> On Jul 25, 2022, at 6:04 AM, ÉRDI Gergő <ge...@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? > _______________________________________________ > ghc-devs mailing list > ghc-devs@haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs _______________________________________________ ghc-devs mailing list ghc-devs@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
