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{-# LANGUAGE Safe #-}
{-# LANGUAGE TypeOperators #-}
-----------------------------------------------------------------------------
-- |
-- 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, liftM2)
import Data.Monoid
import Data.Proxy
import GHC.Generics
-- | `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
-- | @since 4.3.1.0
instance MonadZip [] where
mzip = zip
mzipWith = zipWith
munzip = unzip
-- | @since 4.8.0.0
instance MonadZip Dual where
-- Cannot use coerce, it's unsafe
mzipWith = liftM2
-- | @since 4.8.0.0
instance MonadZip Sum where
mzipWith = liftM2
-- | @since 4.8.0.0
instance MonadZip Product where
mzipWith = liftM2
-- | @since 4.8.0.0
instance MonadZip Maybe where
mzipWith = liftM2
-- | @since 4.8.0.0
instance MonadZip First where
mzipWith = liftM2
-- | @since 4.8.0.0
instance MonadZip Last where
mzipWith = liftM2
-- | @since 4.8.0.0
instance MonadZip f => MonadZip (Alt f) where
mzipWith f (Alt ma) (Alt mb) = Alt (mzipWith f ma mb)
-- | @since 4.9.0.0
instance MonadZip Proxy where
mzipWith _ _ _ = Proxy
-- Instances for GHC.Generics
-- | @since 4.9.0.0
instance MonadZip U1 where
mzipWith _ _ _ = U1
-- | @since 4.9.0.0
instance MonadZip Par1 where
mzipWith = liftM2
-- | @since 4.9.0.0
instance MonadZip f => MonadZip (Rec1 f) where
mzipWith f (Rec1 fa) (Rec1 fb) = Rec1 (mzipWith f fa fb)
-- | @since 4.9.0.0
instance MonadZip f => MonadZip (M1 i c f) where
mzipWith f (M1 fa) (M1 fb) = M1 (mzipWith f fa fb)
-- | @since 4.9.0.0
instance (MonadZip f, MonadZip g) => MonadZip (f :*: g) where
mzipWith f (x1 :*: y1) (x2 :*: y2) = mzipWith f x1 x2 :*: mzipWith f y1 y2
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