[Haskell-cafe] Problem with existential quantification

Dear all, I've written a function with the following type: build :: Bifunctor s => (forall b. (s a b -> b) -> b) -> Fix s a When I try to compile I get the following error: Illegal polymorphic or qualified type: forall b. (s a b -> b) -> b In the type signature for `build': build :: (Bifunctor s) => (forall b. (s a b -> b) -> b) -> Fix s a What's happening? E.M.