-{-# OPTIONS -fno-implicit-prelude #-}
+{-# OPTIONS_GHC -XNoImplicitPrelude #-}
-----------------------------------------------------------------------------
-- |
-- Module : Control.Monad
-- Stability : provisional
-- Portability : portable
--
------------------------------------------------------------------------------
+-- The 'Functor', 'Monad' and 'MonadPlus' classes,
+-- with some useful operations on monads.
module Control.Monad
- ( MonadPlus ( -- class context: Monad
- mzero -- :: (MonadPlus m) => m a
- , mplus -- :: (MonadPlus m) => m a -> m a -> m a
- )
+ (
+ -- * 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 @Monad@ functions
+
+ , mapM -- :: (Monad m) => (a -> m b) -> [a] -> m [b]
+ , mapM_ -- :: (Monad m) => (a -> m b) -> [a] -> m ()
+ , forM -- :: (Monad m) => [a] -> (a -> m b) -> m [b]
+ , forM_ -- :: (Monad m) => [a] -> (a -> m b) -> m ()
+ , sequence -- :: (Monad m) => [m a] -> m [a]
+ , sequence_ -- :: (Monad m) => [m a] -> m ()
+ , (=<<) -- :: (Monad m) => (a -> m b) -> m a -> m b
+ , (>=>) -- :: (Monad m) => (a -> m b) -> (b -> m c) -> (a -> m c)
+ , (<=<) -- :: (Monad m) => (b -> m c) -> (a -> m b) -> (a -> m c)
+ , forever -- :: (Monad m) => m a -> m b
+ , void
+
+ -- ** Generalisations of list functions
+
, 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 ()
, 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
+
, 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)
+ , ap -- :: (Monad m) => m (a -> b) -> m a -> m b
- , 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 ()
- , replicateM -- :: (Monad m) => Int -> m a -> m [a]
- , replicateM_ -- :: (Monad m) => Int -> m a -> m ()
- , (=<<) -- :: (Monad m) => (a -> m b) -> m a -> m b
) where
import Data.Maybe
import GHC.Base
#endif
-#ifndef __HUGS__
+#ifdef __GLASGOW_HASKELL__
infixr 1 =<<
-- -----------------------------------------------------------------------------
-- Prelude monad functions
+-- | Same as '>>=', but with the arguments interchanged.
{-# SPECIALISE (=<<) :: (a -> [b]) -> [a] -> [b] #-}
(=<<) :: Monad m => (a -> m b) -> m a -> m b
-f =<< x = x >>= f
+f =<< x = x >>= f
+-- | Evaluate each action in the sequence from left to right,
+-- and collect the results.
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) }
+ where
+ k m m' = do { x <- m; xs <- m'; return (x:xs) }
+-- | Evaluate each action in the sequence from left to right,
+-- and ignore the results.
sequence_ :: Monad m => [m a] -> m ()
{-# INLINE sequence_ #-}
sequence_ ms = foldr (>>) (return ()) ms
+-- | @'mapM' f@ is equivalent to @'sequence' . 'map' f@.
mapM :: Monad m => (a -> m b) -> [a] -> m [b]
{-# INLINE mapM #-}
mapM f as = sequence (map f as)
+-- | @'mapM_' f@ is equivalent to @'sequence_' . 'map' f@.
mapM_ :: Monad m => (a -> m b) -> [a] -> m ()
{-# INLINE mapM_ #-}
mapM_ f as = sequence_ (map f as)
-#endif /* __HUGS__ */
+
+#endif /* __GLASGOW_HASKELL__ */
-- -----------------------------------------------------------------------------
--- Monadic classes: MonadPlus
+-- The MonadPlus class definition
+-- | Monads that also support choice and failure.
class Monad m => MonadPlus m where
- mzero :: m a
+ -- | the identity of 'mplus'. It should also satisfy the equations
+ --
+ -- > mzero >>= f = mzero
+ -- > v >> mzero = mzero
+ --
+ -- (but the instance for 'System.IO.IO' defined in Control.Monad.Error
+ -- in the mtl package does not satisfy the second one).
+ mzero :: m a
+ -- | an associative operation
mplus :: m a -> m a -> m a
instance MonadPlus [] where
-- -----------------------------------------------------------------------------
-- Functions mandated by the Prelude
+-- | @'guard' b@ is @'return' ()@ if @b@ is 'True',
+-- and 'mzero' if @b@ is 'False'.
guard :: (MonadPlus m) => Bool -> m ()
guard True = return ()
guard False = mzero
--- This subsumes the list-based filter function.
+-- | This generalizes the list-based 'filter' function.
filterM :: (Monad m) => (a -> m Bool) -> [a] -> m [a]
filterM _ [] = return []
ys <- filterM p xs
return (if flg then x:ys else ys)
--- This subsumes the list-based concat function.
+-- | 'forM' is 'mapM' with its arguments flipped
+forM :: Monad m => [a] -> (a -> m b) -> m [b]
+{-# INLINE forM #-}
+forM = flip mapM
+
+-- | 'forM_' is 'mapM_' with its arguments flipped
+forM_ :: Monad m => [a] -> (a -> m b) -> m ()
+{-# INLINE forM_ #-}
+forM_ = flip mapM_
+
+-- | This generalizes the list-based 'concat' function.
msum :: MonadPlus m => [m a] -> m a
{-# INLINE msum #-}
msum = foldr mplus mzero
+infixr 1 <=<, >=>
+
+-- | Left-to-right Kleisli composition of monads.
+(>=>) :: Monad m => (a -> m b) -> (b -> m c) -> (a -> m c)
+f >=> g = \x -> f x >>= g
+
+-- | Right-to-left Kleisli composition of monads. @('>=>')@, with the arguments flipped
+(<=<) :: Monad m => (b -> m c) -> (a -> m b) -> (a -> m c)
+(<=<) = flip (>=>)
+
+-- | @'forever' act@ repeats the action infinitely.
+forever :: (Monad m) => m a -> m b
+forever a = a >> forever a
+
+-- | @'void' value@ discards or ignores the result of evaluation, such as the return value of an 'IO' action.
+void :: Functor f => f a -> f ()
+void = fmap (const ())
+
-- -----------------------------------------------------------------------------
-- 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 generalizes '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
+-- | Like 'foldM', but discards the result.
foldM_ :: (Monad m) => (a -> b -> m a) -> a -> [b] -> m ()
foldM_ f a xs = foldM f a xs >> return ()
+-- | @'replicateM' n act@ performs the action @n@ times,
+-- gathering the results.
replicateM :: (Monad m) => Int -> m a -> m [a]
replicateM n x = sequence (replicate n x)
+-- | Like 'replicateM', but discards the result.
replicateM_ :: (Monad m) => Int -> m a -> m ()
replicateM_ n x = sequence_ (replicate n x)
-unless :: (Monad m) => Bool -> m () -> m ()
-unless p s = if p then return () else s
+{- | 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 ()
-ap :: (Monad m) => m (a -> b) -> m a -> m b
-ap = liftM2 id
+-- | The reverse of 'when'.
-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
+unless :: (Monad m) => Bool -> m () -> m ()
+unless p s = if p then return () else s
+-- | Promote a function to a monad.
+liftM :: (Monad m) => (a1 -> r) -> m a1 -> m r
liftM f m1 = do { x1 <- m1; return (f x1) }
+
+-- | Promote a function to a monad, scanning the monadic arguments from
+-- left to right. For example,
+--
+-- > liftM2 (+) [0,1] [0,2] = [0,2,1,3]
+-- > liftM2 (+) (Just 1) Nothing = Nothing
+--
+liftM2 :: (Monad m) => (a1 -> a2 -> r) -> m a1 -> m a2 -> m r
liftM2 f m1 m2 = do { x1 <- m1; x2 <- m2; return (f x1 x2) }
+
+-- | Promote a function to a monad, scanning the monadic arguments from
+-- left to right (cf. 'liftM2').
+liftM3 :: (Monad m) => (a1 -> a2 -> a3 -> r) -> m a1 -> m a2 -> m a3 -> m r
liftM3 f m1 m2 m3 = do { x1 <- m1; x2 <- m2; x3 <- m3; return (f x1 x2 x3) }
+
+-- | Promote a function to a monad, scanning the monadic arguments from
+-- left to right (cf. 'liftM2').
+liftM4 :: (Monad m) => (a1 -> a2 -> a3 -> a4 -> r) -> m a1 -> m a2 -> m a3 -> m a4 -> m r
liftM4 f m1 m2 m3 m4 = do { x1 <- m1; x2 <- m2; x3 <- m3; x4 <- m4; return (f x1 x2 x3 x4) }
+
+-- | Promote a function to a monad, scanning the monadic arguments from
+-- left to right (cf. 'liftM2').
+liftM5 :: (Monad m) => (a1 -> a2 -> a3 -> a4 -> a5 -> r) -> m a1 -> m a2 -> m a3 -> m a4 -> m a5 -> m r
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:
+ The monad type constructor @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, for example:
+
+> sequence :: Monad m => [m a] -> m [a]
+> sequence_ :: Monad m => [m a] -> m ()
+
+* A prefix \'@m@\' generalizes 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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