[Haskell-cafe] Re: Type parameters in type families

Hugo Pacheco hpacheco at gmail.com
Mon Mar 17 18:59:59 EDT 2008

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

Sorry for the cascaded messages,
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