    {-# 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 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
        {-# MINIMAL mzip | mzipWith #-}

        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

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