2 % (c) The AQUA Project, Glasgow University, 1994-1999
4 \section[Monad]{Module @Monad@}
8 ( MonadPlus ( -- class context: Monad
9 mzero -- :: (MonadPlus m) => m a
10 , mplus -- :: (MonadPlus m) => m a -> m a -> m a
12 , join -- :: (Monad m) => m (m a) -> m a
13 , guard -- :: (MonadPlus m) => Bool -> m ()
14 , when -- :: (Monad m) => Bool -> m () -> m ()
15 , unless -- :: (Monad m) => Bool -> m () -> m ()
16 , ap -- :: (Monad m) => (m (a -> b)) -> (m a) -> m b
17 , msum -- :: (MonadPlus m) => [m a] -> m a
18 , filterM -- :: (Monad m) => (a -> m Bool) -> [m a] -> m [a]
19 , mapAndUnzipM -- :: (Monad m) => (a -> m (b,c)) -> [a] -> m ([b], [c])
20 , zipWithM -- :: (Monad m) => (a -> b -> m c) -> [a] -> [b] -> m [c]
21 , zipWithM_ -- :: (Monad m) => (a -> b -> m c) -> [a] -> [b] -> m ()
22 , foldM -- :: (Monad m) => (a -> b -> m a) -> a -> [b] -> m a
24 , liftM -- :: (Monad m) => (a -> b) -> (m a -> m b)
25 , liftM2 -- :: (Monad m) => (a -> b -> c) -> (m a -> m b -> m c)
30 , Monad((>>=), (>>), return, fail)
33 , mapM -- :: (Monad m) => (a -> m b) -> [a] -> m [b]
34 , mapM_ -- :: (Monad m) => (a -> m b) -> [a] -> m ()
35 , sequence -- :: (Monad m) => [m a] -> m [a]
36 , sequence_ -- :: (Monad m) => [m a] -> m ()
37 , (=<<) -- :: (Monad m) => (a -> m b) -> m a -> m b
43 %*********************************************************
45 \subsection{Monadic classes: @MonadPlus@}
47 %*********************************************************
51 class Monad m => MonadPlus m where
53 mplus :: m a -> m a -> m a
55 instance MonadPlus [] where
59 instance MonadPlus Maybe where
62 Nothing `mplus` ys = ys
67 %*********************************************************
69 \subsection{Functions mandated by the Prelude}
71 %*********************************************************
74 guard :: MonadPlus m => Bool -> m ()
79 -- This subsumes the list-based filter function.
81 filterM :: (Monad m) => ( a -> m Bool ) -> [a] -> m [a]
82 filterM _predM [] = return []
83 filterM predM (x:xs) = do
85 ys <- filterM predM xs
86 return (if flg then x:ys else ys)
88 -- This subsumes the list-based concat function.
90 msum :: MonadPlus m => [m a] -> m a
92 msum = foldr mplus mzero
96 %*********************************************************
98 \subsection{Other monad functions}
100 %*********************************************************
103 join :: (Monad m) => m (m a) -> m a
106 mapAndUnzipM :: (Monad m) => (a -> m (b,c)) -> [a] -> m ([b], [c])
107 mapAndUnzipM f xs = sequence (map f xs) >>= return . unzip
109 zipWithM :: (Monad m) => (a -> b -> m c) -> [a] -> [b] -> m [c]
110 zipWithM f xs ys = sequence (zipWith f xs ys)
112 zipWithM_ :: (Monad m) => (a -> b -> m c) -> [a] -> [b] -> m ()
113 zipWithM_ f xs ys = sequence_ (zipWith f xs ys)
115 foldM :: (Monad m) => (a -> b -> m a) -> a -> [b] -> m a
116 foldM _ a [] = return a
117 foldM f a (x:xs) = f a x >>= \fax -> foldM f fax xs
119 unless :: (Monad m) => Bool -> m () -> m ()
120 unless p s = if p then return () else s
122 when :: (Monad m) => Bool -> m () -> m ()
123 when p s = if p then s else return ()
125 ap :: (Monad m) => m (a->b) -> m a -> m b
128 liftM :: (Monad m) => (a1 -> r) -> m a1 -> m r
129 liftM2 :: (Monad m) => (a1 -> a2 -> r) -> m a1 -> m a2 -> m r
130 liftM3 :: (Monad m) => (a1 -> a2 -> a3 -> r) -> m a1 -> m a2 -> m a3 -> m r
131 liftM4 :: (Monad m) => (a1 -> a2 -> a3 -> a4 -> r) -> m a1 -> m a2 -> m a3 -> m a4 -> m r
132 liftM5 :: (Monad m) => (a1 -> a2 -> a3 -> a4 -> a5 -> r) -> m a1 -> m a2 -> m a3 -> m a4 -> m a5 -> m r
134 liftM f m1 = do { x1 <- m1; return (f x1) }
135 liftM2 f m1 m2 = do { x1 <- m1; x2 <- m2; return (f x1 x2) }
136 liftM3 f m1 m2 m3 = do { x1 <- m1; x2 <- m2; x3 <- m3; return (f x1 x2 x3) }
137 liftM4 f m1 m2 m3 m4 = do { x1 <- m1; x2 <- m2; x3 <- m3; x4 <- m4; return (f x1 x2 x3 x4) }
138 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) }