Re: Getting rid of -XImpredicativeTypes

Elsewhere in the thread, you said

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.

but here you're talking about 'xs :: [forall a . a->a]' being possible with VTA - is the idea that such types will be possible but only with both explicit signatures and VTA? On 30/09/2016 16:29, Simon Peyton Jones via ghc-devs wrote:

Alejandro: excellent point. I mis-spoke before. In my proposal we WILL allow types like (Tree (forall a. a->a)).

I’m trying to get round to writing a proposal (would someone else like to write it – it should be short), but the idea is this:

*When you have -XImpredicativeTypes*

· *You can write a polytype in a visible type argument; eg. f @(forall a. a->a)*

· *You can write a polytype as an argument of a type in a signature e.g. f :: [forall a. a->a] -> Int*

* *

*And that’s all. A unification variable STILL CANNOT be unified with a polytype. The only way you can call a polymorphic function at a polytype is to use Visible Type Application.*

* *

*So using impredicative types might be tiresome. E.g.*

* type SID = forall a. a->a*

* *

* xs :: [forall a. a->a]*

* xs = (:) @SID id ( (:) @SID id ([] @ SID))*

* *

*In short, if you call a function at a polytype, you must use VTA. Simple, easy, predictable; and doubtless annoying. But possible*.

Simon

*From:*Alejandro Serrano Mena [mailto:trupill@gmail.com] *Sent:* 26 September 2016 08:13 *To:* Simon Peyton Jones *Cc:* ghc-users@haskell.org; ghc-devs@haskell.org *Subject:* Re: Getting rid of -XImpredicativeTypes

What would be the story for the types of the arguments. Would I be allowed to write the following?

...

f (lst :: [forall a. a -> a]) = head @(forall a. a -> a) lst 3

Regards,

Alejandro

2016-09-25 20:05 GMT+02:00 Simon Peyton Jones via ghc-devs mailto:ghc-devs@haskell.org>:

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-devs@haskell.org mailto:ghc-devs@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%7Cd4eb1fd61d0148cea9f508d3e5dca6fe%7C72f988bf86f141af91ab2d7cd011db47%7C1&sdata=ZM3djztmpA09J6x1DmmV0LEeftsA1FhjPhjwLuG5w%2FE%3D&reserved=0

_______________________________________________ ghc-devs mailing list ghc-devs@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs