08/11/02 13:03:16
URLﾘﾝｸ(d.hatena.ne.jp)
ブログ持ってないのでここで。酒井さんのパクリ。
{-# OPTIONS -fglasgow-exts #-}
import Control.Monad (liftM2)
import Data.Either
import Test.QuickCheck
infix 4 :<->:
type a :<->: b = (a -> b, b -> a)
(.>) :: a:<->:b -> b:<->:c -> a:<->:c
(f1,g1) .> (f2,g2) = (f2 . f1, g1 . g2)
un :: a:<->:b -> b:<->:a
un (f,g) = (g,f)
type a :+: b = Either a b
infixr 5 :+:
alt :: a:+:b :<->: b:+:a
alt = (f,f) where f = either Right Left
swap :: a:+:b:+:c :<->: b:+:a:+:c
swap = (f,f) where f = either (Right . Left) (either Left (Right . Right))
rt :: b :<->: c -> a:+:b :<->: a:+:c
rt (f,g) = (r f, r g) where r = either Left . (Right .)