#11719: Cannot use higher-rank kinds with type families -------------------------------------+------------------------------------- Reporter: ocharles | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.8.1 Component: Compiler (Type | Version: 8.0.1-rc2 checker) | Resolution: | Keywords: TypeInType Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: GHC rejects | Test Case: valid program | dependent/should_compile/T11719 Blocked By: | Blocking: Related Tickets: #13913, #14268 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by RyanGlScott): * related: #13913 => #13913, #14268 Comment: I've found a workaround for the original issue in this ticket after much persistence. The trick is to associate `BaseType` with a type class: {{{#!hs class CBaseType (k :: forall a. a ~> Type) (x :: b) where type BaseType k x :: Type }}} Then, define a flexible instance like so, using an explicit `forall` to give `k` the appropriate higher-rank kind: {{{#!hs instance forall b (k :: forall a. a ~> Type) (x :: b). CBaseType k x where type BaseType k x = (@@) k x }}} That's it! Now you can use `BaseType` like you'd expect: {{{ λ> data ProxySym0 :: forall a. a ~> Type λ> type instance ProxySym0 @@ x = Proxy x λ> :kind! BaseType ProxySym0 Bool BaseType ProxySym0 Bool :: * = Proxy Bool λ> :kind! BaseType ProxySym0 Maybe BaseType ProxySym0 Maybe :: * = Proxy Maybe λ> :kind! BaseType ProxySym0 'Just BaseType ProxySym0 'Just :: * = Proxy 'Just }}} It's a bit gnarly, but this is the best that you can do until #14268 is implemented. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11719#comment:22 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler