■ [Haskell] rank-2 Polymorphism

f :: ([a] -> Int) -> Int
f g = g [1,2,3] + g ['a','b','c']

はだめだけれど、

f' :: (forall a. [a] -> Int) -> Int
f' g = g [1,2,3] + g ['a','b','c']

はOK。