FP Complete

This post is aimed at Haskellers who are roughly aware of how build infrastructure works for Haskell.

But the topic may have general audience outside of the Haskell community, so this post will briefly describe each part of the infrastructure from the bottom up: compiling modules, building and configuring packages, to downloading and storing those packages online.

This post is a semi-continuation from last week’s post on Casa.


GHC is the de facto standard Haskell compiler. It knows how to load packages and compile files, and produce binary libraries and executables. It has a small database of installed packages, with a simple command-line interface for registering and querying them:

$ ghc-pkg register yourpackage
$ ghc-pkg list

Apart from that, it doesn’t know anything else about how to build packages or where to get them.


Cabal is the library which builds Haskell packages from a .cabal file package description, which consists of a name, version, package dependencies and build flags. To build a Haskell package, you create a file (typically Setup.hs), with contents roughly like:

import Distribution.Simple -- from the Cabal library
main = defaultMain

This (referred to as a “Simple” build), creates a program that you can run to configure, build and install your package.

$ ghc Setup.hs
$ ./Setup configure # Checks dependencies via ghc-pkg
$ ./Setup build # Compiles the modules with GHC
$ ./Setup install # Runs the register step via ghc-pkg

This file tends to be included in the source repository of your package. And modern package build tools tend to create this file automatically if it doesn’t already exist. The reason the build system works like this is so that you can have custom build setups: you can make pre/post build hooks and things like that.

But the Cabal library doesn’t download packages or manage projects consisting of multiple packages, etc.


Hackage is an online archive of versioned package tarballs. Anyone can upload packages to this archive, where the package must have a version associated with it, so that you can later download a specific instance of the package that you want, e.g. text- Each package is restricted to a set of maintainers (such as the author) who is able to upload to it.

The Hackage admins and authors are able to revise the .cabal package description without publishing a new version, and regularly do. These new revisions supersede previous revisions of the cabal files, while the original revisions still remain available if specifically requested (if supported by tooling being used).


There is a program called cabal-install which is able to download packages from Hackage automatically and does some constraint solving to produce a build plan. A build plan is when the tool picks what versions of package dependencies your package needs to build.

It might look like:

Version bounds (<2.1 and >1.3) are used by cabal-install as heuristics to do the solving. It isn’t actually known whether any of these packages build together, or that the build plan will succeed. It’s a best guess.

Finally, once it has a build plan, it uses both GHC and the Cabal library to build Haskell packages, by creating the aforementioned Setup.hs automatically if it doesn’t already exist, and running the ./Setup configure, build, etc. step.


As mentioned, the build plans produced by cabal-install are a best guess based on constraint solving of version bounds. There is a matrix of possible build plans, and the particular one you get may be entirely novel, that no one has ever tried before. Some call this “version hell”.

To rectify this situation, Stackage is a “stable Hackage” service, which publishes known subsets of Hackage that are known to build and pass tests together, called snapshots. There are nightly snapshots published, and long-term snapshots called lts-1.0, lts-2.2, etc. which tend to steadily roll along with the GHC release cycle. These LTS releases are intended to be what people put in source control for their projects.

The Stackage initiative has been running since it was announced in 2012.


The stack program was created to specifically make reproducible build plans based on Stackage. Authors include a stack.yaml file in their project root, which looks like this:

snapshot: lts-1.2
packages: [mypackage1, mypackage2]

This tells stack that:

  1. We want to use the lts-1.2 snapshot, therefore any package dependencies that we need for this project will come from there.
  2. That within this directory, there are two package directories that we want to build.

The snapshot also indicates which version of GHC is used to build that snapshot; so stack also automatically downloads, installs and manages the GHC version for the user. GHC releases tend to come out every 6 months to one year, depending on scheduling, so it’s common to have several GHC versions installed on your machine at once. This is handled transparently out of the box with stack.

Additionally, we can add extra dependencies for when we have patched versions of upstream libraries, which happens a lot in the fast-moving world of Haskell:

snapshot: lts-1.2
packages: [mypackage1, mypackage2]
extra-deps: ["bifunctors-5.5.4"]

The build plan for Stack is easy: the snapshot is already a build plan. We just need to add our source packages and extra dependencies on top of the pristine build plan.

Finally, once it has a build plan, it uses both GHC and the Cabal library to build Haskell packages, by creating the aforementioned Setup.hs automatically if it doesn’t already exist, and running the ./Setup configure, build, etc. step.


Since new revisions of cabal files can be made available at any time, a package identifier like bifunctors-5.5.4 is not reproducible. Its meaning can change over time as new revisions become available. In order to get reproducible build plans, we have to track “revisions” such as bifunctors-5.5.4@rev:1.

Stack has a library called Pantry to store all of this package metadata into an sqlite database on the developer’s machine. It does so in a content-addressable way (CAS), so that every variation on version and revision of a package has a unique SHA256 cryptographic hash summarising both the .cabal package description, and the complete contents of the package.

This lets Stackage be exactly precise. Stackage snapshots used to look like this:

- hackage: List-0.5.2
- hackage: ListLike-4.2.1

Now it looks like this:

- hackage: ALUT-,4118
    size: 1562
    sha256: c9968ebed74fd3956ec7fb67d68e23266b52f55b2d53745defeae20fbcba5579
- hackage: ANum-,1075
    size: 355
    sha256: ba7baa3fadf0a733517fd49c73116af23ccb2e243e08b3e09848dcc40de6bc90

So we’re able to CAS identify the .cabal file by a hash and length,


And we’re able to CAS identify the contents of the package:

  size: 355
  sha256: ba7baa3fadf0a733517fd49c73116af23ccb2e243e08b3e09848dcc40de6bc90

Additionally, each and every file within the package is CAS-stored. The “pantry-tree” refers to a list of CAS hash-len keys (which is also serialised to a binary blob and stored in the same CAS store as the files inside the tarball themselves). With every file stored, we remove a lot of duplication that we had storing a whole tarball for every single variation of a package.

Parenthetically, the 01-index.tar that Hackage serves up with all the latest .cabal files and revisions has to be downloaded every time. As this file is quite large this is slow and wasteful.

Another side point: Hackage Security is not needed or consulted for this. CAS already allows us to know in advance whether what we are receiving is correct or not, as stated elsewhere.

When switching to a newer snapshot, lots of packages will be updated, but within each package, only a few files will have changed. Therefore we only need to download those few files that are different. However, to achieve that, we need an online service capable of serving up those blobs by their SHA256…

Enter Casa

As announced in our casa post, Casa stands for “content-addressable storage archive”, and also means “home” in romance languages, and it is an online service we’re announcing to store packages in a content-addressable way.

Now, the same process which produces Stackage snapshots, can also:

Stack can now download all its assets needed to build a package from Casa:

Furthermore, the snapshot format of Stackage supports specifying locations other than Hackage, such as a git repository at a given commit, or a URL with a tarball. These would also be automatically pushed to Casa, and Stack would download them from Casa automatically like any other package. Parenthetically, Stackage does not currently include packages from outside of Hackage, but Stack’s custom snapshots–which use the same format–do support that.

Internal Company Casas

Companies often run their own Hackage on their own network (or IP-limited public server) and upload their custom packages to it, to be used by everyone in the company.

With the advent of Stack, this became less needed because it’s trivial to fork any package on GitHub and then link to the Git repo in a stack.yaml. Plus, it’s more reproducible, because you refer to a hash rather than a mutable version. Combined with the additional Pantry-based SHA256+length described above, you don’t have to trust GitHub to serve the right content, either.

The Casa repository is here which includes both the server and a (Haskell) client library with which you can push arbitrary files to the casa service. Additionally, to populate your Casa server with everything from a given snapshot, or all of Hackage, you can use casa-curator from the curator repo, which is what we use ourselves.

If you’re a company interested in running your own Casa server, please contact us. Or, if you’d like to discuss the possibility of caching packages in binary form and therefore skipping the build step altogther, please contact us. Also contact us if you would like to discuss storing GHC binary releases into Casa and have Stack pull from it, to allow for a completely Casa-enabled toolchain.


Here’s what we’ve brought to Haskell build infrastructure:

When you upgrade to Stack master or the next release of Stack, you will automatically be using the Casa server.

We believe this CAS architecture has use in other language ecosystems, not just Haskell. See the Casa post for more details.

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