#12564: Type family in type pattern kind -------------------------------------+------------------------------------- Reporter: int-index | Owner: goldfire Type: bug | Status: new Priority: high | Milestone: 8.4.1 Component: Compiler (Type | Version: 8.0.1 checker) | Keywords: TypeInType, Resolution: | TypeFamilies Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: 14119 | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by RyanGlScott): After reading comment:13, I'm left wondering about how this idea would work in practice. I've wanted to define instances like these before: {{{#!hs type family Sing :: k -> Type type instance Sing @Type = TypeRep type instance Sing @(Sing a) = Sing }}} Currently, GHC prevents me from writing the `Sing @(Sing a)` instance since its argument mentions the `Sing` type family. If I understand the plan in comment:13, then GHC would turn this instance into something that looks roughly like this: {{{#!hs type instance (x ~ Sing a) => Sing @x = Sing }}} However, this might be a problem. We have both a `Sing @x` and a `Sing @Type` instance defined—wouldn't these instances conflict? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12564#comment:19 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler