[Haskell-cafe] Problem with existential quantification

[Eric]

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.