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I'll start with the specific case I've seen pop up a few times recently, and then expand to the general. If you're a package author who has been affected by this, please note: I'm putting this information into a blog post since it's easier to state this once and link to it rather than rewrite an explanation on lots of different bug trackers.

hlint is a great tool for getting advice on improving your Haskell codebase (another great Neil Mitchell product). And as such tools go, hlint has new versions which improve its ability to provide useful advice. This means that, sometimes, code which triggered no hlint warnings previously may suddenly present with such warnings under a new hlint version.

Twice recently in my Stackage curation, I've seen a number of test suites fail, even though the code for those packages was unmodified. It turns out that the upgrade to a new version of hlint caused a previously successful test suite to now fail. Clearly the code isn't suddenly broken because a new version of hlint has been released, but as far as the diagnostics of test suite failures are concerned, that's exactly what happened.

Recommendation

I do strongly recommend projects use hlint to get code improvements. And I've seen some great results with using it as part of the CI process, such as on Stack. (For the record: it wasn't my idea and I didn't implement it. I was just pleasantly surprised when my PRs failed because I had some style errors.) However, making the test suite for the entire package fail because of a new version of hlint is too much. Therefore:

DO Have some way to run hlint from your CI process, if you want these warnings to block PRs. There are two approaches I can think of: The way Stack does it: have a separate part of the build matrix just for style errors. The cabal file for the project itself knows nothing about hlint. Via a test suite in your cabal file which is disabled by default. Then: turn on that test suite with a flag from your CI configuration.

DON'T Set up your package which is uploaded to Hackage/built by Stackage such that it will fail if a style-based error occurs.

General recommendation

The general takeaway from this is: when you're building your code on CI, be as strict as you want. Set high standards, block PRs, call master broken, for whatever trivial or non-trivial issues you deem worthy. Turn on -Wall -Werror , respect hlint, error out if someone uses tabs* or includes trailing whitespace. That's all good.

* Cue necessary tabs-vs-spaces argument

However, when you're releasing your code elsewhere, make the tests as lenient as possible on optional features. If the code fails to build: that's a problem. If the code builds, but returns incorrect runtime results: that's a problem. These should stop build systems like Stackage from including your package. But stylistic issues, or newly introduced warnings from the compiler, or myriad other issues, should not trigger a failure for downstream consumers of your package.

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