Simon Peyton Jones via ghc-devs
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GHC has a flag -XImpredicativeTypes that makes a half-hearted attempt to support impredicative polymorphism. But it is vestigial.... if it works, it's really a fluke. We don't really have a systematic story here at all.
Out of curiosity, what ever happened to the most recent attempt at addressing impredicativity [1]? As far as the proposal process is concerned, it would likely be a good idea to put together a proposal so we have somewhere to collect comments. That being said, it needn't be terribly lengthy given that we are merely removing a feature. Incidentally, that reminds me that I have some documentation fixes to the ghc-proposals repository that I should proof-read and push. Cheers, - Ben [1] https://ghc.haskell.org/trac/ghc/wiki/ImpredicativePolymorphism/Impredicativ...