Wildcards in type synonyms

[Richard Eisenberg]

> 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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