+++ /dev/null
-% -----------------------------------------------------------------------------
-% $Id: Monad.lhs,v 1.13 2001/05/18 16:54:05 simonmar Exp $
-%
-% (c) The University of Glasgow, 1994-2000
-%
-
-\section[Monad]{Module @Monad@}
-
-\begin{code}
-{-# OPTIONS -fno-implicit-prelude #-}
-
-module Monad
- ( MonadPlus ( -- class context: Monad
- mzero -- :: (MonadPlus m) => m a
- , mplus -- :: (MonadPlus m) => m a -> m a -> m a
- )
- , join -- :: (Monad m) => m (m a) -> m a
- , guard -- :: (MonadPlus m) => Bool -> m ()
- , when -- :: (Monad m) => Bool -> m () -> m ()
- , unless -- :: (Monad m) => Bool -> m () -> m ()
- , ap -- :: (Monad m) => m (a -> b) -> m a -> m b
- , msum -- :: (MonadPlus m) => [m a] -> m a
- , filterM -- :: (Monad m) => (a -> m Bool) -> [a] -> m [a]
- , mapAndUnzipM -- :: (Monad m) => (a -> m (b,c)) -> [a] -> m ([b], [c])
- , zipWithM -- :: (Monad m) => (a -> b -> m c) -> [a] -> [b] -> m [c]
- , zipWithM_ -- :: (Monad m) => (a -> b -> m c) -> [a] -> [b] -> m ()
- , foldM -- :: (Monad m) => (a -> b -> m a) -> a -> [b] -> m a
-
- , liftM -- :: (Monad m) => (a -> b) -> (m a -> m b)
- , liftM2 -- :: (Monad m) => (a -> b -> c) -> (m a -> m b -> m c)
- , liftM3 -- :: ...
- , liftM4 -- :: ...
- , liftM5 -- :: ...
-
- , Monad((>>=), (>>), return, fail)
- , Functor(fmap)
-
- , mapM -- :: (Monad m) => (a -> m b) -> [a] -> m [b]
- , mapM_ -- :: (Monad m) => (a -> m b) -> [a] -> m ()
- , sequence -- :: (Monad m) => [m a] -> m [a]
- , sequence_ -- :: (Monad m) => [m a] -> m ()
- , (=<<) -- :: (Monad m) => (a -> m b) -> m a -> m b
- ) where
-
-import PrelList
-import PrelMaybe
-import PrelBase
-
-infixr 1 =<<
-\end{code}
-
-%*********************************************************
-%* *
-\subsection{Prelude monad functions}
-%* *
-%*********************************************************
-
-\begin{code}
-{-# SPECIALISE (=<<) :: (a -> [b]) -> [a] -> [b] #-}
-(=<<) :: Monad m => (a -> m b) -> m a -> m b
-f =<< x = x >>= f
-
-sequence :: Monad m => [m a] -> m [a]
-{-# INLINE sequence #-}
-sequence ms = foldr k (return []) ms
- where
- k m m' = do { x <- m; xs <- m'; return (x:xs) }
-
-sequence_ :: Monad m => [m a] -> m ()
-{-# INLINE sequence_ #-}
-sequence_ ms = foldr (>>) (return ()) ms
-
-mapM :: Monad m => (a -> m b) -> [a] -> m [b]
-{-# INLINE mapM #-}
-mapM f as = sequence (map f as)
-
-mapM_ :: Monad m => (a -> m b) -> [a] -> m ()
-{-# INLINE mapM_ #-}
-mapM_ f as = sequence_ (map f as)
-\end{code}
-
-%*********************************************************
-%* *
-\subsection{Monadic classes: @MonadPlus@}
-%* *
-%*********************************************************
-
-
-\begin{code}
-class Monad m => MonadPlus m where
- mzero :: m a
- mplus :: m a -> m a -> m a
-
-instance MonadPlus [] where
- mzero = []
- mplus = (++)
-
-instance MonadPlus Maybe where
- mzero = Nothing
-
- Nothing `mplus` ys = ys
- xs `mplus` _ys = xs
-\end{code}
-
-
-%*********************************************************
-%* *
-\subsection{Functions mandated by the Prelude}
-%* *
-%*********************************************************
-
-\begin{code}
-guard :: (MonadPlus m) => Bool -> m ()
-guard True = return ()
-guard False = mzero
-
--- This subsumes the list-based filter function.
-
-filterM :: (Monad m) => (a -> m Bool) -> [a] -> m [a]
-filterM _ [] = return []
-filterM p (x:xs) = do
- flg <- p x
- ys <- filterM p xs
- return (if flg then x:ys else ys)
-
--- This subsumes the list-based concat function.
-
-msum :: MonadPlus m => [m a] -> m a
-{-# INLINE msum #-}
-msum = foldr mplus mzero
-\end{code}
-
-
-%*********************************************************
-% *
-\subsection{Other monad functions}
-%* *
-%*********************************************************
-
-\begin{code}
-join :: (Monad m) => m (m a) -> m a
-join x = x >>= id
-
-mapAndUnzipM :: (Monad m) => (a -> m (b,c)) -> [a] -> m ([b], [c])
-mapAndUnzipM f xs = sequence (map f xs) >>= return . unzip
-
-zipWithM :: (Monad m) => (a -> b -> m c) -> [a] -> [b] -> m [c]
-zipWithM f xs ys = sequence (zipWith f xs ys)
-
-zipWithM_ :: (Monad m) => (a -> b -> m c) -> [a] -> [b] -> m ()
-zipWithM_ f xs ys = sequence_ (zipWith f xs ys)
-
-foldM :: (Monad m) => (a -> b -> m a) -> a -> [b] -> m a
-foldM _ a [] = return a
-foldM f a (x:xs) = f a x >>= \fax -> foldM f fax xs
-
-unless :: (Monad m) => Bool -> m () -> m ()
-unless p s = if p then return () else s
-
-when :: (Monad m) => Bool -> m () -> m ()
-when p s = if p then s else return ()
-
-ap :: (Monad m) => m (a -> b) -> m a -> m b
-ap = liftM2 id
-
-liftM :: (Monad m) => (a1 -> r) -> m a1 -> m r
-liftM2 :: (Monad m) => (a1 -> a2 -> r) -> m a1 -> m a2 -> m r
-liftM3 :: (Monad m) => (a1 -> a2 -> a3 -> r) -> m a1 -> m a2 -> m a3 -> m r
-liftM4 :: (Monad m) => (a1 -> a2 -> a3 -> a4 -> r) -> m a1 -> m a2 -> m a3 -> m a4 -> m r
-liftM5 :: (Monad m) => (a1 -> a2 -> a3 -> a4 -> a5 -> r) -> m a1 -> m a2 -> m a3 -> m a4 -> m a5 -> m r
-
-liftM f m1 = do { x1 <- m1; return (f x1) }
-liftM2 f m1 m2 = do { x1 <- m1; x2 <- m2; return (f x1 x2) }
-liftM3 f m1 m2 m3 = do { x1 <- m1; x2 <- m2; x3 <- m3; return (f x1 x2 x3) }
-liftM4 f m1 m2 m3 m4 = do { x1 <- m1; x2 <- m2; x3 <- m3; x4 <- m4; return (f x1 x2 x3 x4) }
-liftM5 f m1 m2 m3 m4 m5 = do { x1 <- m1; x2 <- m2; x3 <- m3; x4 <- m4; x5 <- m5; return (f x1 x2 x3 x4 x5) }
-
-\end{code}
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