Wildcards in type synonyms

[Gergő Érdi]

So my idea from my last email is to avoid the need for `substHsTy`:
When `tc_hs_type` encounters a macro occurrence,  I would `tcLHsType`
the rhs then and there (thereby getting fresh metas for each
wildcard), use `substTy` to instantiate with the given type arguments.
Then `tc_hs_type` returns that.

Note that this is NOT about typechecking the rhs *for the definition*,
but rather, using `tcLHsType` as the function that creates fresh metas
for each wildcard.

On Mon, Jul 25, 2022 at 8:58 PM Richard Eisenberg <lists at richarde.dev> wrote:
>
>
>
> > On Jul 25, 2022, at 6:04 AM, ÉRDI Gergő <gergo at erdi.hu> wrote:
> >
> > On Mon, 25 Jul 2022, Simon Peyton Jones wrote:
> >
> >>      Do we have an existing way of substituting types over type variables, *in
> >>      HsType instead of Core Type*?
> >> I'm afraid not. Currently HsType is not processed much -- just renamed and typechecked
> >> into a Type.
> >
> > I wonder if, instead, I could expand the rhs, typecheck it "abstractly" (i.e. in the context of the synonym's binders), and THEN do the substitution.
>
> Why type-check the RHS at all? Presumably, to give nice error messages. But it looks like this aspect of the plan is inessential. To be clear, I *do* think you should type-check the RHS, but I'm also checking my understanding here. If type-checking the RHS is indeed inessential, then the result of that type-checking (a desugared `Type`) should be discarded.
>
> > If I typecheck the rhs for every occurrence, I should get fresh metavars for each wildcard, which is pretty much what I want. I just
> > have to make sure I don't zonk before the substitution.
> >
>
> I see this substitution as happening before any type-checking, so zonking shouldn't be an issue. That is, I would expect a
>
> substHsTy :: UniqFM Name (HsType GhcRn) -> HsType GhcRn -> HsType GhcRn
>
> to do the work, entirely before type-checking.
>
> (Presumably, you don't want the macro-like behavior to extend to fixity resolution. That is, if we have
>
> type macro T a = a + b
>
> and then write `f :: T Int * Double`, we want `f :: (Int + b) * Double`, not `f :: Int + (b * Double)`. If you indeed want the latter (strange days!), then you'd need to be careful to do the substitution before fixity resolution, just after renaming.)
>
> Richard
>
> > Does this make sense?
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