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<pre><a name="line-1"></a><span class='hs-comment'>{-# OPTIONS_GHC -XNoImplicitPrelude #-}</span>
<a name="line-2"></a><span class='hs-comment'>-----------------------------------------------------------------------------</span>
<a name="line-3"></a><span class='hs-comment'>-- |</span>
<a name="line-4"></a><span class='hs-comment'>-- Module : Control.Monad</span>
<a name="line-5"></a><span class='hs-comment'>-- Copyright : (c) The University of Glasgow 2001</span>
<a name="line-6"></a><span class='hs-comment'>-- License : BSD-style (see the file libraries/base/LICENSE)</span>
<a name="line-7"></a><span class='hs-comment'>-- </span>
<a name="line-8"></a><span class='hs-comment'>-- Maintainer : libraries@haskell.org</span>
<a name="line-9"></a><span class='hs-comment'>-- Stability : provisional</span>
<a name="line-10"></a><span class='hs-comment'>-- Portability : portable</span>
<a name="line-11"></a><span class='hs-comment'>--</span>
<a name="line-12"></a><span class='hs-comment'>-- The 'Functor', 'Monad' and 'MonadPlus' classes,</span>
<a name="line-13"></a><span class='hs-comment'>-- with some useful operations on monads.</span>
<a name="line-14"></a>
<a name="line-15"></a><span class='hs-keyword'>module</span> <span class='hs-conid'>Control</span><span class='hs-varop'>.</span><span class='hs-conid'>Monad</span>
<a name="line-16"></a> <span class='hs-layout'>(</span>
<a name="line-17"></a> <span class='hs-comment'>-- * Functor and monad classes</span>
<a name="line-18"></a>
<a name="line-19"></a> <span class='hs-conid'>Functor</span><span class='hs-layout'>(</span><span class='hs-varid'>fmap</span><span class='hs-layout'>)</span>
<a name="line-20"></a> <span class='hs-layout'>,</span> <span class='hs-conid'>Monad</span><span class='hs-layout'>(</span><span class='hs-layout'>(</span><span class='hs-varop'>>>=</span><span class='hs-layout'>)</span><span class='hs-layout'>,</span> <span class='hs-layout'>(</span><span class='hs-varop'>>></span><span class='hs-layout'>)</span><span class='hs-layout'>,</span> <span class='hs-varid'>return</span><span class='hs-layout'>,</span> <span class='hs-varid'>fail</span><span class='hs-layout'>)</span>
<a name="line-21"></a>
<a name="line-22"></a> <span class='hs-layout'>,</span> <span class='hs-conid'>MonadPlus</span> <span class='hs-layout'>(</span> <span class='hs-comment'>-- class context: Monad</span>
<a name="line-23"></a> <span class='hs-varid'>mzero</span> <span class='hs-comment'>-- :: (MonadPlus m) => m a</span>
<a name="line-24"></a> <span class='hs-layout'>,</span> <span class='hs-varid'>mplus</span> <span class='hs-comment'>-- :: (MonadPlus m) => m a -> m a -> m a</span>
<a name="line-25"></a> <span class='hs-layout'>)</span>
<a name="line-26"></a> <span class='hs-comment'>-- * Functions</span>
<a name="line-27"></a>
<a name="line-28"></a> <span class='hs-comment'>-- ** Naming conventions</span>
<a name="line-29"></a> <span class='hs-comment'>-- $naming</span>
<a name="line-30"></a>
<a name="line-31"></a> <span class='hs-comment'>-- ** Basic @Monad@ functions</span>
<a name="line-32"></a>
<a name="line-33"></a> <span class='hs-layout'>,</span> <span class='hs-varid'>mapM</span> <span class='hs-comment'>-- :: (Monad m) => (a -> m b) -> [a] -> m [b]</span>
<a name="line-34"></a> <span class='hs-layout'>,</span> <span class='hs-varid'>mapM_</span> <span class='hs-comment'>-- :: (Monad m) => (a -> m b) -> [a] -> m ()</span>
<a name="line-35"></a> <span class='hs-layout'>,</span> <span class='hs-varid'>forM</span> <span class='hs-comment'>-- :: (Monad m) => [a] -> (a -> m b) -> m [b]</span>
<a name="line-36"></a> <span class='hs-layout'>,</span> <span class='hs-varid'>forM_</span> <span class='hs-comment'>-- :: (Monad m) => [a] -> (a -> m b) -> m ()</span>
<a name="line-37"></a> <span class='hs-layout'>,</span> <span class='hs-varid'>sequence</span> <span class='hs-comment'>-- :: (Monad m) => [m a] -> m [a]</span>
<a name="line-38"></a> <span class='hs-layout'>,</span> <span class='hs-varid'>sequence_</span> <span class='hs-comment'>-- :: (Monad m) => [m a] -> m ()</span>
<a name="line-39"></a> <span class='hs-layout'>,</span> <span class='hs-layout'>(</span><span class='hs-varop'>=<<</span><span class='hs-layout'>)</span> <span class='hs-comment'>-- :: (Monad m) => (a -> m b) -> m a -> m b</span>
<a name="line-40"></a> <span class='hs-layout'>,</span> <span class='hs-layout'>(</span><span class='hs-varop'>>=></span><span class='hs-layout'>)</span> <span class='hs-comment'>-- :: (Monad m) => (a -> m b) -> (b -> m c) -> (a -> m c)</span>
<a name="line-41"></a> <span class='hs-layout'>,</span> <span class='hs-layout'>(</span><span class='hs-varop'><=<</span><span class='hs-layout'>)</span> <span class='hs-comment'>-- :: (Monad m) => (b -> m c) -> (a -> m b) -> (a -> m c)</span>
<a name="line-42"></a> <span class='hs-layout'>,</span> <span class='hs-varid'>forever</span> <span class='hs-comment'>-- :: (Monad m) => m a -> m b</span>
<a name="line-43"></a> <span class='hs-layout'>,</span> <span class='hs-varid'>void</span>
<a name="line-44"></a>
<a name="line-45"></a> <span class='hs-comment'>-- ** Generalisations of list functions</span>
<a name="line-46"></a>
<a name="line-47"></a> <span class='hs-layout'>,</span> <span class='hs-varid'>join</span> <span class='hs-comment'>-- :: (Monad m) => m (m a) -> m a</span>
<a name="line-48"></a> <span class='hs-layout'>,</span> <span class='hs-varid'>msum</span> <span class='hs-comment'>-- :: (MonadPlus m) => [m a] -> m a</span>
<a name="line-49"></a> <span class='hs-layout'>,</span> <span class='hs-varid'>mfilter</span> <span class='hs-comment'>-- :: (MonadPlus m) => (a -> Bool) -> m a -> m a</span>
<a name="line-50"></a> <span class='hs-layout'>,</span> <span class='hs-varid'>filterM</span> <span class='hs-comment'>-- :: (Monad m) => (a -> m Bool) -> [a] -> m [a]</span>
<a name="line-51"></a> <span class='hs-layout'>,</span> <span class='hs-varid'>mapAndUnzipM</span> <span class='hs-comment'>-- :: (Monad m) => (a -> m (b,c)) -> [a] -> m ([b], [c])</span>
<a name="line-52"></a> <span class='hs-layout'>,</span> <span class='hs-varid'>zipWithM</span> <span class='hs-comment'>-- :: (Monad m) => (a -> b -> m c) -> [a] -> [b] -> m [c]</span>
<a name="line-53"></a> <span class='hs-layout'>,</span> <span class='hs-varid'>zipWithM_</span> <span class='hs-comment'>-- :: (Monad m) => (a -> b -> m c) -> [a] -> [b] -> m ()</span>
<a name="line-54"></a> <span class='hs-layout'>,</span> <span class='hs-varid'>foldM</span> <span class='hs-comment'>-- :: (Monad m) => (a -> b -> m a) -> a -> [b] -> m a </span>
<a name="line-55"></a> <span class='hs-layout'>,</span> <span class='hs-varid'>foldM_</span> <span class='hs-comment'>-- :: (Monad m) => (a -> b -> m a) -> a -> [b] -> m ()</span>
<a name="line-56"></a> <span class='hs-layout'>,</span> <span class='hs-varid'>replicateM</span> <span class='hs-comment'>-- :: (Monad m) => Int -> m a -> m [a]</span>
<a name="line-57"></a> <span class='hs-layout'>,</span> <span class='hs-varid'>replicateM_</span> <span class='hs-comment'>-- :: (Monad m) => Int -> m a -> m ()</span>
<a name="line-58"></a>
<a name="line-59"></a> <span class='hs-comment'>-- ** Conditional execution of monadic expressions</span>
<a name="line-60"></a>
<a name="line-61"></a> <span class='hs-layout'>,</span> <span class='hs-varid'>guard</span> <span class='hs-comment'>-- :: (MonadPlus m) => Bool -> m ()</span>
<a name="line-62"></a> <span class='hs-layout'>,</span> <span class='hs-varid'>when</span> <span class='hs-comment'>-- :: (Monad m) => Bool -> m () -> m ()</span>
<a name="line-63"></a> <span class='hs-layout'>,</span> <span class='hs-varid'>unless</span> <span class='hs-comment'>-- :: (Monad m) => Bool -> m () -> m ()</span>
<a name="line-64"></a>
<a name="line-65"></a> <span class='hs-comment'>-- ** Monadic lifting operators</span>
<a name="line-66"></a>
<a name="line-67"></a> <span class='hs-layout'>,</span> <span class='hs-varid'>liftM</span> <span class='hs-comment'>-- :: (Monad m) => (a -> b) -> (m a -> m b)</span>
<a name="line-68"></a> <span class='hs-layout'>,</span> <span class='hs-varid'>liftM2</span> <span class='hs-comment'>-- :: (Monad m) => (a -> b -> c) -> (m a -> m b -> m c)</span>
<a name="line-69"></a> <span class='hs-layout'>,</span> <span class='hs-varid'>liftM3</span> <span class='hs-comment'>-- :: ...</span>
<a name="line-70"></a> <span class='hs-layout'>,</span> <span class='hs-varid'>liftM4</span> <span class='hs-comment'>-- :: ...</span>
<a name="line-71"></a> <span class='hs-layout'>,</span> <span class='hs-varid'>liftM5</span> <span class='hs-comment'>-- :: ...</span>
<a name="line-72"></a>
<a name="line-73"></a> <span class='hs-layout'>,</span> <span class='hs-varid'>ap</span> <span class='hs-comment'>-- :: (Monad m) => m (a -> b) -> m a -> m b</span>
<a name="line-74"></a>
<a name="line-75"></a> <span class='hs-layout'>)</span> <span class='hs-keyword'>where</span>
<a name="line-76"></a>
<a name="line-77"></a><span class='hs-keyword'>import</span> <span class='hs-conid'>Data</span><span class='hs-varop'>.</span><span class='hs-conid'>Maybe</span>
<a name="line-78"></a>
<a name="line-79"></a><span class='hs-cpp'>#ifdef __GLASGOW_HASKELL__</span>
<a name="line-80"></a><span class='hs-keyword'>import</span> <span class='hs-conid'>GHC</span><span class='hs-varop'>.</span><span class='hs-conid'>List</span>
<a name="line-81"></a><span class='hs-keyword'>import</span> <span class='hs-conid'>GHC</span><span class='hs-varop'>.</span><span class='hs-conid'>Base</span>
<a name="line-82"></a><span class='hs-cpp'>#endif</span>
<a name="line-83"></a>
<a name="line-84"></a><span class='hs-cpp'>#ifdef __GLASGOW_HASKELL__</span>
<a name="line-85"></a><span class='hs-keyword'>infixr</span> <span class='hs-num'>1</span> <span class='hs-varop'>=<<</span>
<a name="line-86"></a>
<a name="line-87"></a><span class='hs-comment'>-- -----------------------------------------------------------------------------</span>
<a name="line-88"></a><span class='hs-comment'>-- Prelude monad functions</span>
<a name="line-89"></a>
<a name="line-90"></a><a name="=<<"></a><span class='hs-comment'>-- | Same as '>>=', but with the arguments interchanged.</span>
<a name="line-91"></a><span class='hs-comment'>{-# SPECIALISE (=<<) :: (a -> [b]) -> [a] -> [b] #-}</span>
<a name="line-92"></a><span class='hs-layout'>(</span><span class='hs-varop'>=<<</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>::</span> <span class='hs-conid'>Monad</span> <span class='hs-varid'>m</span> <span class='hs-keyglyph'>=></span> <span class='hs-layout'>(</span><span class='hs-varid'>a</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-varid'>b</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-varid'>b</span>
<a name="line-93"></a><a name="f"></a><span class='hs-definition'>f</span> <span class='hs-varop'>=<<</span> <span class='hs-varid'>x</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>x</span> <span class='hs-varop'>>>=</span> <span class='hs-varid'>f</span>
<a name="line-94"></a>
<a name="line-95"></a><a name="sequence"></a><span class='hs-comment'>-- | Evaluate each action in the sequence from left to right,</span>
<a name="line-96"></a><span class='hs-comment'>-- and collect the results.</span>
<a name="line-97"></a><span class='hs-definition'>sequence</span> <span class='hs-keyglyph'>::</span> <span class='hs-conid'>Monad</span> <span class='hs-varid'>m</span> <span class='hs-keyglyph'>=></span> <span class='hs-keyglyph'>[</span><span class='hs-varid'>m</span> <span class='hs-varid'>a</span><span class='hs-keyglyph'>]</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-keyglyph'>[</span><span class='hs-varid'>a</span><span class='hs-keyglyph'>]</span>
<a name="line-98"></a><span class='hs-comment'>{-# INLINE sequence #-}</span>
<a name="line-99"></a><span class='hs-definition'>sequence</span> <span class='hs-varid'>ms</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>foldr</span> <span class='hs-varid'>k</span> <span class='hs-layout'>(</span><span class='hs-varid'>return</span> <span class='hs-conid'>[]</span><span class='hs-layout'>)</span> <span class='hs-varid'>ms</span>
<a name="line-100"></a> <span class='hs-keyword'>where</span>
<a name="line-101"></a> <span class='hs-varid'>k</span> <span class='hs-varid'>m</span> <span class='hs-varid'>m'</span> <span class='hs-keyglyph'>=</span> <span class='hs-keyword'>do</span> <span class='hs-layout'>{</span> <span class='hs-varid'>x</span> <span class='hs-keyglyph'><-</span> <span class='hs-varid'>m</span><span class='hs-layout'>;</span> <span class='hs-varid'>xs</span> <span class='hs-keyglyph'><-</span> <span class='hs-varid'>m'</span><span class='hs-layout'>;</span> <span class='hs-varid'>return</span> <span class='hs-layout'>(</span><span class='hs-varid'>x</span><span class='hs-conop'>:</span><span class='hs-varid'>xs</span><span class='hs-layout'>)</span> <span class='hs-layout'>}</span>
<a name="line-102"></a>
<a name="line-103"></a><a name="sequence_"></a><span class='hs-comment'>-- | Evaluate each action in the sequence from left to right,</span>
<a name="line-104"></a><span class='hs-comment'>-- and ignore the results.</span>
<a name="line-105"></a><span class='hs-definition'>sequence_</span> <span class='hs-keyglyph'>::</span> <span class='hs-conid'>Monad</span> <span class='hs-varid'>m</span> <span class='hs-keyglyph'>=></span> <span class='hs-keyglyph'>[</span><span class='hs-varid'>m</span> <span class='hs-varid'>a</span><span class='hs-keyglyph'>]</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-conid'>()</span>
<a name="line-106"></a><span class='hs-comment'>{-# INLINE sequence_ #-}</span>
<a name="line-107"></a><span class='hs-definition'>sequence_</span> <span class='hs-varid'>ms</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>foldr</span> <span class='hs-layout'>(</span><span class='hs-varop'>>></span><span class='hs-layout'>)</span> <span class='hs-layout'>(</span><span class='hs-varid'>return</span> <span class='hs-conid'>()</span><span class='hs-layout'>)</span> <span class='hs-varid'>ms</span>
<a name="line-108"></a>
<a name="line-109"></a><a name="mapM"></a><span class='hs-comment'>-- | @'mapM' f@ is equivalent to @'sequence' . 'map' f@.</span>
<a name="line-110"></a><span class='hs-definition'>mapM</span> <span class='hs-keyglyph'>::</span> <span class='hs-conid'>Monad</span> <span class='hs-varid'>m</span> <span class='hs-keyglyph'>=></span> <span class='hs-layout'>(</span><span class='hs-varid'>a</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-varid'>b</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>-></span> <span class='hs-keyglyph'>[</span><span class='hs-varid'>a</span><span class='hs-keyglyph'>]</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-keyglyph'>[</span><span class='hs-varid'>b</span><span class='hs-keyglyph'>]</span>
<a name="line-111"></a><span class='hs-comment'>{-# INLINE mapM #-}</span>
<a name="line-112"></a><span class='hs-definition'>mapM</span> <span class='hs-varid'>f</span> <span class='hs-keyword'>as</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>sequence</span> <span class='hs-layout'>(</span><span class='hs-varid'>map</span> <span class='hs-varid'>f</span> <span class='hs-keyword'>as</span><span class='hs-layout'>)</span>
<a name="line-113"></a>
<a name="line-114"></a><a name="mapM_"></a><span class='hs-comment'>-- | @'mapM_' f@ is equivalent to @'sequence_' . 'map' f@.</span>
<a name="line-115"></a><span class='hs-definition'>mapM_</span> <span class='hs-keyglyph'>::</span> <span class='hs-conid'>Monad</span> <span class='hs-varid'>m</span> <span class='hs-keyglyph'>=></span> <span class='hs-layout'>(</span><span class='hs-varid'>a</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-varid'>b</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>-></span> <span class='hs-keyglyph'>[</span><span class='hs-varid'>a</span><span class='hs-keyglyph'>]</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-conid'>()</span>
<a name="line-116"></a><span class='hs-comment'>{-# INLINE mapM_ #-}</span>
<a name="line-117"></a><span class='hs-definition'>mapM_</span> <span class='hs-varid'>f</span> <span class='hs-keyword'>as</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>sequence_</span> <span class='hs-layout'>(</span><span class='hs-varid'>map</span> <span class='hs-varid'>f</span> <span class='hs-keyword'>as</span><span class='hs-layout'>)</span>
<a name="line-118"></a>
<a name="line-119"></a><span class='hs-cpp'>#endif /* __GLASGOW_HASKELL__ */</span>
<a name="line-120"></a>
<a name="line-121"></a><span class='hs-comment'>-- -----------------------------------------------------------------------------</span>
<a name="line-122"></a><span class='hs-comment'>-- The MonadPlus class definition</span>
<a name="line-123"></a>
<a name="line-124"></a><a name="MonadPlus"></a><span class='hs-comment'>-- | Monads that also support choice and failure.</span>
<a name="line-125"></a><a name="MonadPlus"></a><span class='hs-keyword'>class</span> <span class='hs-conid'>Monad</span> <span class='hs-varid'>m</span> <span class='hs-keyglyph'>=></span> <span class='hs-conid'>MonadPlus</span> <span class='hs-varid'>m</span> <span class='hs-keyword'>where</span>
<a name="line-126"></a> <span class='hs-comment'>-- | the identity of 'mplus'. It should also satisfy the equations</span>
<a name="line-127"></a> <span class='hs-comment'>--</span>
<a name="line-128"></a> <span class='hs-comment'>-- > mzero >>= f = mzero</span>
<a name="line-129"></a> <span class='hs-comment'>-- > v >> mzero = mzero</span>
<a name="line-130"></a> <span class='hs-comment'>--</span>
<a name="line-131"></a> <span class='hs-varid'>mzero</span> <span class='hs-keyglyph'>::</span> <span class='hs-varid'>m</span> <span class='hs-varid'>a</span>
<a name="line-132"></a> <span class='hs-comment'>-- | an associative operation</span>
<a name="line-133"></a> <span class='hs-varid'>mplus</span> <span class='hs-keyglyph'>::</span> <span class='hs-varid'>m</span> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-varid'>a</span>
<a name="line-134"></a>
<a name="line-135"></a><span class='hs-keyword'>instance</span> <span class='hs-conid'>MonadPlus</span> <span class='hs-conid'>[]</span> <span class='hs-keyword'>where</span>
<a name="line-136"></a> <span class='hs-varid'>mzero</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>[]</span>
<a name="line-137"></a> <span class='hs-varid'>mplus</span> <span class='hs-keyglyph'>=</span> <span class='hs-layout'>(</span><span class='hs-varop'>++</span><span class='hs-layout'>)</span>
<a name="line-138"></a>
<a name="line-139"></a><span class='hs-keyword'>instance</span> <span class='hs-conid'>MonadPlus</span> <span class='hs-conid'>Maybe</span> <span class='hs-keyword'>where</span>
<a name="line-140"></a> <span class='hs-varid'>mzero</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>Nothing</span>
<a name="line-141"></a>
<a name="line-142"></a> <span class='hs-conid'>Nothing</span> <span class='hs-varop'>`mplus`</span> <span class='hs-varid'>ys</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>ys</span>
<a name="line-143"></a> <span class='hs-varid'>xs</span> <span class='hs-varop'>`mplus`</span> <span class='hs-sel'>_ys</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>xs</span>
<a name="line-144"></a>
<a name="line-145"></a><span class='hs-comment'>-- -----------------------------------------------------------------------------</span>
<a name="line-146"></a><span class='hs-comment'>-- Functions mandated by the Prelude</span>
<a name="line-147"></a>
<a name="line-148"></a><a name="guard"></a><span class='hs-comment'>-- | @'guard' b@ is @'return' ()@ if @b@ is 'True',</span>
<a name="line-149"></a><span class='hs-comment'>-- and 'mzero' if @b@ is 'False'.</span>
<a name="line-150"></a><span class='hs-definition'>guard</span> <span class='hs-keyglyph'>::</span> <span class='hs-layout'>(</span><span class='hs-conid'>MonadPlus</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=></span> <span class='hs-conid'>Bool</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-conid'>()</span>
<a name="line-151"></a><span class='hs-definition'>guard</span> <span class='hs-conid'>True</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>return</span> <span class='hs-conid'>()</span>
<a name="line-152"></a><span class='hs-definition'>guard</span> <span class='hs-conid'>False</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>mzero</span>
<a name="line-153"></a>
<a name="line-154"></a><span class='hs-comment'>-- | This generalizes the list-based 'filter' function.</span>
<a name="line-155"></a>
<a name="line-156"></a><a name="filterM"></a><span class='hs-definition'>filterM</span> <span class='hs-keyglyph'>::</span> <span class='hs-layout'>(</span><span class='hs-conid'>Monad</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=></span> <span class='hs-layout'>(</span><span class='hs-varid'>a</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-conid'>Bool</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>-></span> <span class='hs-keyglyph'>[</span><span class='hs-varid'>a</span><span class='hs-keyglyph'>]</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-keyglyph'>[</span><span class='hs-varid'>a</span><span class='hs-keyglyph'>]</span>
<a name="line-157"></a><span class='hs-definition'>filterM</span> <span class='hs-keyword'>_</span> <span class='hs-conid'>[]</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>return</span> <span class='hs-conid'>[]</span>
<a name="line-158"></a><span class='hs-definition'>filterM</span> <span class='hs-varid'>p</span> <span class='hs-layout'>(</span><span class='hs-varid'>x</span><span class='hs-conop'>:</span><span class='hs-varid'>xs</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=</span> <span class='hs-keyword'>do</span>
<a name="line-159"></a> <span class='hs-varid'>flg</span> <span class='hs-keyglyph'><-</span> <span class='hs-varid'>p</span> <span class='hs-varid'>x</span>
<a name="line-160"></a> <span class='hs-varid'>ys</span> <span class='hs-keyglyph'><-</span> <span class='hs-varid'>filterM</span> <span class='hs-varid'>p</span> <span class='hs-varid'>xs</span>
<a name="line-161"></a> <span class='hs-varid'>return</span> <span class='hs-layout'>(</span><span class='hs-keyword'>if</span> <span class='hs-varid'>flg</span> <span class='hs-keyword'>then</span> <span class='hs-varid'>x</span><span class='hs-conop'>:</span><span class='hs-varid'>ys</span> <span class='hs-keyword'>else</span> <span class='hs-varid'>ys</span><span class='hs-layout'>)</span>
<a name="line-162"></a>
<a name="line-163"></a><a name="forM"></a><span class='hs-comment'>-- | 'forM' is 'mapM' with its arguments flipped</span>
<a name="line-164"></a><span class='hs-definition'>forM</span> <span class='hs-keyglyph'>::</span> <span class='hs-conid'>Monad</span> <span class='hs-varid'>m</span> <span class='hs-keyglyph'>=></span> <span class='hs-keyglyph'>[</span><span class='hs-varid'>a</span><span class='hs-keyglyph'>]</span> <span class='hs-keyglyph'>-></span> <span class='hs-layout'>(</span><span class='hs-varid'>a</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-varid'>b</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-keyglyph'>[</span><span class='hs-varid'>b</span><span class='hs-keyglyph'>]</span>
<a name="line-165"></a><span class='hs-comment'>{-# INLINE forM #-}</span>
<a name="line-166"></a><span class='hs-definition'>forM</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>flip</span> <span class='hs-varid'>mapM</span>
<a name="line-167"></a>
<a name="line-168"></a><a name="forM_"></a><span class='hs-comment'>-- | 'forM_' is 'mapM_' with its arguments flipped</span>
<a name="line-169"></a><span class='hs-definition'>forM_</span> <span class='hs-keyglyph'>::</span> <span class='hs-conid'>Monad</span> <span class='hs-varid'>m</span> <span class='hs-keyglyph'>=></span> <span class='hs-keyglyph'>[</span><span class='hs-varid'>a</span><span class='hs-keyglyph'>]</span> <span class='hs-keyglyph'>-></span> <span class='hs-layout'>(</span><span class='hs-varid'>a</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-varid'>b</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-conid'>()</span>
<a name="line-170"></a><span class='hs-comment'>{-# INLINE forM_ #-}</span>
<a name="line-171"></a><span class='hs-definition'>forM_</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>flip</span> <span class='hs-varid'>mapM_</span>
<a name="line-172"></a>
<a name="line-173"></a><span class='hs-comment'>-- | This generalizes the list-based 'concat' function.</span>
<a name="line-174"></a>
<a name="line-175"></a><a name="msum"></a><span class='hs-definition'>msum</span> <span class='hs-keyglyph'>::</span> <span class='hs-conid'>MonadPlus</span> <span class='hs-varid'>m</span> <span class='hs-keyglyph'>=></span> <span class='hs-keyglyph'>[</span><span class='hs-varid'>m</span> <span class='hs-varid'>a</span><span class='hs-keyglyph'>]</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-varid'>a</span>
<a name="line-176"></a><span class='hs-comment'>{-# INLINE msum #-}</span>
<a name="line-177"></a><span class='hs-definition'>msum</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>foldr</span> <span class='hs-varid'>mplus</span> <span class='hs-varid'>mzero</span>
<a name="line-178"></a>
<a name="line-179"></a><span class='hs-keyword'>infixr</span> <span class='hs-num'>1</span> <span class='hs-varop'><=<</span><span class='hs-layout'>,</span> <span class='hs-varop'>>=></span>
<a name="line-180"></a>
<a name="line-181"></a><a name=">=>"></a><span class='hs-comment'>-- | Left-to-right Kleisli composition of monads.</span>
<a name="line-182"></a><span class='hs-layout'>(</span><span class='hs-varop'>>=></span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>::</span> <span class='hs-conid'>Monad</span> <span class='hs-varid'>m</span> <span class='hs-keyglyph'>=></span> <span class='hs-layout'>(</span><span class='hs-varid'>a</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-varid'>b</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>-></span> <span class='hs-layout'>(</span><span class='hs-varid'>b</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-varid'>c</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>-></span> <span class='hs-layout'>(</span><span class='hs-varid'>a</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-varid'>c</span><span class='hs-layout'>)</span>
<a name="line-183"></a><span class='hs-definition'>f</span> <span class='hs-varop'>>=></span> <span class='hs-varid'>g</span> <span class='hs-keyglyph'>=</span> <span class='hs-keyglyph'>\</span><span class='hs-varid'>x</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>f</span> <span class='hs-varid'>x</span> <span class='hs-varop'>>>=</span> <span class='hs-varid'>g</span>
<a name="line-184"></a>
<a name="line-185"></a><a name="<=<"></a><span class='hs-comment'>-- | Right-to-left Kleisli composition of monads. @('>=>')@, with the arguments flipped</span>
<a name="line-186"></a><span class='hs-layout'>(</span><span class='hs-varop'><=<</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>::</span> <span class='hs-conid'>Monad</span> <span class='hs-varid'>m</span> <span class='hs-keyglyph'>=></span> <span class='hs-layout'>(</span><span class='hs-varid'>b</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-varid'>c</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>-></span> <span class='hs-layout'>(</span><span class='hs-varid'>a</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-varid'>b</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>-></span> <span class='hs-layout'>(</span><span class='hs-varid'>a</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-varid'>c</span><span class='hs-layout'>)</span>
<a name="line-187"></a><span class='hs-layout'>(</span><span class='hs-varop'><=<</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>flip</span> <span class='hs-layout'>(</span><span class='hs-varop'>>=></span><span class='hs-layout'>)</span>
<a name="line-188"></a>
<a name="line-189"></a><a name="forever"></a><span class='hs-comment'>-- | @'forever' act@ repeats the action infinitely.</span>
<a name="line-190"></a><span class='hs-definition'>forever</span> <span class='hs-keyglyph'>::</span> <span class='hs-layout'>(</span><span class='hs-conid'>Monad</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=></span> <span class='hs-varid'>m</span> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-varid'>b</span>
<a name="line-191"></a><span class='hs-definition'>forever</span> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>a</span> <span class='hs-varop'>>></span> <span class='hs-varid'>forever</span> <span class='hs-varid'>a</span>
<a name="line-192"></a>
<a name="line-193"></a><a name="void"></a><span class='hs-comment'>-- | @'void' value@ discards or ignores the result of evaluation, such as the return value of an 'IO' action.</span>
<a name="line-194"></a><span class='hs-definition'>void</span> <span class='hs-keyglyph'>::</span> <span class='hs-conid'>Functor</span> <span class='hs-varid'>f</span> <span class='hs-keyglyph'>=></span> <span class='hs-varid'>f</span> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>f</span> <span class='hs-conid'>()</span>
<a name="line-195"></a><span class='hs-definition'>void</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>fmap</span> <span class='hs-layout'>(</span><span class='hs-varid'>const</span> <span class='hs-conid'>()</span><span class='hs-layout'>)</span>
<a name="line-196"></a>
<a name="line-197"></a><span class='hs-comment'>-- -----------------------------------------------------------------------------</span>
<a name="line-198"></a><span class='hs-comment'>-- Other monad functions</span>
<a name="line-199"></a>
<a name="line-200"></a><a name="join"></a><span class='hs-comment'>-- | The 'join' function is the conventional monad join operator. It is used to</span>
<a name="line-201"></a><span class='hs-comment'>-- remove one level of monadic structure, projecting its bound argument into the</span>
<a name="line-202"></a><span class='hs-comment'>-- outer level.</span>
<a name="line-203"></a><span class='hs-definition'>join</span> <span class='hs-keyglyph'>::</span> <span class='hs-layout'>(</span><span class='hs-conid'>Monad</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=></span> <span class='hs-varid'>m</span> <span class='hs-layout'>(</span><span class='hs-varid'>m</span> <span class='hs-varid'>a</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-varid'>a</span>
<a name="line-204"></a><span class='hs-definition'>join</span> <span class='hs-varid'>x</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>x</span> <span class='hs-varop'>>>=</span> <span class='hs-varid'>id</span>
<a name="line-205"></a>
<a name="line-206"></a><a name="mapAndUnzipM"></a><span class='hs-comment'>-- | The 'mapAndUnzipM' function maps its first argument over a list, returning</span>
<a name="line-207"></a><span class='hs-comment'>-- the result as a pair of lists. This function is mainly used with complicated</span>
<a name="line-208"></a><span class='hs-comment'>-- data structures or a state-transforming monad.</span>
<a name="line-209"></a><span class='hs-definition'>mapAndUnzipM</span> <span class='hs-keyglyph'>::</span> <span class='hs-layout'>(</span><span class='hs-conid'>Monad</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=></span> <span class='hs-layout'>(</span><span class='hs-varid'>a</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-layout'>(</span><span class='hs-varid'>b</span><span class='hs-layout'>,</span><span class='hs-varid'>c</span><span class='hs-layout'>)</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>-></span> <span class='hs-keyglyph'>[</span><span class='hs-varid'>a</span><span class='hs-keyglyph'>]</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-layout'>(</span><span class='hs-keyglyph'>[</span><span class='hs-varid'>b</span><span class='hs-keyglyph'>]</span><span class='hs-layout'>,</span> <span class='hs-keyglyph'>[</span><span class='hs-varid'>c</span><span class='hs-keyglyph'>]</span><span class='hs-layout'>)</span>
<a name="line-210"></a><span class='hs-definition'>mapAndUnzipM</span> <span class='hs-varid'>f</span> <span class='hs-varid'>xs</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>sequence</span> <span class='hs-layout'>(</span><span class='hs-varid'>map</span> <span class='hs-varid'>f</span> <span class='hs-varid'>xs</span><span class='hs-layout'>)</span> <span class='hs-varop'>>>=</span> <span class='hs-varid'>return</span> <span class='hs-varop'>.</span> <span class='hs-varid'>unzip</span>
<a name="line-211"></a>
<a name="line-212"></a><a name="zipWithM"></a><span class='hs-comment'>-- | The 'zipWithM' function generalizes 'zipWith' to arbitrary monads.</span>
<a name="line-213"></a><span class='hs-definition'>zipWithM</span> <span class='hs-keyglyph'>::</span> <span class='hs-layout'>(</span><span class='hs-conid'>Monad</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=></span> <span class='hs-layout'>(</span><span class='hs-varid'>a</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>b</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-varid'>c</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>-></span> <span class='hs-keyglyph'>[</span><span class='hs-varid'>a</span><span class='hs-keyglyph'>]</span> <span class='hs-keyglyph'>-></span> <span class='hs-keyglyph'>[</span><span class='hs-varid'>b</span><span class='hs-keyglyph'>]</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-keyglyph'>[</span><span class='hs-varid'>c</span><span class='hs-keyglyph'>]</span>
<a name="line-214"></a><span class='hs-definition'>zipWithM</span> <span class='hs-varid'>f</span> <span class='hs-varid'>xs</span> <span class='hs-varid'>ys</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>sequence</span> <span class='hs-layout'>(</span><span class='hs-varid'>zipWith</span> <span class='hs-varid'>f</span> <span class='hs-varid'>xs</span> <span class='hs-varid'>ys</span><span class='hs-layout'>)</span>
<a name="line-215"></a>
<a name="line-216"></a><a name="zipWithM_"></a><span class='hs-comment'>-- | 'zipWithM_' is the extension of 'zipWithM' which ignores the final result.</span>
<a name="line-217"></a><span class='hs-definition'>zipWithM_</span> <span class='hs-keyglyph'>::</span> <span class='hs-layout'>(</span><span class='hs-conid'>Monad</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=></span> <span class='hs-layout'>(</span><span class='hs-varid'>a</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>b</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-varid'>c</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>-></span> <span class='hs-keyglyph'>[</span><span class='hs-varid'>a</span><span class='hs-keyglyph'>]</span> <span class='hs-keyglyph'>-></span> <span class='hs-keyglyph'>[</span><span class='hs-varid'>b</span><span class='hs-keyglyph'>]</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-conid'>()</span>
<a name="line-218"></a><span class='hs-definition'>zipWithM_</span> <span class='hs-varid'>f</span> <span class='hs-varid'>xs</span> <span class='hs-varid'>ys</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>sequence_</span> <span class='hs-layout'>(</span><span class='hs-varid'>zipWith</span> <span class='hs-varid'>f</span> <span class='hs-varid'>xs</span> <span class='hs-varid'>ys</span><span class='hs-layout'>)</span>
<a name="line-219"></a>
<a name="line-220"></a><span class='hs-comment'>{- | The 'foldM' function is analogous to 'foldl', except that its result is
<a name="line-221"></a>encapsulated in a monad. Note that 'foldM' works from left-to-right over
<a name="line-222"></a>the list arguments. This could be an issue where @('>>')@ and the `folded
<a name="line-223"></a>function' are not commutative.
<a name="line-224"></a>
<a name="line-225"></a>
<a name="line-226"></a>> foldM f a1 [x1, x2, ..., xm]
<a name="line-227"></a>
<a name="line-228"></a>==
<a name="line-229"></a>
<a name="line-230"></a>> do
<a name="line-231"></a>> a2 <- f a1 x1
<a name="line-232"></a>> a3 <- f a2 x2
<a name="line-233"></a>> ...
<a name="line-234"></a>> f am xm
<a name="line-235"></a>
<a name="line-236"></a>If right-to-left evaluation is required, the input list should be reversed.
<a name="line-237"></a>-}</span>
<a name="line-238"></a>
<a name="line-239"></a><a name="foldM"></a><span class='hs-definition'>foldM</span> <span class='hs-keyglyph'>::</span> <span class='hs-layout'>(</span><span class='hs-conid'>Monad</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=></span> <span class='hs-layout'>(</span><span class='hs-varid'>a</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>b</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-varid'>a</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>-></span> <span class='hs-keyglyph'>[</span><span class='hs-varid'>b</span><span class='hs-keyglyph'>]</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-varid'>a</span>
<a name="line-240"></a><span class='hs-definition'>foldM</span> <span class='hs-keyword'>_</span> <span class='hs-varid'>a</span> <span class='hs-conid'>[]</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>return</span> <span class='hs-varid'>a</span>
<a name="line-241"></a><span class='hs-definition'>foldM</span> <span class='hs-varid'>f</span> <span class='hs-varid'>a</span> <span class='hs-layout'>(</span><span class='hs-varid'>x</span><span class='hs-conop'>:</span><span class='hs-varid'>xs</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>f</span> <span class='hs-varid'>a</span> <span class='hs-varid'>x</span> <span class='hs-varop'>>>=</span> <span class='hs-keyglyph'>\</span><span class='hs-varid'>fax</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>foldM</span> <span class='hs-varid'>f</span> <span class='hs-varid'>fax</span> <span class='hs-varid'>xs</span>
<a name="line-242"></a>
<a name="line-243"></a><a name="foldM_"></a><span class='hs-comment'>-- | Like 'foldM', but discards the result.</span>
<a name="line-244"></a><span class='hs-definition'>foldM_</span> <span class='hs-keyglyph'>::</span> <span class='hs-layout'>(</span><span class='hs-conid'>Monad</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=></span> <span class='hs-layout'>(</span><span class='hs-varid'>a</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>b</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-varid'>a</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>-></span> <span class='hs-keyglyph'>[</span><span class='hs-varid'>b</span><span class='hs-keyglyph'>]</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-conid'>()</span>
<a name="line-245"></a><span class='hs-definition'>foldM_</span> <span class='hs-varid'>f</span> <span class='hs-varid'>a</span> <span class='hs-varid'>xs</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>foldM</span> <span class='hs-varid'>f</span> <span class='hs-varid'>a</span> <span class='hs-varid'>xs</span> <span class='hs-varop'>>></span> <span class='hs-varid'>return</span> <span class='hs-conid'>()</span>
<a name="line-246"></a>
<a name="line-247"></a><a name="replicateM"></a><span class='hs-comment'>-- | @'replicateM' n act@ performs the action @n@ times,</span>
<a name="line-248"></a><span class='hs-comment'>-- gathering the results.</span>
<a name="line-249"></a><span class='hs-definition'>replicateM</span> <span class='hs-keyglyph'>::</span> <span class='hs-layout'>(</span><span class='hs-conid'>Monad</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=></span> <span class='hs-conid'>Int</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-keyglyph'>[</span><span class='hs-varid'>a</span><span class='hs-keyglyph'>]</span>
<a name="line-250"></a><span class='hs-definition'>replicateM</span> <span class='hs-varid'>n</span> <span class='hs-varid'>x</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>sequence</span> <span class='hs-layout'>(</span><span class='hs-varid'>replicate</span> <span class='hs-varid'>n</span> <span class='hs-varid'>x</span><span class='hs-layout'>)</span>
<a name="line-251"></a>
<a name="line-252"></a><a name="replicateM_"></a><span class='hs-comment'>-- | Like 'replicateM', but discards the result.</span>
<a name="line-253"></a><span class='hs-definition'>replicateM_</span> <span class='hs-keyglyph'>::</span> <span class='hs-layout'>(</span><span class='hs-conid'>Monad</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=></span> <span class='hs-conid'>Int</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-conid'>()</span>
<a name="line-254"></a><span class='hs-definition'>replicateM_</span> <span class='hs-varid'>n</span> <span class='hs-varid'>x</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>sequence_</span> <span class='hs-layout'>(</span><span class='hs-varid'>replicate</span> <span class='hs-varid'>n</span> <span class='hs-varid'>x</span><span class='hs-layout'>)</span>
<a name="line-255"></a>
<a name="line-256"></a><span class='hs-comment'>{- | Conditional execution of monadic expressions. For example,
<a name="line-257"></a>
<a name="line-258"></a>> when debug (putStr "Debugging\n")
<a name="line-259"></a>
<a name="line-260"></a>will output the string @Debugging\\n@ if the Boolean value @debug@ is 'True',
<a name="line-261"></a>and otherwise do nothing.
<a name="line-262"></a>-}</span>
<a name="line-263"></a>
<a name="line-264"></a><a name="when"></a><span class='hs-definition'>when</span> <span class='hs-keyglyph'>::</span> <span class='hs-layout'>(</span><span class='hs-conid'>Monad</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=></span> <span class='hs-conid'>Bool</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-conid'>()</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-conid'>()</span>
<a name="line-265"></a><span class='hs-definition'>when</span> <span class='hs-varid'>p</span> <span class='hs-varid'>s</span> <span class='hs-keyglyph'>=</span> <span class='hs-keyword'>if</span> <span class='hs-varid'>p</span> <span class='hs-keyword'>then</span> <span class='hs-varid'>s</span> <span class='hs-keyword'>else</span> <span class='hs-varid'>return</span> <span class='hs-conid'>()</span>
<a name="line-266"></a>
<a name="line-267"></a><span class='hs-comment'>-- | The reverse of 'when'.</span>
<a name="line-268"></a>
<a name="line-269"></a><a name="unless"></a><span class='hs-definition'>unless</span> <span class='hs-keyglyph'>::</span> <span class='hs-layout'>(</span><span class='hs-conid'>Monad</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=></span> <span class='hs-conid'>Bool</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-conid'>()</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-conid'>()</span>
<a name="line-270"></a><span class='hs-definition'>unless</span> <span class='hs-varid'>p</span> <span class='hs-varid'>s</span> <span class='hs-keyglyph'>=</span> <span class='hs-keyword'>if</span> <span class='hs-varid'>p</span> <span class='hs-keyword'>then</span> <span class='hs-varid'>return</span> <span class='hs-conid'>()</span> <span class='hs-keyword'>else</span> <span class='hs-varid'>s</span>
<a name="line-271"></a>
<a name="line-272"></a><a name="liftM"></a><span class='hs-comment'>-- | Promote a function to a monad.</span>
<a name="line-273"></a><span class='hs-definition'>liftM</span> <span class='hs-keyglyph'>::</span> <span class='hs-layout'>(</span><span class='hs-conid'>Monad</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=></span> <span class='hs-layout'>(</span><span class='hs-varid'>a1</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>r</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-varid'>a1</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-varid'>r</span>
<a name="line-274"></a><span class='hs-definition'>liftM</span> <span class='hs-varid'>f</span> <span class='hs-varid'>m1</span> <span class='hs-keyglyph'>=</span> <span class='hs-keyword'>do</span> <span class='hs-layout'>{</span> <span class='hs-varid'>x1</span> <span class='hs-keyglyph'><-</span> <span class='hs-varid'>m1</span><span class='hs-layout'>;</span> <span class='hs-varid'>return</span> <span class='hs-layout'>(</span><span class='hs-varid'>f</span> <span class='hs-varid'>x1</span><span class='hs-layout'>)</span> <span class='hs-layout'>}</span>
<a name="line-275"></a>
<a name="line-276"></a><a name="liftM2"></a><span class='hs-comment'>-- | Promote a function to a monad, scanning the monadic arguments from</span>
<a name="line-277"></a><span class='hs-comment'>-- left to right. For example,</span>
<a name="line-278"></a><span class='hs-comment'>--</span>
<a name="line-279"></a><span class='hs-comment'>-- > liftM2 (+) [0,1] [0,2] = [0,2,1,3]</span>
<a name="line-280"></a><span class='hs-comment'>-- > liftM2 (+) (Just 1) Nothing = Nothing</span>
<a name="line-281"></a><span class='hs-comment'>--</span>
<a name="line-282"></a><span class='hs-definition'>liftM2</span> <span class='hs-keyglyph'>::</span> <span class='hs-layout'>(</span><span class='hs-conid'>Monad</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=></span> <span class='hs-layout'>(</span><span class='hs-varid'>a1</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>a2</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>r</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-varid'>a1</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-varid'>a2</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-varid'>r</span>
<a name="line-283"></a><span class='hs-definition'>liftM2</span> <span class='hs-varid'>f</span> <span class='hs-varid'>m1</span> <span class='hs-varid'>m2</span> <span class='hs-keyglyph'>=</span> <span class='hs-keyword'>do</span> <span class='hs-layout'>{</span> <span class='hs-varid'>x1</span> <span class='hs-keyglyph'><-</span> <span class='hs-varid'>m1</span><span class='hs-layout'>;</span> <span class='hs-varid'>x2</span> <span class='hs-keyglyph'><-</span> <span class='hs-varid'>m2</span><span class='hs-layout'>;</span> <span class='hs-varid'>return</span> <span class='hs-layout'>(</span><span class='hs-varid'>f</span> <span class='hs-varid'>x1</span> <span class='hs-varid'>x2</span><span class='hs-layout'>)</span> <span class='hs-layout'>}</span>
<a name="line-284"></a>
<a name="line-285"></a><a name="liftM3"></a><span class='hs-comment'>-- | Promote a function to a monad, scanning the monadic arguments from</span>
<a name="line-286"></a><span class='hs-comment'>-- left to right (cf. 'liftM2').</span>
<a name="line-287"></a><span class='hs-definition'>liftM3</span> <span class='hs-keyglyph'>::</span> <span class='hs-layout'>(</span><span class='hs-conid'>Monad</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=></span> <span class='hs-layout'>(</span><span class='hs-varid'>a1</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>a2</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>a3</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>r</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-varid'>a1</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-varid'>a2</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-varid'>a3</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-varid'>r</span>
<a name="line-288"></a><span class='hs-definition'>liftM3</span> <span class='hs-varid'>f</span> <span class='hs-varid'>m1</span> <span class='hs-varid'>m2</span> <span class='hs-varid'>m3</span> <span class='hs-keyglyph'>=</span> <span class='hs-keyword'>do</span> <span class='hs-layout'>{</span> <span class='hs-varid'>x1</span> <span class='hs-keyglyph'><-</span> <span class='hs-varid'>m1</span><span class='hs-layout'>;</span> <span class='hs-varid'>x2</span> <span class='hs-keyglyph'><-</span> <span class='hs-varid'>m2</span><span class='hs-layout'>;</span> <span class='hs-varid'>x3</span> <span class='hs-keyglyph'><-</span> <span class='hs-varid'>m3</span><span class='hs-layout'>;</span> <span class='hs-varid'>return</span> <span class='hs-layout'>(</span><span class='hs-varid'>f</span> <span class='hs-varid'>x1</span> <span class='hs-varid'>x2</span> <span class='hs-varid'>x3</span><span class='hs-layout'>)</span> <span class='hs-layout'>}</span>
<a name="line-289"></a>
<a name="line-290"></a><a name="liftM4"></a><span class='hs-comment'>-- | Promote a function to a monad, scanning the monadic arguments from</span>
<a name="line-291"></a><span class='hs-comment'>-- left to right (cf. 'liftM2').</span>
<a name="line-292"></a><span class='hs-definition'>liftM4</span> <span class='hs-keyglyph'>::</span> <span class='hs-layout'>(</span><span class='hs-conid'>Monad</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=></span> <span class='hs-layout'>(</span><span class='hs-varid'>a1</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>a2</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>a3</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>a4</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>r</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-varid'>a1</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-varid'>a2</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-varid'>a3</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-varid'>a4</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-varid'>r</span>
<a name="line-293"></a><span class='hs-definition'>liftM4</span> <span class='hs-varid'>f</span> <span class='hs-varid'>m1</span> <span class='hs-varid'>m2</span> <span class='hs-varid'>m3</span> <span class='hs-varid'>m4</span> <span class='hs-keyglyph'>=</span> <span class='hs-keyword'>do</span> <span class='hs-layout'>{</span> <span class='hs-varid'>x1</span> <span class='hs-keyglyph'><-</span> <span class='hs-varid'>m1</span><span class='hs-layout'>;</span> <span class='hs-varid'>x2</span> <span class='hs-keyglyph'><-</span> <span class='hs-varid'>m2</span><span class='hs-layout'>;</span> <span class='hs-varid'>x3</span> <span class='hs-keyglyph'><-</span> <span class='hs-varid'>m3</span><span class='hs-layout'>;</span> <span class='hs-varid'>x4</span> <span class='hs-keyglyph'><-</span> <span class='hs-varid'>m4</span><span class='hs-layout'>;</span> <span class='hs-varid'>return</span> <span class='hs-layout'>(</span><span class='hs-varid'>f</span> <span class='hs-varid'>x1</span> <span class='hs-varid'>x2</span> <span class='hs-varid'>x3</span> <span class='hs-varid'>x4</span><span class='hs-layout'>)</span> <span class='hs-layout'>}</span>
<a name="line-294"></a>
<a name="line-295"></a><a name="liftM5"></a><span class='hs-comment'>-- | Promote a function to a monad, scanning the monadic arguments from</span>
<a name="line-296"></a><span class='hs-comment'>-- left to right (cf. 'liftM2').</span>
<a name="line-297"></a><span class='hs-definition'>liftM5</span> <span class='hs-keyglyph'>::</span> <span class='hs-layout'>(</span><span class='hs-conid'>Monad</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=></span> <span class='hs-layout'>(</span><span class='hs-varid'>a1</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>a2</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>a3</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>a4</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>a5</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>r</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-varid'>a1</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-varid'>a2</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-varid'>a3</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-varid'>a4</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-varid'>a5</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-varid'>r</span>
<a name="line-298"></a><span class='hs-definition'>liftM5</span> <span class='hs-varid'>f</span> <span class='hs-varid'>m1</span> <span class='hs-varid'>m2</span> <span class='hs-varid'>m3</span> <span class='hs-varid'>m4</span> <span class='hs-varid'>m5</span> <span class='hs-keyglyph'>=</span> <span class='hs-keyword'>do</span> <span class='hs-layout'>{</span> <span class='hs-varid'>x1</span> <span class='hs-keyglyph'><-</span> <span class='hs-varid'>m1</span><span class='hs-layout'>;</span> <span class='hs-varid'>x2</span> <span class='hs-keyglyph'><-</span> <span class='hs-varid'>m2</span><span class='hs-layout'>;</span> <span class='hs-varid'>x3</span> <span class='hs-keyglyph'><-</span> <span class='hs-varid'>m3</span><span class='hs-layout'>;</span> <span class='hs-varid'>x4</span> <span class='hs-keyglyph'><-</span> <span class='hs-varid'>m4</span><span class='hs-layout'>;</span> <span class='hs-varid'>x5</span> <span class='hs-keyglyph'><-</span> <span class='hs-varid'>m5</span><span class='hs-layout'>;</span> <span class='hs-varid'>return</span> <span class='hs-layout'>(</span><span class='hs-varid'>f</span> <span class='hs-varid'>x1</span> <span class='hs-varid'>x2</span> <span class='hs-varid'>x3</span> <span class='hs-varid'>x4</span> <span class='hs-varid'>x5</span><span class='hs-layout'>)</span> <span class='hs-layout'>}</span>
<a name="line-299"></a>
<a name="line-300"></a><span class='hs-comment'>{- | In many situations, the 'liftM' operations can be replaced by uses of
<a name="line-301"></a>'ap', which promotes function application.
<a name="line-302"></a>
<a name="line-303"></a>> return f `ap` x1 `ap` ... `ap` xn
<a name="line-304"></a>
<a name="line-305"></a>is equivalent to
<a name="line-306"></a>
<a name="line-307"></a>> liftMn f x1 x2 ... xn
<a name="line-308"></a>
<a name="line-309"></a>-}</span>
<a name="line-310"></a>
<a name="line-311"></a><a name="ap"></a><span class='hs-definition'>ap</span> <span class='hs-keyglyph'>::</span> <span class='hs-layout'>(</span><span class='hs-conid'>Monad</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=></span> <span class='hs-varid'>m</span> <span class='hs-layout'>(</span><span class='hs-varid'>a</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>b</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-varid'>b</span>
<a name="line-312"></a><span class='hs-definition'>ap</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>liftM2</span> <span class='hs-varid'>id</span>
<a name="line-313"></a>
<a name="line-314"></a>
<a name="line-315"></a><span class='hs-comment'>-- -----------------------------------------------------------------------------</span>
<a name="line-316"></a><span class='hs-comment'>-- Other MonadPlus functions</span>
<a name="line-317"></a>
<a name="line-318"></a><span class='hs-comment'>-- | Direct 'MonadPlus' equivalent of 'filter'</span>
<a name="line-319"></a><span class='hs-comment'>-- @'filter'@ = @(mfilter:: (a -> Bool) -> [a] -> [a]@</span>
<a name="line-320"></a><span class='hs-comment'>-- applicable to any 'MonadPlus', for example</span>
<a name="line-321"></a><span class='hs-comment'>-- @mfilter odd (Just 1) == Just 1@</span>
<a name="line-322"></a><span class='hs-comment'>-- @mfilter odd (Just 2) == Nothing@</span>
<a name="line-323"></a>
<a name="line-324"></a><a name="mfilter"></a><span class='hs-definition'>mfilter</span> <span class='hs-keyglyph'>::</span> <span class='hs-layout'>(</span><span class='hs-conid'>MonadPlus</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=></span> <span class='hs-layout'>(</span><span class='hs-varid'>a</span> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>Bool</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-varid'>a</span>
<a name="line-325"></a><span class='hs-definition'>mfilter</span> <span class='hs-varid'>p</span> <span class='hs-varid'>ma</span> <span class='hs-keyglyph'>=</span> <span class='hs-keyword'>do</span>
<a name="line-326"></a> <span class='hs-varid'>a</span> <span class='hs-keyglyph'><-</span> <span class='hs-varid'>ma</span>
<a name="line-327"></a> <span class='hs-keyword'>if</span> <span class='hs-varid'>p</span> <span class='hs-varid'>a</span> <span class='hs-keyword'>then</span> <span class='hs-varid'>return</span> <span class='hs-varid'>a</span> <span class='hs-keyword'>else</span> <span class='hs-varid'>mzero</span>
<a name="line-328"></a>
<a name="line-329"></a><span class='hs-comment'>{- $naming
<a name="line-330"></a>
<a name="line-331"></a>The functions in this library use the following naming conventions:
<a name="line-332"></a>
<a name="line-333"></a>* A postfix \'@M@\' always stands for a function in the Kleisli category:
<a name="line-334"></a> The monad type constructor @m@ is added to function results
<a name="line-335"></a> (modulo currying) and nowhere else. So, for example,
<a name="line-336"></a>
<a name="line-337"></a>> filter :: (a -> Bool) -> [a] -> [a]
<a name="line-338"></a>> filterM :: (Monad m) => (a -> m Bool) -> [a] -> m [a]
<a name="line-339"></a>
<a name="line-340"></a>* A postfix \'@_@\' changes the result type from @(m a)@ to @(m ())@.
<a name="line-341"></a> Thus, for example:
<a name="line-342"></a>
<a name="line-343"></a>> sequence :: Monad m => [m a] -> m [a]
<a name="line-344"></a>> sequence_ :: Monad m => [m a] -> m ()
<a name="line-345"></a>
<a name="line-346"></a>* A prefix \'@m@\' generalizes an existing function to a monadic form.
<a name="line-347"></a> Thus, for example:
<a name="line-348"></a>
<a name="line-349"></a>> sum :: Num a => [a] -> a
<a name="line-350"></a>> msum :: MonadPlus m => [m a] -> m a
<a name="line-351"></a>
<a name="line-352"></a>-}</span>
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