Get Involved with Nixpkgs Haskell

2022-11-16

This post explains how you can get started with contributing to the Haskell subsystem of Nixpkgs. This will mainly be helpful for people who are happily using Haskell binaries and Haskell libraries from Nixpkgs, but want to start contributing additional fixes, or helping the Haskell subsystem in Nixpkgs advance faster.

This post lists a few different ways to get involved, from easiest to hardest.

Method 1: Join our Matrix Channel

The easiest way to get involved is to join the Matrix channel for the Haskell infrastructure in Nixpkgs: #haskell:nixos.org. Here you can talk to the Haskell maintainers in Nixpkgs, as well as other users. This is a great place to ask casual questions.

Method 2: Answer Haskell- and Nix-related Questions on Other Sites

People often ask questions related to the Haskell subsystem of Nixpkgs on other websites. The maintainers in Nixpkgs are not always watching these other websites, so it is very helpful for other members of the community to answer questions.

Here's a non-exhaustive list of other sites where people frequently ask questions:

Method 3: Add Yourself to the Maintainers List for Your Haskell Packages in Nixpkgs

Haskell packages in Nixpkgs have a notion of a maintainer that is specific to Nixpkgs. The maintainer in Nixpkgs could be different from the person who uploads the packages to Hackage. When Haskell packages break in Nixpkgs, the maintainers are notified on GitHub. Here's an example of a notification. You can see that a couple well-known packages were broken, and the maintainers were pinged.

If you add yourself to the maintainer list for a given Haskell package, you'll be pinged on GitHub when it breaks and stops compiling. You'll have time to send a PR fixing it before Nixpkgs moves on with the broken package.

To add yourself to the maintainers list for specific packages, send a PR adding your GitHub handle and a list of Haskell packages here.

Again, you can add yourself as a maintainer for any Haskell package you use. You don't have to be the original author of the package.

Method 4: Watch the haskell Label in Nixpkgs

Haskell-related issues and PRs get tagged with the haskell label in Nixpkgs. Responding to issues and PRs can take a lot of work, so we really appreciate when other users help with responding to issues, or do reviews of PRs.

Even if you feel like you're not knowledgable enough with how the Haskell ecosystem works in Nixpkgs, we still really appreciate comments like "I can confirm this is a problem, it fails for me too!", or "I've tried building the derivation from this PR and I can confirm it works!"

After reviewing even 5 to 10 Haskell PRs, you should have a good idea of the common sources of problems, and how we fix them.1 You'll be able to make suggestions to new PRs without us needing to be involved. It is so much easier for us to just be able to click the "Merge" button on GitHub than having to go back and forth on a PR.

Method 5: Send PRs Fixing Broken Haskell Packages in Nixpkgs

Every week or two In Nixpkgs, we branch off master with the haskell-updates branch and update our Stackage and Hackage pins. This often causes some Haskell packages to break. We spend a couple days trying to fix as much of the breakage as possible. When the haskell-updates branch is in a good state, we merge it back in to master and repeat the process. Here is an example of one of these haskell-updates PRs. The specific details of this process are documented in HACKING.md in the Nixpkgs repo.

We have a daily build-report that shows which packages are broken. We always appreciate when people watch this build report, and send fixes for broken packages. This really helps us be able to merge the haskell-updates branch faster, and get the most up-to-date libraries and compilers out to our users. If you've ever heard someone complain about Nixpkgs not having the latest GHC, you should really point them to this blog post and ask them to help out!2

In theory, all you should need to do is watch this daily build-report, but in practice, you will likely also want to watch the haskell-updates jobset on Hydra, and fix any evaluation errors that are present. These also prevent us from merging in the haskell-updates branch to master.

Conclusion

If you're a happy Haskell and Nixpkgs user (or even better, an unhappy user), we'd really appreciate help keeping Nixpkgs in good shape. Please feel free to join us on Matrix, or dive in and start sending PRs!

Footnotes

  1. We don't currently have a guide for how to fix a given broken Haskell package, but watching incoming PRs is a really good way to figure out how to unbreak packages.↩︎

  2. To add a little more explanation to this, here's the general route for end-users to be able to use Haskell-related changes in Nixpkgs:

    1. We ask that all Haskell-related changes in Nixpkgs to be sent as PRs to the haskell-updates branch. This includes things like new GHC versions. We review PRs to the haskell-updates branch and merge them in.
    2. The Hydra jobset for haskell-updates builds all Haskell-related packages. We can merge the haskell-updates branch into master as soon as CI is clean.
    3. After merging haskell-updates into master, the various Hydra jobsets for the nixpkgs-unstable and nixos-unstable channels go to work. Once these are finished, end-users are easily able to use new Haskell packages and compilers.
    4. We restart this process.
    ↩︎

tags: haskell, nixos