Getting rid of -XImpredicativeTypes

[Dan Doel]

I don't use the extension, because it's more pleasant to use newtypes with
polymorphic contents. But here are some questions:

1) ImpredicativeTypes enables types like `Maybe (forall a. a)`. Do those
just disappear, or are they also enabled anyway? (I would guess the former.)

2) There was a sketch drawn up around a year ago (I think) aiming to
actually fix ImpredicativeTypes. I don't recall who was working on it, but
I think when I mentioned it in the context of something else, you didn't
seem to be aware of it. I guess it's safe to say that nothing ever came of
it, at least inasmuch as no one ever showed you their proposal for a
properly functioning ImpredicativeTypes?

Anyhow, if it can't be fixed, I think not having the extension is superior
to its current state. And really, I think even if fixing it were on the
roadmap, it'd be better to get rid of it until it were actually fixed.

-- Dan

On Sun, Sep 25, 2016 at 2:05 PM, Simon Peyton Jones via ghc-devs <
ghc-devs at haskell.org> wrote:

> 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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>
>
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