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

On Thu, 28 May 2020, Ben Gamari wrote:

Henning Thielemann writes:

...

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

* 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?

* Using GHC's HIE output to implement a linter which would identify above-mentioned dictionaries.