Re: Getting rid of -XImpredicativeTypes

[Ganesh Sittampalam]

Oops, I completely missed you saying that despite reading your post
multiple times and actually quoting it. Sorry about that.

But yes, that makes it very clear, thanks. Doable, even if a pain in the
neck. The motivation for my question was that I vaguely recalled
encountering code that uses impredicative instantation when upgrading
darcs to support GHC 8.0. Using VTA will at least make it feasible to
migrate even if it requires CPP, so I'm no longer worried about having
to rewrite hurriedly.

Given the type inference problems, I appreciate it's better to just give
up than try to support something half baked.

On 03/10/2016 10:14, Simon Peyton Jones wrote:
> 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:gan...@earth.li]
> *Sent:* 02 October 2016 12:07
> *To:* Simon Peyton Jones <simo...@microsoft.com>; Alejandro Serrano Mena
> <trup...@gmail.com>
> *Cc:* ghc-devs@haskell.org; ghc-us...@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:trup...@gmail.com]
> *Sent:* 26 September 2016 08:13
> *To:* Simon Peyton Jones <simo...@microsoft.com>
> <mailto:simo...@microsoft.com>
> *Cc:* ghc-us...@haskell.org <mailto:ghc-us...@haskell.org>;
> ghc-devs@haskell.org <mailto: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 <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
>   

Re: Getting rid of -XImpredicativeTypes

[Ben Gamari]

Carter Schonwald  writes:

> 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 ?
>
I don't believe this will have any effect on the behavior of ($). That
is, unless you don't mind giving up the ability to write runST $ do ...

Cheers,

- Ben


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Re: Getting rid of -XImpredicativeTypes

[Richard Eisenberg]

> 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

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Re: Getting rid of -XImpredicativeTypes

[Baldur Blöndal]

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

2016-09-30 15:29 GMT+00:00 Simon Peyton Jones via ghc-devs <
ghc-devs@haskell.org>:

> 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:trup...@gmail.com]
> *Sent:* 26 September 2016 08:13
> *To:* Simon Peyton Jones <simo...@microsoft.com>
> *Cc:* ghc-us...@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 <
> 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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RE: Getting rid of -XImpredicativeTypes

[Simon Peyton Jones via ghc-devs]

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-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 
<ghc-devs@haskell.org<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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Re: Getting rid of -XImpredicativeTypes

[Alejandro Serrano Mena]

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>:

> 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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Re: Getting rid of -XImpredicativeTypes

[Alejandro Serrano Mena]

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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Re: Getting rid of -XImpredicativeTypes

[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]?

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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Re: 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@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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Re: Getting rid of -XImpredicativeTypes

[Edward Z. Yang]

A ghc-proposals is a good way to solicit feedback and publicize more
widely.  At least a proposal is worth it (and I am in favor of removing
ImpredicativeTypes, FWIW).

Edward

Excerpts from Simon Peyton Jones via ghc-devs's message of 2016-09-25 18:05:38 
+:
> 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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Re: Getting rid of -XImpredicativeTypes

[Carter Schonwald]

Sounds good to me. Such a change actually probably be good for reducing ghc
support load around flags that don't work and increase reasons why using
explicit type application will be awesome / more expressive than what I
would otherwise be able to do with proxy arguments


Tldr +1
A) reduces amount of community support load around unsupported flags
B) makes visible type application extension meaningfully stronger / more
powerful than proxy value approaches

On Sunday, September 25, 2016, Simon Peyton Jones via ghc-devs <
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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Getting rid of -XImpredicativeTypes

[Simon Peyton Jones via ghc-devs]

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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