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

(from thread: http://www.haskell.org/pipermail/beginners/2013-October/012703.html) Why type inference can't resolve this code?

{-# LANGUAGE RankNTypes, ConstraintKinds #-}

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 compiles ok

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) -- This doesn't compile because foo' does not typecheck

-- Kim-Ee pointed out that this works: type F tc a b = (tc a, tc b) => (forall c. tc c => c -> c) -> Either a b -> Either a b foo' = (foo :: F Num a b) (+2)