Re: catch instance Foldable ((, ) a) (Was: What Haskell users are actively maintaining or deving software using ghc <8)

Henning Thielemann writes:

On Thu, 28 May 2020, Ben Gamari wrote:

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Henning Thielemann writes:

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What linter can easily catch "instance Foldable ((,) a)"?

GHC cannot warn about it, so far.

Another solution would be to separate 'base' from GHC. I could then work with a patched 'base' with forbidden "instance Foldable ((,) a)".

For what it's worth, I suspect there are a few solutions to the problem of disallowing use of particular instances which may be close-at-hand:

* Providing a compiler flag or pragma to "poison" an instance, locally preventing the compiler from using it as evidence; this would require a GHC proposal and may not be entirely trivial to implement, but it's certainly feasible.

https://gitlab.haskell.org/ghc/ghc/issues/11796

Indeed. All we need is someone to implement. Admittedly, I don't have much insight into how difficult this would be. Richard or Simon would likely be more helpful in this regard.

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* Providing a GHC source plugin that would look for dictionaries of type (e.g.) Foldable ((,) a) in the desugared Core of a module and report them as errors

Interesting idea. Or would it be better implemented in a type-checker plugin?

That would be another approach but I don't believe this is possible with our current typechecker plugin mechanism.. Specifically, I don't think you can currently hide existing instances. In principle your plugin could solve the unwanted instance with a TypeError but I'm not sure whether this would work reliably with GHC's current implementation: I believe that typechecker plugins are currently a fallback looked at after the usual paths have failed. Moreover, even if it does work we don't that guarantee solver behavior won't change in the future. Cheers, - Ben