On Tue, 24 Nov 2020, Viktor Dukhovni wrote:
Rather than waiting, it is perhaps more pragmatic to go with a custom Prelude, which one can have now, even for versions of GHC/base, that won't have the feature in question:
I have such a Prelude: https://hackage.haskell.org/package/prelude-compat It helps a bit, but does not fully solve the problem. If you use Fold.all on a Map and then switch from Map k a to (Map k a, b) you will again not encounter a warning nor a type error. I had the idea of implementing forbidden instances like: instance Unsatisfiable a => Foldable ((,) a) where If I use them (or import them?) they would conflict with the one from FTP-Prelude. I have not tried that idea. It might exclude importing code that uses Foldable on pair intentionally.