[Haskell-beginners] RankNTypes + ConstraintKinds to use Either as a "union"

Is there a way to compile this code? If not, why? {-# LANGUAGE RankNTypes, ConstraintKinds #-} -- This compiles bar :: (Num a, Num b) => (forall c. Num c => c -> c) -> Either a b -> Either a b bar f (Left a) = Left (f a) bar f (Right b) = Right (f b) bar' = bar (+ 2) -- This doesn't compile because foo' does not typecheck foo :: (tc a, tc b) => (forall c. tc c => c -> c) -> Either a b -> Either a b foo f (Left a) = Left (f a) foo f (Right b) = Right (f b) foo' = foo (+ 2)