2 % (c) The AQUA Project, Glasgow University, 1994-1996
5 \section[Monad]{Module @Monad@}
9 Functor(..), Monad(..), MonadZero(..), MonadPlus(..),
11 -- Prelude monad functions
12 accumulate, sequence, mapM, mapM_, guard, filter, concat, applyM,
14 -- Other monad functions
15 join, mapAndUnzipM, zipWithM, foldM, when, unless, ap, unless, when,
16 liftM, liftM2, liftM3, liftM4, liftM5
25 %*********************************************************
27 \subsection{Functions mandated by the Prelude}
29 %*********************************************************
32 accumulate :: Monad m => [m a] -> m [a]
33 accumulate [] = return []
34 accumulate (m:ms) = do { x <- m; xs <- accumulate ms; return (x:xs) }
36 sequence :: Monad m => [m a] -> m ()
37 sequence = foldr (>>) (return ())
39 mapM :: Monad m => (a -> m b) -> [a] -> m [b]
40 mapM f as = accumulate (map f as)
42 mapM_ :: Monad m => (a -> m b) -> [a] -> m ()
43 mapM_ f as = sequence (map f as)
45 guard :: MonadZero m => Bool -> m ()
46 guard p = if p then return () else zero
48 -- This subsumes the list-based filter function.
50 filter :: MonadZero m => (a -> Bool) -> m a -> m a
51 filter p = applyM (\x -> if p x then return x else zero)
53 -- This subsumes the list-based concat function.
55 concat :: MonadPlus m => [m a] -> m a
56 concat = foldr (++) zero
58 applyM :: Monad m => (a -> m b) -> m a -> m b
63 %*********************************************************
65 \subsection{Other monad functions}
67 %*********************************************************
70 join :: (Monad m) => m (m a) -> m a
73 mapAndUnzipM :: (Monad m) => (a -> m (b,c)) -> [a] -> m ([b], [c])
74 mapAndUnzipM f xs = accumulate (map f xs) >>= return . unzip
76 zipWithM :: (Monad m) => (a -> b -> m c) -> [a] -> [b] -> m [c]
77 zipWithM f xs ys = accumulate (zipWith f xs ys)
79 foldM :: (Monad m) => (a -> b -> m a) -> a -> [b] -> m a
80 foldM f a [] = return a
81 foldM f a (x:xs) = f a x >>= \fax -> foldM f fax xs
83 unless :: (Monad m) => Bool -> m () -> m ()
84 unless p s = if p then return () else s
86 when :: (Monad m) => Bool -> m () -> m ()
87 when p s = if p then s else return ()
89 liftM :: (Monad m) => (a1 -> r) -> m a1 -> m r
90 liftM2 :: (Monad m) => (a1 -> a2 -> r) -> m a1 -> m a2 -> m r
91 liftM3 :: (Monad m) => (a1 -> a2 -> a3 -> r) -> m a1 -> m a2 -> m a3 -> m r
92 liftM4 :: (Monad m) => (a1 -> a2 -> a3 -> a4 -> r) -> m a1 -> m a2 -> m a3 -> m a4 -> m r
93 liftM5 :: (Monad m) => (a1 -> a2 -> a3 -> a4 -> a5 -> r) -> m a1 -> m a2 -> m a3 -> m a4 -> m a5 -> m r
95 liftM f m1 = do { x1 <- m1; return (f x1) }
96 liftM2 f m1 m2 = do { x1 <- m1; x2 <- m2; return (f x1 x2) }
97 liftM3 f m1 m2 m3 = do { x1 <- m1; x2 <- m2; x3 <- m3; return (f x1 x2 x3) }
98 liftM4 f m1 m2 m3 m4 = do { x1 <- m1; x2 <- m2; x3 <- m3; x4 <- m4; return (f x1 x2 x3 x4) }
99 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) }
101 ap :: (Monad m) => m (a->b) -> m a -> m b