#9171: Can't match type family with kind polymorphism ----------------------------------------------+---------------------------- Reporter: MikeIzbicki | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects valid program | Unknown/Multiple Test Case: | Difficulty: Unknown Blocking: | Blocked By: | Related Tickets: ----------------------------------------------+---------------------------- Comment (by kosmikus): Consider this small variation of your example: {{{ data Base type family GetParam (p::k1) (t::k2) :: k3 type instance GetParam Base Int = Int type instance GetParam Base Int = Maybe type instance GetParam Base Maybe = Bool test1 = undefined :: GetParam Base (GetParam Base Int :: *) test2 = undefined :: GetParam Base (GetParam Base Int :: * -> *) }}} Now `test1` has type `Int` and `test2` has type `Bool`, and the only difference is the intermediate kind of `GetParam Base Int` which you haven't specified. So I think GHC is right to complain. The kind of the result of `GetParam Base Int` is ambiguous without annotation, and its choice can in general make a difference. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9171#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler