utils/Pair.hs

    {-
    A simple homogeneous pair type with useful Functor, Applicative, and
    Traversable instances.
    -}

    {-# LANGUAGE CPP #-}

    module Pair ( Pair(..), unPair, toPair, swap ) where

    #include "HsVersions.h"

    import Outputable
    #if __GLASGOW_HASKELL__ < 709
    import Control.Applicative
    import Data.Foldable
    import Data.Monoid
    import Data.Traversable
    #endif

    data Pair a = Pair { pFst :: a, pSnd :: a }
    -- Note that Pair is a *unary* type constructor
    -- whereas (,) is binary

    -- The important thing about Pair is that it has a *homogenous*
    -- Functor instance, so you can easily apply the same function
    -- to both components
    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 Foldable Pair where
      foldMap f (Pair x y) = f x `mappend` f y

    instance Traversable Pair where
      traverse f (Pair x y) = Pair <$> f x <*> f y

    instance Outputable a => Outputable (Pair a) where
      ppr (Pair a b) = ppr a <+> char '~' <+> ppr b

    unPair :: Pair a -> (a,a)
    unPair (Pair x y) = (x,y)

    toPair :: (a,a) -> Pair a
    toPair (x,y) = Pair x y

    swap :: Pair a -> Pair a
    swap (Pair x y) = Pair y x