gistfile1.hs

{-# LANGUAGE FlexibleInstances #-}

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

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

instance Category F where
  id  = F Prelude.id
  F f . F g = F (f Prelude.. g)

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

type Endo a = F a a