Control/Monad/Zip.hs

{-# LANGUAGE Safe #-}

-----------------------------------------------------------------------------
-- |
-- Module : Control.Monad.Zip
-- Copyright : (c) Nils Schweinsberg 2011,
-- (c) George Giorgidze 2011
-- (c) University Tuebingen 2011
-- License : BSD-style (see the file libraries/base/LICENSE)
-- Maintainer : libraries@haskell.org
-- Stability : experimental
-- Portability : portable
--
-- Monadic zipping (used for monad comprehensions)
--
-----------------------------------------------------------------------------

module Control.Monad.Zip where

import Prelude
import Control.Monad (liftM)

-- | `MonadZip` type class. Minimal definition: `mzip` or `mzipWith`
--
-- Instances should satisfy the laws:
--
-- * Naturality :
--
-- > liftM (f *** g) (mzip ma mb) = mzip (liftM f ma) (liftM g mb)
--
-- * Information Preservation:
--
-- > liftM (const ()) ma = liftM (const ()) mb
-- > ==>
-- > munzip (mzip ma mb) = (ma, mb)
--
class Monad m => MonadZip m where

 mzip :: m a -> m b -> m (a,b)
 mzip = mzipWith (,)

 mzipWith :: (a -> b -> c) -> m a -> m b -> m c
 mzipWith f ma mb = liftM (uncurry f) (mzip ma mb)

 munzip :: m (a,b) -> (m a, m b)
 munzip mab = (liftM fst mab, liftM snd mab)
 -- munzip is a member of the class because sometimes
 -- you can implement it more efficiently than the
 -- above default code. See Trac #4370 comment by giorgidze
 {-# MINIMAL mzip | mzipWith #-}

instance MonadZip [] where
 mzip = zip
 mzipWith = zipWith
 munzip = unzip