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monad comprehensions: Group and Zip monad

ticket #4370
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1 parent 68871b8 commit 41839f14458d79ac02ad11087885c7f4f1144a73 @mcmaniac mcmaniac committed with simonpj Apr 29, 2011
Showing with 81 additions and 0 deletions.
  1. +33 −0 Control/Monad/Group.hs
  2. +46 −0 Control/Monad/Zip.hs
  3. +2 −0 base.cabal
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33 Control/Monad/Group.hs
@@ -0,0 +1,33 @@
+-----------------------------------------------------------------------------
+-- |
+-- Module : Control.Monad.Group
+-- Copyright : (c) Nils Schweinsberg 2011,
+-- (c) University Tuebingen 2011
+-- License : BSD-style (see the file libraries/base/LICENSE)
+-- Maintainer : libraries@haskell.org
+-- Stability : experimental
+-- Portability : non-portable
+--
+-- Monadic grouping (used for monad comprehensions)
+--
+-----------------------------------------------------------------------------
+
+{-# LANGUAGE CPP, MultiParamTypeClasses, FlexibleInstances #-}
+
+module Control.Monad.Group where
+
+import Prelude
+#if defined(__GLASGOW_HASKELL__)
+import GHC.Exts (groupWith)
+#endif
+
+-- | `MonadGroup` type class without restrictions on the type `t`
+class Monad m => MonadGroup m t where
+ mgroupWith :: (a -> t) -> m a -> m (m a)
+
+#if defined(__GLASGOW_HASKELL__)
+-- | Grouping instance for lists using the `groupWith` function from the
+-- "GHC.Exts" library
+instance Ord t => MonadGroup [] t where
+ mgroupWith = groupWith
+#endif
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46 Control/Monad/Zip.hs
@@ -0,0 +1,46 @@
+-----------------------------------------------------------------------------
+-- |
+-- Module : Control.Monad.Zip
+-- Copyright : (c) Nils Schweinsberg 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)
+
+instance MonadZip [] where
+ mzip = zip
+
+munzip :: MonadZip m => m (a,b) -> (m a, m b)
+munzip mab = (liftM fst mab, liftM snd mab)
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2 base.cabal
@@ -121,6 +121,8 @@ Library {
Control.Monad.ST
Control.Monad.ST.Lazy
Control.Monad.ST.Strict
+ Control.Monad.Group
+ Control.Monad.Zip
Data.Bits,
Data.Bool,
Data.Char,

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