Unpacking coercions

[Simon Peyton Jones]

I think the intention is to have that proposal (which proposes a language change) be superseded by this idea (which does not change the language).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>
Sent: 05 September 2018 15:26
To: Simon Peyton Jones <simonpj at microsoft.com>
Cc: 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I think the intention is to have that proposal (which proposes a language change) be superseded by this idea (which does not change the language).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%7Cab6e886b24b548eab26608d6133b91b5%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C636717543898919689&sdata=XKBwJiLM%2FcH5FeRLuodH3SKXQUppYT0QYDojH4fO7Tg%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.

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/ghc-devs/attachments/20180905/8c88cce5/attachment.html>