Gah.  We have no way to be polymorphic over all pointers (both lifted and unlifted) but not over Int# etc.

As you say, this is too much of a special case to make an invasive change.

I’m quite dubious about making weak poitners to unlifted heap-allocated objects.  I can’t say it’s wrong but it feels dodgy to me.

Simon

From: David Feuer <david.feuer@gmail.com>
Sent: 22 August 2018 14:59
To: Ben Gamari <ben@smart-cactus.org>
Cc: Simon Peyton Jones <simonpj@microsoft.com>; David Feuer <david@well-typed.com>; ghc-devs <ghc-devs@haskell.org>
Subject: Re: Unlifted primop types

The problem is that this also accepts things that aren't pointers at all! We could fix that *for primops* by changing RuntimeRep to something like

data RuntimeRep

  = PtrRep Liftedness

   | ...

But that would only work for primops (at least for now) so it may not be worth the breakage.

On Wed, Aug 22, 2018, 7:45 AM Ben Gamari <ben@smart-cactus.org> wrote:

Simon Peyton Jones <simonpj@microsoft.com> writes:

> |  Huh! It looks like what we currently do for some primops is just use a
> |  totally bogus kind. For example, mkWeak# will happily accept an Int# as
> |  its first argument.
>
> Well, I see
>   primop  MkWeakOp "mkWeak#" GenPrimOp
>      o -> b -> (State# RealWorld -> (# State# RealWorld, c #))
>
> and I believe (from Ben's message) that the "o" means "open type variable",
> which is the old terminology for what we now call levity-polymorphic.
>
Right; currently (largely for historical reasons) we use `o` to
accommodate cases that accept both lifted and unlifted pointers.

Cheers,

- Ben