Getting rid of -XImpredicativeTypes

[Edward Z. Yang]

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