Unpacking coercions

Simon Peyton Jones simonpj at microsoft.com
Wed Sep 5 21:24:28 UTC 2018 

OK – a good thread.   From it I have learned


  *   What the goal is
  *   That exposing (~#) and (~R#) is not just lexically tricky; it’s deeper than that (Richard’s note below).
  *   That there is a viable, implementation plan; not a thing of beauty perhaps, but ok if carefully documented

Sounds good to me!

My suggestion, if someone is motivated enough to pursue this:

  *   write a wiki page to summarise the above points
  *   plus a ticket to track progress
  *   implement the design

Happy to help review.

Sorry to have been slow to understand.

Simon



From: Richard Eisenberg <rae at cs.brynmawr.edu>
Sent: 05 September 2018 16:26
To: Ryan Scott <ryan.gl.scott at gmail.com>
Cc: Simon Peyton Jones <simonpj at microsoft.com>; ghc-devs at haskell.org
Subject: Re: Unpacking coercions

I don't think that wiki reference is really about this problem.

Instead, I think that we'd need Constraint# to be able to offer users ~# and ~R#. The problem, then, is that there is no answer to this question: Constraint is to Type as Constraint# is to what? Currently, if ~# and ~R# are the only two generators of Constraint#, then Constraint# is like TYPE (TupleRep '[]). But I think wiring that in would be short-sighted.

Instead, this road leads to CONSTRAINT :: RuntimeRep -> Type, which is like TYPE. Today's Constraint would be CONSTRAINT LiftedRep. And we would have (~#), (~R#) :: forall k1 k2. k1 -> k2 -> CONSTRAINT (TupleRep '[]). In the Glorious Future, we'll have CONSTRAINT ~R TYPE, but we're not there yet.

This all feel like the Right Answer to me, but it's unclear if we should start down this road before we have roles in kinds.

Richard

On Sep 5, 2018, at 10:54 AM, Ryan Scott <ryan.gl.scott at gmail.com<mailto:ryan.gl.scott at gmail.com>> wrote:

> We could utterly lie and say
>
>               data Coercion a b where
>
>                   Coercion :: Coercion a a
>
>
>
> That would generate the right bits in in the .o file, and we’d totally ignore the .hi file.  Gruesome but I think it would work.

This was precisely what I had in mind. We already perform this trick for several things defined in ghc-prim (e.g., the (~) and (~~) classes), so this would just be another example of that.

> But rather than all this circumlocution, why don’t we just make it possible to write ~# and ~R# directly.  Even if we dodge the need right now, it’ll surely come back.

Hah, the reason David is suggesting his idea in the first place is to avoid having to do this! My understanding is that making it possible to write (~#) and (~R#) directly would involve quite a number of complications, as Richard's wiki entry on the subject [1] demonstrates.

That's not to say that I wouldn't like to see that happen some day. But a mere mortal like myself couldn't possibly implement this, whereas David's idea is actually within reach.

Ryan S.
-----
[1] https://ghc.haskell.org/trac/ghc/wiki/DependentHaskell/Internal#Liftedvs.Unliftedequality

On Wed, Sep 5, 2018 at 10:45 AM, Simon Peyton Jones <simonpj at microsoft.com<mailto:simonpj at microsoft.com>> wrote:
I think the intention is to have that proposal (which proposes a language change) be superseded by this idea (which does not change the language).
Oh, I did not know that.   I’ll ignore the proposal for now, in that case.

All that would take is putting Coercion in TysWiredIn, and moving Coercion from Data.Type.Coercion to somewhere in ghc-prim.

I don’t think it’s quite as simple as that.  Yes, we can wire it into the compiler; but we still need a module that defines the info table, curried data constructor etc for the type.  And we have no way to do that.

We could utterly lie and say
              data Coercion a b where
                  Coercion :: Coercion a a

That would generate the right bits in in the .o file, and we’d totally ignore the .hi file.  Gruesome but I think it would work.


But rather than all this circumlocution, why don’t we just make it possible to write ~# and ~R# directly.  Even if we dodge the need right now, it’ll surely come back.

If that is lexically tiresome, we could I suppose provide builtin-aliases for them, as if we had
              type NomEq# = (~#)
             type ReprEq# = (~R#)

Simon

From: Ryan Scott <ryan.gl.scott at gmail.com<mailto:ryan.gl.scott at gmail.com>>
Sent: 05 September 2018 15:26
To: Simon Peyton Jones <simonpj at microsoft.com<mailto:simonpj at microsoft.com>>
Cc: ghc-devs at haskell.org<mailto:ghc-devs at haskell.org>
Subject: Re: Unpacking coercions

> Simple is good.  But what is this dead simple idea?

I'm referring to David's first e-mail on this thread: https://mail.haskell.org/pipermail/ghc-devs/2018-September/016191.html

All that would take is putting Coercion in TysWiredIn, and moving Coercion from Data.Type.Coercion to somewhere in ghc-prim.

> Maybe this thread belongs with the proposal, unless I’m misunderstanding.

I think the intention is to have that proposal (which proposes a language change) be superseded by this idea (which does not change the language).

Ryan S.


On Wed, Sep 5, 2018 at 10:20 AM, Simon Peyton Jones <simonpj at microsoft.com<mailto:simonpj at microsoft.com>> wrote:
Simple is good.  But what is this dead simple idea?

Perhaps: https://github.com/ghc-proposals/ghc-proposals/pull/116<https://na01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Fghc-proposals%2Fghc-proposals%2Fpull%2F116&data=02%7C01%7Csimonpj%40microsoft.com%7C51cff02dc13341258db108d61343d184%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C636717579340406159&sdata=1VQIATP4YeqvffmUsqIE6m5aDOe55nlf4NE6WOqWZ%2BU%3D&reserved=0>
But that proposal lists several possible alternatives.  Which one did you mean?

And all of them are language changes. Making evidence strict would require no language changes to solve the original problem.

Maybe this thread belongs with the proposal, unless I’m misunderstanding.

Simon

From: ghc-devs <ghc-devs-bounces at haskell.org<mailto:ghc-devs-bounces at haskell.org>> On Behalf Of Ryan Scott
Sent: 05 September 2018 15:15
To: ghc-devs at haskell.org<mailto:ghc-devs at haskell.org>
Subject: Re: Unpacking coercions

These aren't mutually exclusive ideas. While I'm sure there's many ways we could solve this problem, David's idea has the distinct advantage of being dead simple. I'd rather not block his vision on some other large refactor that may never materialize. (And if it _does_ materialize, we could revert any wiring-in of Coercible quite easily.)

Ryan S.


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