1) ImpredicativeTypes enables types like `Maybe (forall a. a)`. Do those just disappear, or are they also enabled anyway? (I would guess the former.)
Yes, they’d disappear.
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?
It’s just a swamp. I have tried multiple times to fix ImpredicativeTypes, and failed every time. Which is not to say that someone shouldn’t try again, with new thinking.
Simon
From: Dan Doel [mailto:dan.doel@gmail.com]
Sent: 26 September 2016 00:54
To: Simon Peyton Jones
Cc: ghc-users@haskell.org; ghc-devs@haskell.org
Subject: Re: Getting rid of -XImpredicativeTypes
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 mailto:ghc-devs@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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