content: {-# LANGUAGE Safe, 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.Functor.Identity
  import Data.Monoid
  import Data.Proxy
  import qualified Data.List.NonEmpty as NE
  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 4.9.0.0
  instance MonadZip NE.NonEmpty where
    mzip     = NE.zip
    mzipWith = NE.zipWith
    munzip   = NE.unzip
  
  -- | @since 4.8.0.0
  instance MonadZip Identity where
      mzipWith                 = liftM2
      munzip (Identity (a, b)) = (Identity a, Identity b)
  
  -- | @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
  