Re: [Haskell-cafe] foldl in terms of foldr

f :: a -> b -> c is a function that takes an a, a b, and returns a c.

Except that f and g are not isomorphic. In fact, there exists no defined fuction g :: (a -> b) -> c (what type would (g id) be?

The types are isomorphic. They both have the same extension. Both types are empty. How do you make a function that returns an ununtyped value? You can't.