After.hs

class Fold a where
  fold :: a

instance Fold (b -> (a -> b) -> Maybe a -> b) where
  fold nothing just = \case
    Nothing -> nothing
    Just a -> just a

instance Fold ((a -> c) -> (b -> c) -> Either a b -> c) where
  fold l r = \case
    Left a -> l a
    Right b -> r b