Data.Monoid

(regurgitating http://www.haskell.org/pipermail/libraries/2005-July/004057.html) I propose to replace the instance instance Monoid (a -> a) where mempty = id mappend = (.) with newtype Endo a = Endo { runEndo :: a -> a } instance Monoid (Endo a) where mempty = Endo id Endo f `mappend` Endo g = Endo (f . g) instance Monoid b => Monoid (a -> b) where mempty _ = mempty mappend f g x = f x `mappend` g x