#15869: Discrepancy between seemingly equivalent type synonym and type family -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 8.6.2 checker) | Keywords: TypeFamilies, Resolution: | TypeInType Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by RyanGlScott): I wonder if (3) has anything to do with #9269? `KindOf2 A` is a bit weird in that it's an application of a type family that returns `forall a -> a -> Type`, which is a polymorphic type. Normally, GHC disallows that—you wouldn't be able to define `type family FAA where FAA = forall a -> a -> Type`, for instance. Yet `KindOf2` offers a way around this restriction, so perhaps (3) is a manifestation of GHC trying to deal with something it doesn't know how to reason about. Then again, `KindOf2 A` seems to work in other places, as comment:1 demonstrates. It's just in type family equations where things go haywire. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15869#comment:9 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler