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

2016-09-26 9:00 GMT+02:00 Ben Gamari :

Simon Peyton Jones via ghc-devs writes:

...

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.

Out of curiosity, what ever happened to the most recent attempt at addressing impredicativity [1]?

It turned our that the new system being proposed was not better than the current one. In particular, it was almost impossible to know upfront whether a program using impredicative types would need an annotation to typecheck. Furthermore, I think it would imply a humongous amount of changes to GHC, for not a very large gain.

As far as the proposal process is concerned, it would likely be a good idea to put together a proposal so we have somewhere to collect comments. That being said, it needn't be terribly lengthy given that we are merely removing a feature.

Incidentally, that reminds me that I have some documentation fixes to the ghc-proposals repository that I should proof-read and push.

Cheers,

- Ben

[1] https://ghc.haskell.org/trac/ghc/wiki/ImpredicativePolymorphism/ Impredicative-2015

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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.orgmailto:ghc-devs@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devshttps://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

On Sep 30, 2016, at 6:36 PM, Baldur Blöndal wrote:

Shot in the dark: Would extensions like QuantifiedConstraints or ImplicationConstraints, if implemented, help with ImpredicativeTypes?

I don't think so. The challenge with ImpredicativeTypes is retaining predictability of type inference, and I don't see how implication constraints helps with this. Richard

On a more inane front, does this give a path to either making $ less magical, or better user facing errors when folks use compose (.) style code instead and hit impredicativtity issues that $ magic would have handled ? On Sunday, October 2, 2016, Ganesh Sittampalam wrote:

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 javascript:_e(%7B%7D,'cvml','trupill@gmail.com');] *Sent:* 26 September 2016 08:13 *To:* Simon Peyton Jones javascript:_e(%7B%7D,'cvml','simonpj@microsoft.com'); *Cc:* ghc-users@haskell.org javascript:_e(%7B%7D,'cvml','ghc-users@haskell.org');; ghc-devs@haskell.org javascript:_e(%7B%7D,'cvml','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 < ghc-devs@haskell.org javascript:_e(%7B%7D,'cvml','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

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Indeed, as I said “I mis-spoke before: In my proposal we WILL allow types like (Tree (forall a. a->a))”. So yes, such types will be possible in type signatures (with ImpredicativeTypes). But using functions with such type signatures will be tiresome, because you’ll have to use VTA on every occasion. E.g. if xs :: [forall a. a->a] then you can’t say (reverse xs), because that requires impredicative instantiation of reverse’s type argument. You must stay reverse @(forall a. a->a) xs Does that help? Simon From: Ganesh Sittampalam [mailto:ganesh@earth.li] Sent: 02 October 2016 12:07 To: Simon Peyton Jones ; Alejandro Serrano Mena Cc: ghc-devs@haskell.org; ghc-users@haskell.org Subject: 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 mailto:simonpj@microsoft.com Cc: ghc-users@haskell.orgmailto:ghc-users@haskell.org; ghc-devs@haskell.orgmailto: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.orgmailto:ghc-devs@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devshttps://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.orgmailto:ghc-devs@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devshttps://na01.safelinks.protection.outlook.com/?url=http%3A%2F%2Fmail.haskell.org%2Fcgi-bin%2Fmailman%2Flistinfo%2Fghc-devs&data=01%7C01%7Csimonpj%40microsoft.com%7C2a21525b2ae3432ba31e08d3eab43e4c%7C72f988bf86f141af91ab2d7cd011db47%7C1&sdata=Y9f9UGWEd1%2FHwNFLQx2XRrlk35gZeK7Sm2w1NBnU3FY%3D&reserved=0