Commits on Source (3)
-
-
-
This program, from #13971, currently has a rather confusing error message: ```hs class C a where type T a :: k type T a = Int ``` ``` • Kind mis-match on LHS of default declaration for ‘T’ • In the default type instance declaration for ‘T’ In the class declaration for ‘C’ ``` It's not at all obvious why GHC is complaining about the LHS until you realize that the default, when printed with `-fprint-explicit-kinds`, is actually `type T @{k} @* a = Int`. That is to say, the kind of `a` is being instantiated to `Type`, whereas it ought to be a kind variable. The primary thrust of this patch is to weak the error message to make this connection more obvious: ``` • Illegal argument ‘*’ in: ‘type T @{k} @* a = Int’ The arguments to ‘T’ must all be type variables • In the default type instance declaration for ‘T’ In the class declaration for ‘C’ ``` Along the way, I performed some code cleanup suggested by @rae in ghc/ghc#13971 (comment 191287). Before, we were creating a substitution from the default declaration's type variables to the type family tycon's type variables by way of `tcMatchTys`. But this is overkill, since we already know (from the aforementioned validity checking) that all the arguments in a default declaration must be type variables anyway. Therefore, creating the substitution is as simple as using `zipTvSubst`. I took the opportunity to perform this refactoring while I was in town. Fixes #13971.
libraries/base/tests/T16111.hs
0 → 100644
libraries/base/tests/T16111.stderr
0 → 100644