/usr/lib/hugs/packages/base/Control/Monad.hs is in libhugs-base-bundled 98.200609.21-5.3ubuntu1.
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-----------------------------------------------------------------------------
-- |
-- 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 ()
, 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
-- ** 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
, 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
-- | Monads that also support choice and failure.
class Monad m => MonadPlus m where
-- | 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"
-- does not satisfy the second one).
mzero :: m a
-- | an associative operation
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' 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 generalizes 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)
-- | '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
-- -----------------------------------------------------------------------------
-- 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)
{- | 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
-- | 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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