#13192: Ambiguity Caused By PolyKind and Not Helpful Error Messages -------------------------------------+------------------------------------- Reporter: Shayan-Najd | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Poor/confusing | Unknown/Multiple error message | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by goldfire): The type instance in @rwbarton's case expands to `F k (I k a) = a`. This instance requires the two `k`s to be the same. In `identity`'s type signature, we get `F (TYPE r) (I k a) -> F (TYPE r') (I k a)`. GHC (rightly) doesn't know that `k` and `TYPE r` are the same. (The fact that it's `TYPE r` and not `Type` is ''not'' the issue.) -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13192#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler