#8030: FlexibleContexts PolyKinds Type Families bug ---------------------------------+------------------------------------------ Reporter: wvv | Owner: Type: bug | Status: closed Priority: normal | Component: Compiler Version: 7.6.3 | Resolution: invalid Keywords: | Os: Unknown/Multiple Architecture: Unknown/Multiple | Failure: GHC rejects valid program Blockedby: | Blocking: Related: | ---------------------------------+------------------------------------------ Changes (by goldfire): * status: new => closed * resolution: => invalid Comment: This looks like correct behavior to me. The problem is that the type signatures for `mempty` and `mappend` are inherently ambiguous. Because GHC cannot know whether `Pr` is injective, there is no way to get the identity of the type `a` from the identity of `Pr a`. Thus, all uses of `mempty` and `mappend` will be ambiguous. This ambiguity is what is causing the error you see. It's possible that the definitions of `mempty` and `mappend` as given should be rejected, but the error you're seeing looks correct to me. -- Ticket URL: http://hackage.haskell.org/trac/ghc/ticket/8030#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler