#12553: Reference kind in a type instance declaration defined in another instance declaration -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Resolution: | Keywords: TypeInType 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): Recall that type families don't really have a kind because they must always appear saturated. I'll write `~>` for a function argument that must always be provided. (In the language of my [https://github.com/goldfirere/thesis dissertation], `->` used in a kind is ''matchable'' and I will use `~>` for ''unmatchable''. Currently, unmatchable functions in types must always appear saturated. Type families are the only way to write an unmatchable function in a type.) So, with {{{ class Syntactic a where type Domain a :: Sig u -> Type type Internal a :: u }}} I would expect {{{ Domain :: forall u. Type ~> Sig u -> Type Internal :: forall u. Type ~> u }}} Having the `forall` out front does matter, because it means the type family can branch on the choice of `u`. (It should really be a `pi`. And be unmatchable.) If Jack wants things to be different, then he can specify {{{ class Syntactic' a where type Domain' a :: forall u. Sig u -> Type type Internal' a :: forall u. u }}} yielding {{{ Domain' :: Type ~> forall u. Sig u -> Type Internal' :: Type -> forall u. u }}} These `forall`s describe arguments that ''cannot'' be matched against in the type family instances and so are seemingly less useful. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12553#comment:7 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler