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|
{-# 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
instance MonadZip [] where
mzip = zip
mzipWith = zipWith
munzip = unzip
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