as a crate, let me know in the issue tracker.</p>\n<p>To see this in action, let's modify our program above. First, I'll add the dependency to my <code>Cargo.toml</code> file:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#657b83;\">[</span><span style=\"color:#b58900;\">dependencies</span><span style=\"color:#657b83;\">.</span><span style=\"color:#b58900;\">validation</span><span style=\"color:#657b83;\">]\n</span><span style=\"color:#268bd2;\">git </span><span style=\"color:#657b83;\">= </span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">https://github.com/snoyberg/validation-rs</span><span style=\"color:#839496;\">"\n</span><span style=\"color:#268bd2;\">rev </span><span style=\"color:#657b83;\">= </span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">0a7521f7022262bb00aea61761f76c3dd5ccefb5</span><span style=\"color:#839496;\">"\n</span></code></pre>\n<p>And then modify the code to use the <code>Validation</code> enum instead of <code>Result</code>:</p>\n<pre style=\"background-color:#fdf6e3;\">\n<code><span style=\"color:#859900;\">use </span><span style=\"color:#657b83;\">std::collections::HashMap;\n</span><span style=\"color:#859900;\">use </span><span style=\"color:#657b83;\">validation::Validation;\n\n</span><span style=\"color:#268bd2;\">fn </span><span style=\"color:#b58900;\">main</span><span style=\"color:#657b83;\">() {\n </span><span style=\"color:#268bd2;\">let</span><span style=\"color:#657b83;\"> people = </span><span style=\"color:#859900;\">vec!</span><span style=\"color:#657b83;\">[\n </span><span style=\"color:#859900;\">Ok</span><span style=\"color:#657b83;\">((</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">Alice</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">30</span><span style=\"color:#657b83;\">)),\n </span><span style=\"color:#859900;\">Ok</span><span style=\"color:#657b83;\">((</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">Bob</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">35</span><span style=\"color:#657b83;\">)),\n </span><span style=\"color:#859900;\">Err</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">Uh-oh, this didn't work!</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">),\n </span><span style=\"color:#859900;\">Ok</span><span style=\"color:#657b83;\">((</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">Charlies</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, </span><span style=\"color:#6c71c4;\">25</span><span style=\"color:#657b83;\">)),\n </span><span style=\"color:#859900;\">Err</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">And neither did this!</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">),\n ].</span><span style=\"color:#859900;\">into_iter</span><span style=\"color:#657b83;\">().collect::<Validation<HashMap<</span><span style=\"color:#859900;\">_</span><span style=\"color:#657b83;\">, </span><span style=\"color:#859900;\">_</span><span style=\"color:#657b83;\">>, </span><span style=\"color:#859900;\">Vec</span><span style=\"color:#657b83;\"><</span><span style=\"color:#859900;\">&</span><span style=\"color:#268bd2;\">str</span><span style=\"color:#657b83;\">>>>();\n\n </span><span style=\"color:#859900;\">match</span><span style=\"color:#657b83;\"> people.</span><span style=\"color:#859900;\">into_result</span><span style=\"color:#657b83;\">() {\n </span><span style=\"color:#859900;\">Err</span><span style=\"color:#657b83;\">(errs) </span><span style=\"color:#859900;\">=> </span><span style=\"color:#657b83;\">{\n </span><span style=\"color:#859900;\">println!</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">Errors:</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">);\n errs.</span><span style=\"color:#859900;\">into_iter</span><span style=\"color:#657b83;\">().</span><span style=\"color:#859900;\">map</span><span style=\"color:#657b83;\">(|</span><span style=\"color:#268bd2;\">x</span><span style=\"color:#657b83;\">| </span><span style=\"color:#859900;\">println!</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#cb4b16;\">{}</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, x)).</span><span style=\"color:#859900;\">collect</span><span style=\"color:#657b83;\">()\n }\n </span><span style=\"color:#859900;\">Ok</span><span style=\"color:#657b83;\">(people) </span><span style=\"color:#859900;\">=> </span><span style=\"color:#657b83;\">{\n </span><span style=\"color:#859900;\">println!</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">Alice is: </span><span style=\"color:#cb4b16;\">{:?}</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">, people.</span><span style=\"color:#859900;\">get</span><span style=\"color:#657b83;\">(</span><span style=\"color:#839496;\">"</span><span style=\"color:#2aa198;\">Alice</span><span style=\"color:#839496;\">"</span><span style=\"color:#657b83;\">));\n }\n }\n}\n</span></code></pre>\n<p><strong>Bonus</strong> Note the somewhat cheeky usage of <code>map</code> and <code>collect</code> to print out the errors. This is leveraging the <code>()</code> impl of <code>FromIterator</code>, which collects together a stream of <code>()</code> values into a single one.</p>\n<h2 id=\"conclusion\">Conclusion</h2>\n<p>I realize that was a bit of a rambling journey, but hopefully a fun one for Rustaceans, Haskellers, and Scala folk. Here are some of my takeaways:</p>\n<ul>\n<li>The <code>collect</code> method is very flexible</li>\n<li>There's no magic involved in <code>collect</code>, just the <code>FromIterator</code> trait and the behavior of the types that implement it\n<ul>\n<li>This was actually a big takeaway for me. I had somehow forgotten about <code>FromIterator</code> a few months back, and was nervous about what "secret" behavior <code>collect</code> was doing.</li>\n</ul>\n</li>\n<li>The downside of <code>collect</code> is that, since it's not structure preserving like <code>map</code> or <code