Wildcards in type synonyms

[Simon Peyton Jones]

Hi Gergo

I'd like to implement type synonyms containing wildcards. The idea is
> that if you have `type MySyn a = MyType a _ Int`, then during
> typechecking, every occurrence of `MySyn T` would be expanded into
> `MyType T w123 Int`, with a fresh type (meta)variable `w123`.
>

I imagine you mean that if you then write
f :: MySyn a -> a -> Int
then it's exactly as if you wrote (using PartialTypeSignatures)
f :: MyType a _ Int -> a -> Int
today.   So if I understand it right, you intend that these type synonyms
are second-class citizens: *they can occur precisely (and only) where
wildcards are allowed to occur today*.  For example, as Iavor suggests,
you'd reject
data T a = MkT (MySyn a)

If you want to do this in a fork of GHC, that's obviously fine. If you want
to offer it as a language extension, the best thing would be to write a GHC
Proposal.  Also you'd get a lot of useful design feedback that way, which
might save you implementation cycles.

What is the reason for this? I would have expected type synonyms to be
> only relevant during typechecking, and then fully resolved in the
> elaborated Core output.
>

In GHC *an Id has only one type*.   It does not have a "source type" and a
"Core type".  So we allow Core types to contain synonyms so that when we
export that Id the importing scope (e.g. GHCi, and type error messages) can
see it.  Synonyms can also allow types to take less space. E.g.  we have
Type, where (if we fully expanded) we'd have to have (TYPE LiftedRep).  One
could imagine a different design.

I would expect that, by the time typechecking is over, all your wildcard
synonyms are gone.  They really are second class.

Just to mention too that the entire "wildcards in type signatures" story is
(I think) jolly useful, but it also turned out to be pretty tricky to
implement.  If you just macro-expand your new synonyms, you won't disturb
the wildcard story, but I just wanted to advertise that it's a tricky area.

Simon

On Fri, 22 Jul 2022 at 07:30, ÉRDI Gergő <gergo at erdi.hu> wrote:

> Hi,
>
> I'd like to implement type synonyms containing wildcards. The idea is
> that if you have `type MySyn a = MyType a _ Int`, then during
> typechecking, every occurrence of `MySyn T` would be expanded into
> `MyType T w123 Int`, with a fresh type (meta)variable `w123`.
>
> One worrying thing I noticed in my initial exploration of the GHC
> codebase is that the Core representation of `Type`s can still contain
> type synonym occurrences. And this doesn't seem like just an artefact
> of sharing the `Type` representation with `TcType`, since the
> `coreView` function also has code to look through type synonyms.
>
> What is the reason for this? I would have expected type synonyms to be
> only relevant during typechecking, and then fully resolved in the
> elaborated Core output. If that were the case, then a new version of
> `expand_syn` could live in `TcM` and take care of making these fresh
> metavariables.
>
> Beside this concrete question, taking a step back, I would also like
> to hear from people who know their way around this part of GHC, what
> they think about this and how they'd approach implementing it.
>
> Thanks,
>          Gergo
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