#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: | -------------------------------------+------------------------------------- Comment (by RyanGlScott): Now that #14268 is implemented, we can work around this issue in a much less hacky way: {{{#!hs type family BaseType (k :: forall a. a ~> Type) (x :: b) :: Type where forall (k :: forall a. a ~> Type) x. BaseType k x = (@@) k x }}} That being said, the way that I originally thought this type family should have been defined: {{{#!hs type family BaseType (k :: forall a. a ~> Type) (x :: b) :: Type where BaseType (k :: forall a. a ~> Type) x = (@@) k x }}} Still does not kind-check: {{{ $ ghc/inplace/bin/ghc-stage2 Bug.hs [1 of 1] Compiling Bug ( Bug.hs, Bug.o ) Bug.hs:22:13: error: • Expected kind ‘forall a. a ~> *’, but ‘k’ has kind ‘a0 ~> *’ • In the first argument of ‘BaseType’, namely ‘(k :: forall a. a ~> Type)’ In the type family declaration for ‘BaseType’ | 22 | BaseType (k :: forall a. a ~> Type) x = (@@) k x | ^ }}} This feels like it ought to Just Work™, though. Should we keep this ticket open until this is possible? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11719#comment:23 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler