First Open Source Contribution to a Haskell Project

The first pull request I ever sent to a Haskell library changed eleven characters in a Haddock comment. It sat for nine days, got one review comment asking me to run the formatter, and was merged on a Sunday morning. That small, slightly awkward exchange taught me more about how Haskell projects actually work than a month of reading source code had.

Making your first open source contribution in Haskell is less about cleverness with types and more about reading a project's habits correctly. Most libraries on Hackage are kept alive by one to three volunteers, and they judge a newcomer's patch on whether it is easy to review. This guide covers how to pick the project, find a task that will be merged, build it without fighting the toolchain, and write a pull request a busy maintainer can approve in ten minutes.

Pick a Project You Already Depend On

Do not start by browsing lists of beginner friendly repositories. Start with the cabal file of something you have built. Every package in your build-depends list is a project whose API you already understand, whose bugs you may already have hit, and whose documentation you have read with a real question in mind.

Then check three signals on the repository before you invest an evening:

• Recent merges from outsiders. If the last ten merged pull requests all came from the maintainer, outside patches may not be welcome or simply may not get attention.
• Response time on issues. A project that answers within a week is worth more to a beginner than a famous one that answers within a quarter.
• A CONTRIBUTING file or a clear CI setup. Either one tells you the expected formatter, the supported GHC versions and how tests are run.

Mid sized libraries, roughly 2,000 to 15,000 lines, tend to be the sweet spot. If you want background on how these packages are released and kept up, the piece on how the Haskell community builds and maintains its packages explains the Hackage side of the process.

Find a Task That Will Actually Get Merged

The fastest merges I have seen fall into four categories, roughly in order of how likely they are to be accepted:

1. Documentation fixes: a missing example in a Haddock block, a wrong type in a comment, a broken module link.
2. Build fixes for a new GHC release, usually a changed import or a bumped upper bound.
3. Tests for behaviour that already works but has no coverage.
4. Small bug fixes with a clear reproduction in an existing issue.

New features are last on that list for a reason. A feature adds API surface the maintainer must support for years. A doc fix adds nothing to maintain. Filter the tracker by labels such as "good first issue", but also read closed issues from the last six months to learn what the maintainer rejects.

The Common Advice to Skip: Start With a Big Feature to Show You Are Serious

A lot of contributor guides say the best way to get noticed is to tackle something ambitious. In the Haskell ecosystem this backfires more often than it works. A 600 line pull request from an unknown name that adds a new typeclass instance hierarchy will usually stall, not because it is bad, but because reviewing it costs the maintainer an entire weekend.

Trust in open source is built in small deposits. Three merged ten line patches earn you more goodwill than one rejected rewrite. If you do have a big idea, open an issue first and describe the change in plain words, including the type signatures you would add. Ask whether it fits the project before writing the code.

Build the Project Without Fighting the Toolchain

Most newcomers lose their first evening to the build, not the code. Three habits help:

• Read the CI configuration to find which GHC versions are tested, then install one of them with GHCup instead of using whatever your system already has.
• Use the build tool the project uses. If there is a cabal.project file, use cabal. If there is a stack.yaml, use stack. Mixing them creates noise in your diff.
• Run the test suite once before changing anything, so you know which failures were already there.

If cabal refuses to find a build plan, you are not alone. The walkthrough on fixing cabal's could not resolve dependencies error covers the usual causes, from stale index files to an upper bound that excludes your compiler.

Match the Project's Formatting

Check for a fourmolu.yaml, an ormolu setting in CI, a .stylish-haskell.yaml or an hlint configuration. Run exactly the tool the project uses, and only on the files you changed. Reformatting twenty untouched modules hides your real change.

Write the Change So It Is Easy to Review

Haskell gives reviewers a powerful shortcut: they read type signatures first. Keep public signatures unchanged unless changing them is the point of the patch. If you must change one, say so in the first line of your description, because it may force a major version bump under the Package Versioning Policy.

• One logical change per pull request. A bug fix and an unrelated rename belong in two.
• Add a test that fails before your fix and passes after it. For property based projects, a QuickCheck property is usually welcome.
• Add an entry to the changelog if the project keeps one.

Write a Pull Request Description a Maintainer Can Approve Fast

Your description should answer three questions in under 150 words: what changed, why, and how you checked it. A structure that works well:

Then wait. A reasonable rule is to send one polite follow up after two weeks of silence, and nothing more for another month.

Handle Review Comments Without Losing Momentum

Expect at least one round of changes. Typical requests are renaming a local binding, replacing a partial function like head with a pattern match, or splitting a long where block. Push fixes as new commits and only squash if asked.

If you disagree with a comment, explain your reasoning once, briefly, and then defer to the maintainer. It is their project and their support burden. After the merge, look for a second issue in the same repository while the code is fresh. It will take half the time.

FAQ About a First Open Source Contribution in Haskell

Do I need to understand advanced type level Haskell to contribute?

No. Most open issues in everyday libraries involve documentation, tests, error messages or compatibility with a new GHC. Basic comfort with typeclasses, Maybe and Either is enough for the majority of first patches.

Is a documentation fix really a meaningful contribution?

Yes. Haddock pages are the first thing new users read, and a single wrong example can cost hundreds of people time. Maintainers value these fixes because they are cheap to review and clearly improve the package.

How long should I wait before giving up on a pull request?

Give it about six weeks with one polite reminder in between. If there is still no reply, leave a short comment that you are stepping away, so someone else can pick up the work later.

A Practical First Step for This Week

Open the cabal file of your current project, choose one dependency, and read its last twenty closed issues tonight. Pick the smallest open task you understand, build the project on a GHC version from its CI, and send a pull request of under fifty lines with a clear three part description. When it lands, browse more open source writing on this site and choose your next project with the same filter.