    -----------------------------------------------------------------------------
    -- |
    -- Module      :  Control.Category
    -- Copyright   :  (c) Ashley Yakeley 2007
    -- License     :  BSD-style (see the LICENSE file in the distribution)
    --
    -- Maintainer  :  ashley@semantic.org
    -- Stability   :  experimental
    -- Portability :  portable

    -- http://hackage.haskell.org/trac/ghc/ticket/1773

    module Control.Category where

    import qualified Prelude

    infixr 9 .
    infixr 1 >>>, <<<

    -- | A class for categories.
    --   id and (.) must form a monoid.
    class Category cat where
            -- | the identity morphism
            id :: cat a a

            -- | morphism composition
            (.) :: cat b c -> cat a b -> cat a c

    {-# RULES
    "identity/left" forall p .
                    id . p = p
    "identity/right"        forall p .
                    p . id = p
    "association"   forall p q r .
                    (p . q) . r = p . (q . r)
     #-}

    instance Category (->) where
            id = Prelude.id
    #ifndef __HADDOCK__
    -- Haddock 1.x cannot parse this:
            (.) = (Prelude..)
    #endif

    -- | Right-to-left composition
    (<<<) :: Category cat => cat b c -> cat a b -> cat a c
    (<<<) = (.)

    -- | Left-to-right composition
    (>>>) :: Category cat => cat a b -> cat b c -> cat a c
    f >>> g = g . f