content: {-# 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
  
  