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module Monad ( MonadPlus(mzero, mplus), join, guard, when, unless, ap, msum, filterM, mapAndUnzipM, zipWithM, zipWithM_, foldM, liftM, liftM2, liftM3, liftM4, liftM5, -- ...and what the Prelude exports Monad((>>=), (>>), return, fail), Functor(fmap), mapM, mapM_, sequence, sequence_, (=<<), ) where -- The MonadPlus class definition class (Monad m) => MonadPlus m where mzero :: m a mplus :: m a -> m a -> m a -- Instances of MonadPlus instance MonadPlus Maybe where mzero = Nothing Nothing `mplus` ys = ys xs `mplus` ys = xs instance MonadPlus [] where mzero = [] mplus = (++) -- Functions msum :: MonadPlus m => [m a] -> m a msum xs = foldr mplus mzero xs join :: (Monad m) => m (m a) -> m a join x = x >>= id when :: (Monad m) => Bool -> m () -> m () when p s = if p then s else return () unless :: (Monad m) => Bool -> m () -> m () unless p s = when (not p) s ap :: (Monad m) => m (a -> b) -> m a -> m b ap = liftM2 ($) guard :: MonadPlus m => Bool -> m () guard p = if p then return () else mzero 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 f a [] = return a foldM f a (x:xs) = f a x >>= \ y -> foldM f y xs filterM :: Monad m => (a -> m Bool) -> [a] -> m [a] filterM p [] = return [] filterM p (x:xs) = do { b <- p x; ys <- filterM p xs; return (if b then (x:ys) else ys) } liftM :: (Monad m) => (a -> b) -> (m a -> m b) liftM f = \a -> do { a' <- a; return (f a') } liftM2 :: (Monad m) => (a -> b -> c) -> (m a -> m b -> m c) liftM2 f = \a b -> do { a' <- a; b' <- b; return (f a' b') } liftM3 :: (Monad m) => (a -> b -> c -> d) -> (m a -> m b -> m c -> m d) liftM3 f = \a b c -> do { a' <- a; b' <- b; c' <- c; return (f a' b' c') } liftM4 :: (Monad m) => (a -> b -> c -> d -> e) -> (m a -> m b -> m c -> m d -> m e) liftM4 f = \a b c d -> do { a' <- a; b' <- b; c' <- c; d' <- d; return (f a' b' c' d') } liftM5 :: (Monad m) => (a -> b -> c -> d -> e -> f) -> (m a -> m b -> m c -> m d -> m e -> m f) liftM5 f = \a b c d e -> do { a' <- a; b' <- b; c' <- c; d' <- d; e' <- e; return (f a' b' c' d' e') }