I wonder if I am dealing with bugs in the type checker (replying to myself).

Curiously if I have 

class FunctorF d where
   fmapF :: d -> (x -> y) -> F d x -> F d y

fff a = fmapF a id

it compiles correctly. If I infer the type signature of fff I get 

fff :: forall d x. (FunctorF d) => d -> F d x -> F d x

On the other side, it fails to compile when this signature is explicit:

fff :: forall d x. (FunctorF d) => d -> F d x -> F d x
fff a = fmapF a id

I am repeating myself in http://hackage.haskell.org/trac/ghc/ticket/2157#comment:6.


Sorry for the cascaded messages,
hugo