Pairモナドですー＞ω＜　どう役立てるかはあなた次第(｀・ω・´)

example.hs

import Data.Pair

main = print $ (+) <:> (*) <*> pure 2 <*> pair 3 4 -- Pair (5,8)

Pair.hs

{-# LANGUAGE  GeneralizedNewtypeDeriving #-}
module Data.Pair (Pair, pair, (<:>)) where
import Data.Monoid
import Control.Applicative

newtype Pair a = Pair (a, a) deriving (Show, Eq, Monoid)

infixl 5 <:>

pair, (<:>) :: a -> a -> Pair a
pair x y = Pair (x, y)
(<:>) = pair

instance Functor Pair where
  fmap f (Pair (x, y)) = Pair (f x,f y)

instance Applicative Pair where
  pure x = Pair (x, x)
  (Pair (f, g)) <*> (Pair (x, y)) = Pair (f x, g y)

instance  Monad Pair where
  return = pure
  (Pair (x, y)) >>= f = case (f x, f y) of
    (Pair (x', _), Pair (_, y')) -> Pair (x', y')