#15592: Type families without CUSKs cannot be given visible kind variable binders -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.8.1 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: | polykinds/T15592{,b} Blocked By: 14880 | Blocking: Related Tickets: #15591 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by simonpj): * status: closed => new * resolution: fixed => Comment: Concerning comment:16 we need to be careful We certainly have `C :: forall {k}. k -> Constraint`. So `k` is `Inferred` and `a` is `Required`. Now look at `T`. I think we want {{{ T :: forall k (a::k) (f :: k->Type). f a -> Type }}} That is: `k` must be `Specified` (because the user wrote it); but it must precede the `a` which comes from the class. On the other hand, if `T` didn't mention `k` it'd be `Inferred`. What about this? {{{ class D (a::k) where type S a }}} Here `k` is `Specified` for `D`, but is it `Specified` or `Inferred` for `S`? The notes in `Note [Required, Specified, and Inferred for types]` do not really answer this question. This business of thinking about how the class header affects the Inferred/Specified/Required spec for the associated type is desperately tricky. I still wonder if we should instead look at the type declaration alone. An improvement would be this: * Whether the tyvar is Inferred or Specified is determined by looking at the associated type (alone) * But the order in which the Specified variables appear is affected by the class header. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15592#comment:20 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler