Prevent new dead Haskell code #5909
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`allow-import-from-derivation` is true by default in Nix, but if a user has turned it off (as I have recently), our flake will fail without this.
Added Weeder to the default Nix shell. Weeder no longer uses Dhall for its config, replaced it with an equivalent TOML one.
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This is temporary so that weeds can be tracked going forward without having to fix all the existing ones first.
Also ones with debugging functions. I don’t know why, but this caused some additional weeds to be discovered.
To make it more explicit what isn’t being checked.
This doesn’t yet turn off `type-class-roots`, because whitelisting all of those is a pain, but it pre-populates `root-instances` with instances that we would want to keep, even if unused.
aryairani
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Oct 1, 2025
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aryairani
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I'm thinking it should go into check.sh too if it's going into CI?
This script also gets run as part of `scripts/check.sh`. Since `check-weeds` has to do much of the same work as `check-formatting`, I extracted the common code to a separate Bash file. Also made it so the scripts don’t have to be run from the repo root directory.
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Overview
This (re?)enables Weeder, which does whole-program dead-code analysis. No Haskell code is touched by this PR.
There are at least three different areas of the code base where I’ve discovered myself fixing code that isn’t referenced anywhere. While I fixed those areas upon encountering them, having dead code around adds noise and introduces confusion like I’ve run into (especially with there being some more-or-less duplicated subsystems, with the old being incrementally replaced by the new)
There was a Weeder config in the repo, but it used the older Dhall format instead of the newer TOML, and has a minimal-enough config that I’m not sure whether it was ever used, and certainly not recently.
Weeder is now also included in the Nix environment and run in CI.
Implementation notes
This is mostly the first few commits from #5392, which was rightly closed.
Loose ends
This PR doesn’t delete any code. All existing weeds are grandfathered in. #5392 attempted to do too much at once, and so nothing got in. This intends to stop the accumulation of weeds first, with removal of existing weeds to come later.