A ghc-proposals is a good way to solicit feedback and publicize more widely. At least a proposal is worth it (and I am in favor of removing ImpredicativeTypes, FWIW). Edward Excerpts from Simon Peyton Jones via ghc-devs's message of 2016-09-25 18:05:38 +0000:
Friends
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.
I propose, therefore, to remove it entirely. That is, if you use -XImpredicativeTypes, you'll get a warning that it does nothing (ie. complete no-op) and you should remove it.
Before I pull the trigger, does anyone think they are using it in a mission-critical way?
Now that we have Visible Type Application there is a workaround: if you want to call a polymorphic function at a polymorphic type, you can explicitly apply it to that type. For example:
{-# LANGUAGE ImpredicativeTypes, TypeApplications, RankNTypes #-}
module Vta where
f x = id @(forall a. a->a) id @Int x
You can also leave out the @Int part of course.
Currently we have to use -XImpredicativeTypes to allow the @(forall a. a->a). Is that sensible? Or should we allow it regardless? I rather think the latter... if you have Visible Type Application (i.e. -XTypeApplications) then applying to a polytype is nothing special. So I propose to lift that restriction.
I should go through the GHC Proposals Process for this, but I'm on a plane, so I'm going to at least start with an email.
Simon