----------------------------------------------------------------------------- -- | -- Module : Data.Semigroupoid.Dual -- Copyright : (C) 2007-2011 Edward Kmett -- License : BSD-style (see the file LICENSE) -- -- Maintainer : Edward Kmett <ekmett@gmail.com> -- Stability : provisional -- Portability : portable -- -- A semigroupoid satisfies all of the requirements to be a Category except -- for the existence of identity arrows. ---------------------------------------------------------------------------- module Data.Semigroupoid.Dual (Dual(..)) where import Data.Semigroupoid import Control.Category import Prelude () newtype Dual k a b = Dual { getDual :: k b a } instance Semigroupoid k => Semigroupoid (Dual k) where Dual f `o` Dual g = Dual (g `o` f) instance Category k => Category (Dual k) where id = Dual id Dual f . Dual g = Dual (g . f)