Getting rid of -XImpredicativeTypes
Edward Z. Yang
ezyang at mit.edu
Sun Sep 25 18:26:04 UTC 2016
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
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