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.d...@gmail.com] Sent: 26 September 2016 00:54 To: Simon Peyton Jones <simo...@microsoft.com> Cc: ghc-us...@haskell.org; ghc-d...@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 <ghc-d...@haskell.org<mailto:ghc-d...@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 _______________________________________________ ghc-devs mailing list ghc-d...@haskell.org<mailto:ghc-d...@haskell.org> http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs<https://na01.safelinks.protection.outlook.com/?url=http%3A%2F%2Fmail.haskell.org%2Fcgi-bin%2Fmailman%2Flistinfo%2Fghc-devs&data=01%7C01%7Csimonpj%40microsoft.com%7Cf4d41103efc4487796c708d3e59f3720%7C72f988bf86f141af91ab2d7cd011db47%7C1&sdata=Ju37RVvZoNakCzz1dgTj%2F8mO9yz4ImbII14Hw%2FHWRpk%3D&reserved=0>
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