Common type classes

class Eq a where
  eq :: a -> a -> Boolean
-- data Ordering = LT | GT | EQ
class Eq a => Ord a where
  compare :: a -> a -> Ordering
class Foldable f where
  foldl :: forall a b. (b -> a -> b) -> b -> f a -> b
  foldr :: forall a b. (a -> b -> b) -> b -> f a -> b
  foldMap :: forall a m. Monoid m => (a -> m) -> f a -> m