Stackage changes and Stack 2.

Posted by Michael Snoyman - 24 April, 2019

Stackage changes and Stack 2

We’re ramping up for the Stack 2 release, which contains a number of changes. (If you want more information, check out the changelog.) I’m not going to be covering all of those changes in this blog post. Instead, today, I want to talk about what the release process will look like, both for Stack itself and Stackage.

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Stack release process

I’ve created a new Stack 2 milestone on Github to track issues we want resolved before release. I’m also now announcing a 2 week deadline for a feature freeze. If you have a feature you believe belongs in Stack 2, or a bug fix you’d like to see addressed in this release, now’s the time to raise it. As usual, pull requests will be more likely to result in changes happening, and the Stack team is always happy to help people new to the codebase.

As of 2 weeks from today (May 8), we’ll enter a code freeze period. We’ll cut a release branch and begin releasing release candidates. New features will not be added to the release candidate branch, and we will need to weigh carefully which bug fixes are added to this branch to avoid destabilizing things too much.

I strongly encourage Stack users to begin experimenting with the Stack master branch earlier rather than later. You can upgrade to the latest master by running `stack upgrade --git`.

We will plan for at least a 3 week code freeze period, which may be extended based on feedback from users. That would make the earliest release of Stack 2 May 29.

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Changes to Stackage

Last year, I wrote a few blog posts about Pantry. I never really finished that series, and I think some people were left wondering how this would affect Stack and Stackage going forward. Let me give a bit more information, now that we know how Pantry is going to work out. Big thanks to Kirill Zaborsky and Sibi Prabakaran for all of their work on the Pantry changes.

Pantry is a library for representing Haskell source files in a fully deterministic way. Each package ends up with a pantry tree key, which is a hash of its contents, and some description of how to get the source code. Examples would be from Hackage with a specific revision, from a Git repository at a certain commit, or from a tarball.

With Stack 2, we are overhauling the Stackage snapshot file format to directly use Pantry. This fixes a long-standing annoyance: we had different snapshot formats for Stackage itself and for “custom snapshots” in Stack. Those are now unified. It also addresses a long-standing concern in Stackage: the inability to patch packages if necessary. By allowing package sources from outside of Hackage, the Stackage Curator team will now have the ability to patch packages as necessary.

We’ve already been generating new-style Stackage snapshot files, which can be viewed at the commercialhaskell/stackage-snapshots repo. Currently, we’re using the original Stackage build process, generating the original LTS Haskell and Stackage Nightly snapshot formats, and converting to the new format. In the next week or two, we’ll be reversing that: using a new Stackage Curator build tool to produce snapshot files and perform builds, and then for compatibility generating the old file formats.

We don’t have a guarantee of how long we will continue to support the original file format. There are no plans at all to ever remove the existing files, but at some point in the future we will stop generating compatibility files. We recommend tooling authors using the snapshot files directly get in touch to discuss upgrade paths. (I discussed this offline with some of the Nix contributor team, but that work was never completed.)

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Future: Pantry server

Pantry works via content-addressable storage: each tree and file can be accessed via its hash. One plan we have for the future is to write a Pantry server, which will allow Stack (and other tools) to download package contents via the Pantry key instead of, for example, needing to clone a Git repository. For compatibility with tooling already familiar with HTTP(S) and Tarballs, we’ll likely provide such an interface to this store as well.

That said: we are not including this feature in Stack 2. It is slated for a later release. Personally, I think it will be a fun project. If anyone’s interested in working on this, feel free to comment on the issue.

Further details about Pantry

I’m not sure if Pantry is a topic people are interested in hearing more about and using it in their own projects. For now, I’m keeping the details relatively high-level in this post. If there's a demand for more information, hit me up with questions and I can put together another post answering them.

Topics: stack, haskell stack, Stackage, Haskell Software


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