{-# 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