use weeder

[kostmo]

Closes #2043

Local usage notes

I suspect that HLS in VS Code spontaneously pollutes the .hie directory, because after a few successful invocations, weeder started complaining with:

incompatible hie file: /home/kostmo/github/swarm/.hie/Swarm/Web/Auth.hie
    this version of weeder was compiled with GHC version 9064
    the hie files in this project were generated with GHC version 9048
    weeder must be built with the same GHC version as the project it is used on

Fixing false positives

Previously, for each library and executable, the HIE directory was always the same:

ghc-options:
    -hiedir=.hie

However, most of the executables have at least one module path that is the same relative to their hs-source-dirs base: namely, Main.hs. This resulted in all but one of the Main.hie files being overwritten in the end.

To avoid this, I have specified a different -hiedir for each library/executable/test that parallels its hs-source-dirs base. This way, so long as hs-source-dirs are unique across these targets, all of the *.hs files across the entire project will have unique *.hie paths.

Whitelisting exceptions

There are some known limitations with weeder regarding support for Template Haskell and type families. Exceptions are specified in the roots list in weeder.toml.

After removing a handful of dead functions myself, there are approx. 30 "true positive" weeds that I have listed explicitly in weeder.toml. Maintenance of this list of exceptions should eventually be easier with ocharles/weeder#146.

Integration with CI

I found a ready-made GitHub Action for weeder: https://github.com/freckle/weeder-action
I hacked support directly into the generated .github/workflows/haskell-ci.yml file.

Ideally, the generator would have an option for a weeder step. Indeed, there is an open issue for supporting weeder in haskell-ci: haskell-CI/haskell-ci#132

A separate but related functionality that would be nice in haskell-ci is to specify one of the GHC versions in the matrix to do additional validations/builds that would be redundant if performed on the other matrix entries. I suppose weeder is inexpensive enough to redo four times, but the weeder binary is not available for download for all GHC versions (e.g. ghc 9.8.2). Something like haddock we probably only need to build once.
I have hacked this in to the generated file for weeder with a simple if condition.

[xsebek]

I tried it too, but gave up after the false positives. I did not notice that running HLS in VSCode has an impact, maybe I should give it a second try. 🤔

[byorgey]

Great work, thanks!

My concern with editing haskell-ci.yml manually like this is that these changes might be lost next time we automatically regenerate it. At the very least, we should add some clear comments to the top of the file explaining which parts need to be left alone (we already have such a comment, about the custom on.push and on.pull-request conditions).

[xsebek]

Amazing, thanks @kostmo! 👍