Wildcards in type synonyms

[Gergő Érdi]

Thanks Simon, these are very useful hints. My plan is to just push
ahead on a separate fork, with this "macro semantics", and maybe if it
comes out nicer than I'd hope, I'll propose it.

So it seems that instead of shoehorning it into the existing type
synonyms, a better bet would be to branch off to a separate path quite
early (maybe as soon as during renaming), expand them during
typechecking, and *not* touch how/when existing "normal" type synonyms
are resolved.

On Fri, Jul 22, 2022 at 4:14 PM Simon Peyton Jones
<simon.peytonjones at gmail.com> wrote:
>
> 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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>
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