gistfile1.hs

{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE GeneralizedNewtypeDeriving #-}

import Prelude hiding (id, (.))
import Control.Category (Category(..))
import Data.Monoid (Monoid(..))

newtype F a b = F (a -> b) deriving Category

instance Monoid (F a a) where
  mempty  = id
  mappend = (.)

type Endo a = F a a