#15592: Type families without CUSKs cannot be given visible kind variable binders -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.4.3 Resolution: | Keywords: | TypeApplications, TypeFamilies, | CUSKs Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by goldfire): I agree with comment:1. If a user writes the name of the variable, it is `Specified`. Thus, `a` is specified (in both declarations). `k`, unmentioned, is `Inferred`. But I see some trouble ahead. Consider {{{#!hs data VisProxy k (a :: k) = MkVP class D (a :: Proxy j) (b :: Proxy k) c where meth1 :: forall z. D @j @k a b z => z -> Proxy '(a, b) meth2 :: Proxy k j -> Proxy '(a, b, c) }}} The constraint in `meth1` looks like it's redundantly specifying that `D` should be instantiated at `j` and `k`. I say "redundantly" because `D`, without a CUSK, cannot be polymorphically recursive. However, we discover in `meth2` that `j` actually depends on `k`. So (assuming inference succeeds at all, which I don't wish to debate here), we will get `D :: forall (k :: Type) (j :: k). Proxy j -> Proxy k -> Type -> Constraint`. (The `Type` in the third required argument to `D` comes from `z`'s kind in `meth1`.) Bottom line: any use of visible kind application should be considered to be an instance of polymorphic recursion, and thus should be banned in a mutually recursive group on a CUSK-less type. This is true even if the visible kind application is redundant. Do you agree? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15592#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler