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<pre><a name="line-1"></a><span class='hs-comment'>-----------------------------------------------------------------------------</span>
<a name="line-2"></a><span class='hs-comment'>-- |</span>
<a name="line-3"></a><span class='hs-comment'>-- Module : Control.Monad.Trans.List</span>
<a name="line-4"></a><span class='hs-comment'>-- Copyright : (c) Andy Gill 2001,</span>
<a name="line-5"></a><span class='hs-comment'>-- (c) Oregon Graduate Institute of Science and Technology, 2001</span>
<a name="line-6"></a><span class='hs-comment'>-- License : BSD-style (see the file LICENSE)</span>
<a name="line-7"></a><span class='hs-comment'>--</span>
<a name="line-8"></a><span class='hs-comment'>-- Maintainer : ross@soi.city.ac.uk</span>
<a name="line-9"></a><span class='hs-comment'>-- Stability : experimental</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 ListT monad transformer, adding backtracking to a given monad,</span>
<a name="line-13"></a><span class='hs-comment'>-- which must be commutative.</span>
<a name="line-14"></a><span class='hs-comment'>-----------------------------------------------------------------------------</span>
<a name="line-15"></a>
<a name="line-16"></a><span class='hs-keyword'>module</span> <span class='hs-conid'>Control</span><span class='hs-varop'>.</span><span class='hs-conid'>Monad</span><span class='hs-varop'>.</span><span class='hs-conid'>Trans</span><span class='hs-varop'>.</span><span class='hs-conid'>List</span> <span class='hs-layout'>(</span>
<a name="line-17"></a> <span class='hs-comment'>-- * The ListT monad transformer</span>
<a name="line-18"></a> <span class='hs-conid'>ListT</span><span class='hs-layout'>(</span><span class='hs-keyglyph'>..</span><span class='hs-layout'>)</span><span class='hs-layout'>,</span>
<a name="line-19"></a> <span class='hs-varid'>mapListT</span><span class='hs-layout'>,</span>
<a name="line-20"></a> <span class='hs-comment'>-- * Lifting other operations</span>
<a name="line-21"></a> <span class='hs-varid'>liftCallCC</span><span class='hs-layout'>,</span>
<a name="line-22"></a> <span class='hs-varid'>liftCatch</span><span class='hs-layout'>,</span>
<a name="line-23"></a> <span class='hs-layout'>)</span> <span class='hs-keyword'>where</span>
<a name="line-24"></a>
<a name="line-25"></a><span class='hs-keyword'>import</span> <span class='hs-conid'>Control</span><span class='hs-varop'>.</span><span class='hs-conid'>Monad</span><span class='hs-varop'>.</span><span class='hs-conid'>IO</span><span class='hs-varop'>.</span><span class='hs-conid'>Class</span>
<a name="line-26"></a><span class='hs-keyword'>import</span> <span class='hs-conid'>Control</span><span class='hs-varop'>.</span><span class='hs-conid'>Monad</span><span class='hs-varop'>.</span><span class='hs-conid'>Trans</span><span class='hs-varop'>.</span><span class='hs-conid'>Class</span>
<a name="line-27"></a>
<a name="line-28"></a><span class='hs-keyword'>import</span> <span class='hs-conid'>Control</span><span class='hs-varop'>.</span><span class='hs-conid'>Applicative</span>
<a name="line-29"></a><span class='hs-keyword'>import</span> <span class='hs-conid'>Control</span><span class='hs-varop'>.</span><span class='hs-conid'>Monad</span>
<a name="line-30"></a>
<a name="line-31"></a><a name="ListT"></a><span class='hs-comment'>-- | Parameterizable list monad, with an inner monad.</span>
<a name="line-32"></a><a name="ListT"></a><span class='hs-comment'>--</span>
<a name="line-33"></a><a name="ListT"></a><span class='hs-comment'>-- /Note:/ this does not yield a monad unless the argument monad is commutative.</span>
<a name="line-34"></a><a name="ListT"></a><span class='hs-keyword'>newtype</span> <span class='hs-conid'>ListT</span> <span class='hs-varid'>m</span> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>ListT</span> <span class='hs-layout'>{</span> <span class='hs-varid'>runListT</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> <span class='hs-layout'>}</span>
<a name="line-35"></a>
<a name="line-36"></a><a name="mapListT"></a><span class='hs-comment'>-- | Map between 'ListT' computations.</span>
<a name="line-37"></a><span class='hs-comment'>--</span>
<a name="line-38"></a><span class='hs-comment'>-- * @'runListT' ('mapListT' f m) = f ('runListT' m@)</span>
<a name="line-39"></a><span class='hs-definition'>mapListT</span> <span class='hs-keyglyph'>::</span> <span class='hs-layout'>(</span><span class='hs-varid'>m</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'>n</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-conid'>ListT</span> <span class='hs-varid'>m</span> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>ListT</span> <span class='hs-varid'>n</span> <span class='hs-varid'>b</span>
<a name="line-40"></a><span class='hs-definition'>mapListT</span> <span class='hs-varid'>f</span> <span class='hs-varid'>m</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>ListT</span> <span class='hs-varop'>$</span> <span class='hs-varid'>f</span> <span class='hs-layout'>(</span><span class='hs-varid'>runListT</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span>
<a name="line-41"></a>
<a name="line-42"></a><span class='hs-keyword'>instance</span> <span class='hs-layout'>(</span><span class='hs-conid'>Functor</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=></span> <span class='hs-conid'>Functor</span> <span class='hs-layout'>(</span><span class='hs-conid'>ListT</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyword'>where</span>
<a name="line-43"></a> <span class='hs-varid'>fmap</span> <span class='hs-varid'>f</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>mapListT</span> <span class='hs-varop'>$</span> <span class='hs-varid'>fmap</span> <span class='hs-varop'>$</span> <span class='hs-varid'>map</span> <span class='hs-varid'>f</span>
<a name="line-44"></a>
<a name="line-45"></a><span class='hs-keyword'>instance</span> <span class='hs-layout'>(</span><span class='hs-conid'>Applicative</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=></span> <span class='hs-conid'>Applicative</span> <span class='hs-layout'>(</span><span class='hs-conid'>ListT</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyword'>where</span>
<a name="line-46"></a> <span class='hs-varid'>pure</span> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>ListT</span> <span class='hs-varop'>$</span> <span class='hs-varid'>pure</span> <span class='hs-keyglyph'>[</span><span class='hs-varid'>a</span><span class='hs-keyglyph'>]</span>
<a name="line-47"></a> <span class='hs-varid'>f</span> <span class='hs-varop'><*></span> <span class='hs-varid'>v</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>ListT</span> <span class='hs-varop'>$</span> <span class='hs-layout'>(</span><span class='hs-varop'><*></span><span class='hs-layout'>)</span> <span class='hs-varop'><$></span> <span class='hs-varid'>runListT</span> <span class='hs-varid'>f</span> <span class='hs-varop'><*></span> <span class='hs-varid'>runListT</span> <span class='hs-varid'>v</span>
<a name="line-48"></a>
<a name="line-49"></a><span class='hs-keyword'>instance</span> <span class='hs-layout'>(</span><span class='hs-conid'>Applicative</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=></span> <span class='hs-conid'>Alternative</span> <span class='hs-layout'>(</span><span class='hs-conid'>ListT</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyword'>where</span>
<a name="line-50"></a> <span class='hs-varid'>empty</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>ListT</span> <span class='hs-varop'>$</span> <span class='hs-varid'>pure</span> <span class='hs-conid'>[]</span>
<a name="line-51"></a> <span class='hs-varid'>m</span> <span class='hs-varop'><|></span> <span class='hs-varid'>n</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>ListT</span> <span class='hs-varop'>$</span> <span class='hs-layout'>(</span><span class='hs-varop'>++</span><span class='hs-layout'>)</span> <span class='hs-varop'><$></span> <span class='hs-varid'>runListT</span> <span class='hs-varid'>m</span> <span class='hs-varop'><*></span> <span class='hs-varid'>runListT</span> <span class='hs-varid'>n</span>
<a name="line-52"></a>
<a name="line-53"></a><span class='hs-keyword'>instance</span> <span class='hs-layout'>(</span><span class='hs-conid'>Monad</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=></span> <span class='hs-conid'>Monad</span> <span class='hs-layout'>(</span><span class='hs-conid'>ListT</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyword'>where</span>
<a name="line-54"></a> <span class='hs-varid'>return</span> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>ListT</span> <span class='hs-varop'>$</span> <span class='hs-varid'>return</span> <span class='hs-keyglyph'>[</span><span class='hs-varid'>a</span><span class='hs-keyglyph'>]</span>
<a name="line-55"></a> <span class='hs-varid'>m</span> <span class='hs-varop'>>>=</span> <span class='hs-varid'>k</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>ListT</span> <span class='hs-varop'>$</span> <span class='hs-keyword'>do</span>
<a name="line-56"></a> <span class='hs-varid'>a</span> <span class='hs-keyglyph'><-</span> <span class='hs-varid'>runListT</span> <span class='hs-varid'>m</span>
<a name="line-57"></a> <span class='hs-varid'>b</span> <span class='hs-keyglyph'><-</span> <span class='hs-varid'>mapM</span> <span class='hs-layout'>(</span><span class='hs-varid'>runListT</span> <span class='hs-varop'>.</span> <span class='hs-varid'>k</span><span class='hs-layout'>)</span> <span class='hs-varid'>a</span>
<a name="line-58"></a> <span class='hs-varid'>return</span> <span class='hs-layout'>(</span><span class='hs-varid'>concat</span> <span class='hs-varid'>b</span><span class='hs-layout'>)</span>
<a name="line-59"></a> <span class='hs-varid'>fail</span> <span class='hs-keyword'>_</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>ListT</span> <span class='hs-varop'>$</span> <span class='hs-varid'>return</span> <span class='hs-conid'>[]</span>
<a name="line-60"></a>
<a name="line-61"></a><span class='hs-keyword'>instance</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'>MonadPlus</span> <span class='hs-layout'>(</span><span class='hs-conid'>ListT</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyword'>where</span>
<a name="line-62"></a> <span class='hs-varid'>mzero</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>ListT</span> <span class='hs-varop'>$</span> <span class='hs-varid'>return</span> <span class='hs-conid'>[]</span>
<a name="line-63"></a> <span class='hs-varid'>m</span> <span class='hs-varop'>`mplus`</span> <span class='hs-varid'>n</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>ListT</span> <span class='hs-varop'>$</span> <span class='hs-keyword'>do</span>
<a name="line-64"></a> <span class='hs-varid'>a</span> <span class='hs-keyglyph'><-</span> <span class='hs-varid'>runListT</span> <span class='hs-varid'>m</span>
<a name="line-65"></a> <span class='hs-varid'>b</span> <span class='hs-keyglyph'><-</span> <span class='hs-varid'>runListT</span> <span class='hs-varid'>n</span>
<a name="line-66"></a> <span class='hs-varid'>return</span> <span class='hs-layout'>(</span><span class='hs-varid'>a</span> <span class='hs-varop'>++</span> <span class='hs-varid'>b</span><span class='hs-layout'>)</span>
<a name="line-67"></a>
<a name="line-68"></a><span class='hs-keyword'>instance</span> <span class='hs-conid'>MonadTrans</span> <span class='hs-conid'>ListT</span> <span class='hs-keyword'>where</span>
<a name="line-69"></a> <span class='hs-varid'>lift</span> <span class='hs-varid'>m</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>ListT</span> <span class='hs-varop'>$</span> <span class='hs-keyword'>do</span>
<a name="line-70"></a> <span class='hs-varid'>a</span> <span class='hs-keyglyph'><-</span> <span class='hs-varid'>m</span>
<a name="line-71"></a> <span class='hs-varid'>return</span> <span class='hs-keyglyph'>[</span><span class='hs-varid'>a</span><span class='hs-keyglyph'>]</span>
<a name="line-72"></a>
<a name="line-73"></a><span class='hs-keyword'>instance</span> <span class='hs-layout'>(</span><span class='hs-conid'>MonadIO</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=></span> <span class='hs-conid'>MonadIO</span> <span class='hs-layout'>(</span><span class='hs-conid'>ListT</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyword'>where</span>
<a name="line-74"></a> <span class='hs-varid'>liftIO</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>lift</span> <span class='hs-varop'>.</span> <span class='hs-varid'>liftIO</span>
<a name="line-75"></a>
<a name="line-76"></a><a name="liftCallCC"></a><span class='hs-comment'>-- | Lift a @callCC@ operation to the new monad.</span>
<a name="line-77"></a><span class='hs-definition'>liftCallCC</span> <span class='hs-keyglyph'>::</span> <span class='hs-layout'>(</span><span class='hs-layout'>(</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'>m</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'>m</span> <span class='hs-keyglyph'>[</span><span class='hs-varid'>a</span><span class='hs-keyglyph'>]</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'>a</span><span class='hs-keyglyph'>]</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>-></span>
<a name="line-78"></a> <span class='hs-layout'>(</span><span class='hs-layout'>(</span><span class='hs-varid'>a</span> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>ListT</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-conid'>ListT</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-conid'>ListT</span> <span class='hs-varid'>m</span> <span class='hs-varid'>a</span>
<a name="line-79"></a><span class='hs-definition'>liftCallCC</span> <span class='hs-varid'>callCC</span> <span class='hs-varid'>f</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>ListT</span> <span class='hs-varop'>$</span>
<a name="line-80"></a> <span class='hs-varid'>callCC</span> <span class='hs-varop'>$</span> <span class='hs-keyglyph'>\</span><span class='hs-varid'>c</span> <span class='hs-keyglyph'>-></span>
<a name="line-81"></a> <span class='hs-varid'>runListT</span> <span class='hs-layout'>(</span><span class='hs-varid'>f</span> <span class='hs-layout'>(</span><span class='hs-keyglyph'>\</span><span class='hs-varid'>a</span> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>ListT</span> <span class='hs-varop'>$</span> <span class='hs-varid'>c</span> <span class='hs-keyglyph'>[</span><span class='hs-varid'>a</span><span class='hs-keyglyph'>]</span><span class='hs-layout'>)</span><span class='hs-layout'>)</span>
<a name="line-82"></a>
<a name="line-83"></a><a name="liftCatch"></a><span class='hs-comment'>-- | Lift a @catchError@ operation to the new monad.</span>
<a name="line-84"></a><span class='hs-definition'>liftCatch</span> <span class='hs-keyglyph'>::</span> <span class='hs-layout'>(</span><span class='hs-varid'>m</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'>e</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><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'>a</span><span class='hs-keyglyph'>]</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>-></span>
<a name="line-85"></a> <span class='hs-conid'>ListT</span> <span class='hs-varid'>m</span> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>-></span> <span class='hs-layout'>(</span><span class='hs-varid'>e</span> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>ListT</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-conid'>ListT</span> <span class='hs-varid'>m</span> <span class='hs-varid'>a</span>
<a name="line-86"></a><span class='hs-definition'>liftCatch</span> <span class='hs-varid'>catchError</span> <span class='hs-varid'>m</span> <span class='hs-varid'>h</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>ListT</span> <span class='hs-varop'>$</span> <span class='hs-varid'>runListT</span> <span class='hs-varid'>m</span>
<a name="line-87"></a> <span class='hs-varop'>`catchError`</span> <span class='hs-keyglyph'>\</span><span class='hs-varid'>e</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>runListT</span> <span class='hs-layout'>(</span><span class='hs-varid'>h</span> <span class='hs-varid'>e</span><span class='hs-layout'>)</span>
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