{-# OPTIONS -fno-implicit-prelude #-} ----------------------------------------------------------------------------- -- | -- Module : Control.Monad -- Copyright : (c) The University of Glasgow 2001 -- License : BSD-style (see the file libraries/base/LICENSE) -- -- Maintainer : libraries@haskell.org -- Stability : provisional -- Portability : portable -- -- The 'Functor', 'Monad' and 'MonadPlus' classes, -- with some useful operations on monads. module Control.Monad ( -- * Functor and monad classes Functor(fmap) , Monad((>>=), (>>), return, fail) , MonadPlus ( -- class context: Monad mzero -- :: (MonadPlus m) => m a , mplus -- :: (MonadPlus m) => m a -> m a -> m a ) -- * Functions -- ** Naming conventions -- $naming -- ** Basic functions from the "Prelude" , 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 -- ** Generalisations of list functions , join -- :: (Monad m) => m (m a) -> m a , 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 , foldM_ -- :: (Monad m) => (a -> b -> m a) -> a -> [b] -> m () , replicateM -- :: (Monad m) => Int -> m a -> m [a] , replicateM_ -- :: (Monad m) => Int -> m a -> m () -- ** Conditional execution of monadic expressions , guard -- :: (MonadPlus m) => Bool -> m () , when -- :: (Monad m) => Bool -> m () -> m () , unless -- :: (Monad m) => Bool -> m () -> m () -- ** Monadic lifting operators -- $lifting , liftM -- :: (Monad m) => (a -> b) -> (m a -> m b) , liftM2 -- :: (Monad m) => (a -> b -> c) -> (m a -> m b -> m c) , liftM3 -- :: ... , liftM4 -- :: ... , liftM5 -- :: ... , ap -- :: (Monad m) => m (a -> b) -> m a -> m b ) where import Data.Maybe -- ----------------------------------------------------------------------------- -- |The MonadPlus class definition 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 -- ----------------------------------------------------------------------------- -- Functions mandated by the Prelude 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 -- ----------------------------------------------------------------------------- -- Other monad functions -- | The 'join' function is the conventional monad join operator. It is used to -- remove one level of monadic structure, projecting its bound argument into the -- outer level. join :: (Monad m) => m (m a) -> m a join x = x >>= id -- | The 'mapAndUnzipM' function maps its first argument over a list, returning -- the result as a pair of lists. This function is mainly used with complicated -- data structures or a state-transforming monad. mapAndUnzipM :: (Monad m) => (a -> m (b,c)) -> [a] -> m ([b], [c]) mapAndUnzipM f xs = sequence (map f xs) >>= return . unzip -- | The 'zipWithM' function generalises 'zipWith' to arbitrary monads. zipWithM :: (Monad m) => (a -> b -> m c) -> [a] -> [b] -> m [c] zipWithM f xs ys = sequence (zipWith f xs ys) -- | 'zipWithM_' is the extension of 'zipWithM' which ignores the final result. zipWithM_ :: (Monad m) => (a -> b -> m c) -> [a] -> [b] -> m () zipWithM_ f xs ys = sequence_ (zipWith f xs ys) {- | The 'foldM' function is analogous to 'foldl', except that its result is encapsulated in a monad. Note that 'foldM' works from left-to-right over the list arguments. This could be an issue where '(>>)' and the `folded function' are not commutative. > foldM f a1 [x1, x2, ..., xm ] == > do > a2 <- f a1 x1 > a3 <- f a2 x2 > ... > f am xm If right-to-left evaluation is required, the input list should be reversed. -} 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 foldM_ :: (Monad m) => (a -> b -> m a) -> a -> [b] -> m () foldM_ f a xs = foldM f a xs >> return () replicateM :: (Monad m) => Int -> m a -> m [a] replicateM n x = sequence (replicate n x) replicateM_ :: (Monad m) => Int -> m a -> m () replicateM_ n x = sequence_ (replicate n x) {- | Conditional execution of monadic expressions. For example, > when debug (putStr "Debugging\n") will output the string @Debugging\\n@ if the Boolean value @debug@ is 'True', and otherwise do nothing. -} when :: (Monad m) => Bool -> m () -> m () when p s = if p then s else return () -- | The reverse of 'when'. unless :: (Monad m) => Bool -> m () -> m () unless p s = if p then return () else s {- $lifting The monadic lifting operators promote a function to a monad. The function arguments are scanned left to right. For example, > liftM2 (+) [0,1] [0,2] = [0,2,1,3] > liftM2 (+) (Just 1) Nothing = Nothing -} 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) } {- | In many situations, the 'liftM' operations can be replaced by uses of 'ap', which promotes function application. > return f `ap` x1 `ap` ... `ap` xn is equivalent to > liftMn f x1 x2 ... xn -} ap :: (Monad m) => m (a -> b) -> m a -> m b ap = liftM2 id {- $naming The functions in this library use the following naming conventions: * A postfix \`M\' always stands for a function in the Kleisli category: @m@ is added to function results (modulo currying) and nowhere else. So, for example, > filter :: (a -> Bool) -> [a] -> [a] > filterM :: (Monad m) => (a -> m Bool) -> [a] -> m [a] * A postfix \`_\' changes the result type from @(m a)@ to @(m ())@. Thus (in the "Prelude"): > sequence :: Monad m => [m a] -> m [a] > sequence_ :: Monad m => [m a] -> m () * A prefix \`m\' generalises an existing function to a monadic form. Thus, for example: > sum :: Num a => [a] -> a > msum :: MonadPlus m => [m a] -> m a -}
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