#9171: Clarify error messages when there is a kind mismatch ----------------------------------------------+---------------------------- 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 rwbarton): Replying to [comment:3 MikeIzbicki]: The kind of your first `GetParam` would be `forall k3 k1 k2. k1 -> k2 -> k3` meaning that for each triple of kinds (k1,k2,k3), `GetParam` is a type function `k1 -> k2 -> k3`. It doesn't mean that for each pair of kinds (k1,k2) there is a kind k3 which is determined by the type family and a type function `k1 -> k2 -> k3`. It's like the type level `forall c a b. C a b c => a -> b -> c` (though it's not a perfect analogy; since type families can perform type-level "kind-case" but functions can't perform type-case, I added a type class constraint). Or, if you convert the polykindedness to extra arguments to the type family (as ghci will show you is happening behind the scenes if you `:info GetParam`), then your instances become {{{ type instance GetParam * * * Base Int = Int type instance GetParam (* -> *) * * Base Int = Maybe }}} and now you can see that they don't conflict. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9171#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler