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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.Maybe</span>
<a name="line-4"></a><span class='hs-comment'>-- Copyright : (c) 2007 Yitzak Gale, Eric Kidd</span>
<a name="line-5"></a><span class='hs-comment'>-- License : BSD-style (see the file LICENSE)</span>
<a name="line-6"></a><span class='hs-comment'>--</span>
<a name="line-7"></a><span class='hs-comment'>-- Maintainer : ross@soi.city.ac.uk</span>
<a name="line-8"></a><span class='hs-comment'>-- Stability : experimental</span>
<a name="line-9"></a><span class='hs-comment'>-- Portability : portable</span>
<a name="line-10"></a><span class='hs-comment'>--</span>
<a name="line-11"></a><span class='hs-comment'>-- The 'MaybeT' monad transformer adds the ability to fail to a monad.</span>
<a name="line-12"></a><span class='hs-comment'>--</span>
<a name="line-13"></a><span class='hs-comment'>-- A sequence of actions succeeds, producing a value, only if all the</span>
<a name="line-14"></a><span class='hs-comment'>-- actions in the sequence are successful. If one fails, the rest of</span>
<a name="line-15"></a><span class='hs-comment'>-- the sequence is skipped and the composite action fails.</span>
<a name="line-16"></a><span class='hs-comment'>--</span>
<a name="line-17"></a><span class='hs-comment'>-- For a variant allowing a range of error values, see</span>
<a name="line-18"></a><span class='hs-comment'>-- "Control.Monad.Trans.Error".</span>
<a name="line-19"></a><span class='hs-comment'>-----------------------------------------------------------------------------</span>
<a name="line-20"></a>
<a name="line-21"></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'>Maybe</span> <span class='hs-layout'>(</span>
<a name="line-22"></a> <span class='hs-comment'>-- * The MaybeT monad transformer</span>
<a name="line-23"></a> <span class='hs-conid'>MaybeT</span><span class='hs-layout'>(</span><span class='hs-keyglyph'>..</span><span class='hs-layout'>)</span><span class='hs-layout'>,</span>
<a name="line-24"></a> <span class='hs-varid'>mapMaybeT</span><span class='hs-layout'>,</span>
<a name="line-25"></a> <span class='hs-comment'>-- * Lifting other operations</span>
<a name="line-26"></a> <span class='hs-varid'>liftCallCC</span><span class='hs-layout'>,</span>
<a name="line-27"></a> <span class='hs-varid'>liftCatch</span><span class='hs-layout'>,</span>
<a name="line-28"></a> <span class='hs-varid'>liftListen</span><span class='hs-layout'>,</span>
<a name="line-29"></a> <span class='hs-varid'>liftPass</span><span class='hs-layout'>,</span>
<a name="line-30"></a> <span class='hs-layout'>)</span> <span class='hs-keyword'>where</span>
<a name="line-31"></a>
<a name="line-32"></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-33"></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-34"></a>
<a name="line-35"></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-36"></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-layout'>(</span><span class='hs-conid'>MonadPlus</span><span class='hs-layout'>(</span><span class='hs-varid'>mzero</span><span class='hs-layout'>,</span> <span class='hs-varid'>mplus</span><span class='hs-layout'>)</span><span class='hs-layout'>,</span> <span class='hs-varid'>liftM</span><span class='hs-layout'>,</span> <span class='hs-varid'>ap</span><span class='hs-layout'>)</span>
<a name="line-37"></a>
<a name="line-38"></a><a name="MaybeT"></a><span class='hs-comment'>-- | The parameterizable maybe monad, obtained by composing an arbitrary</span>
<a name="line-39"></a><a name="MaybeT"></a><span class='hs-comment'>-- monad with the 'Maybe' monad.</span>
<a name="line-40"></a><a name="MaybeT"></a><span class='hs-comment'>--</span>
<a name="line-41"></a><a name="MaybeT"></a><span class='hs-comment'>-- Computations are actions that may produce a value or fail.</span>
<a name="line-42"></a><a name="MaybeT"></a><span class='hs-comment'>--</span>
<a name="line-43"></a><a name="MaybeT"></a><span class='hs-comment'>-- The 'return' function yields a successful computation, while @>>=@</span>
<a name="line-44"></a><a name="MaybeT"></a><span class='hs-comment'>-- sequences two subcomputations, failing on the first error.</span>
<a name="line-45"></a><a name="MaybeT"></a><span class='hs-keyword'>newtype</span> <span class='hs-conid'>MaybeT</span> <span class='hs-varid'>m</span> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>MaybeT</span> <span class='hs-layout'>{</span> <span class='hs-varid'>runMaybeT</span> <span class='hs-keyglyph'>::</span> <span class='hs-varid'>m</span> <span class='hs-layout'>(</span><span class='hs-conid'>Maybe</span> <span class='hs-varid'>a</span><span class='hs-layout'>)</span> <span class='hs-layout'>}</span>
<a name="line-46"></a>
<a name="line-47"></a><a name="mapMaybeT"></a><span class='hs-comment'>-- | Transform the computation inside a @MaybeT@.</span>
<a name="line-48"></a><span class='hs-definition'>mapMaybeT</span> <span class='hs-keyglyph'>::</span> <span class='hs-layout'>(</span><span class='hs-varid'>m</span> <span class='hs-layout'>(</span><span class='hs-conid'>Maybe</span> <span class='hs-varid'>a</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>n</span> <span class='hs-layout'>(</span><span class='hs-conid'>Maybe</span> <span class='hs-varid'>b</span><span class='hs-layout'>)</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>MaybeT</span> <span class='hs-varid'>m</span> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>MaybeT</span> <span class='hs-varid'>n</span> <span class='hs-varid'>b</span>
<a name="line-49"></a><span class='hs-definition'>mapMaybeT</span> <span class='hs-varid'>f</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>MaybeT</span> <span class='hs-varop'>.</span> <span class='hs-varid'>f</span> <span class='hs-varop'>.</span> <span class='hs-varid'>runMaybeT</span>
<a name="line-50"></a>
<a name="line-51"></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'>MaybeT</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyword'>where</span>
<a name="line-52"></a> <span class='hs-varid'>fmap</span> <span class='hs-varid'>f</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>mapMaybeT</span> <span class='hs-layout'>(</span><span class='hs-varid'>fmap</span> <span class='hs-layout'>(</span><span class='hs-varid'>fmap</span> <span class='hs-varid'>f</span><span class='hs-layout'>)</span><span class='hs-layout'>)</span>
<a name="line-53"></a>
<a name="line-54"></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-conid'>Monad</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'>MaybeT</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyword'>where</span>
<a name="line-55"></a> <span class='hs-varid'>pure</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>return</span>
<a name="line-56"></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'>ap</span>
<a name="line-57"></a>
<a name="line-58"></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-conid'>Monad</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'>MaybeT</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyword'>where</span>
<a name="line-59"></a> <span class='hs-varid'>empty</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>mzero</span>
<a name="line-60"></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'>mplus</span>
<a name="line-61"></a>
<a name="line-62"></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'>MaybeT</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyword'>where</span>
<a name="line-63"></a> <span class='hs-varid'>fail</span> <span class='hs-keyword'>_</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>MaybeT</span> <span class='hs-layout'>(</span><span class='hs-varid'>return</span> <span class='hs-conid'>Nothing</span><span class='hs-layout'>)</span>
<a name="line-64"></a> <span class='hs-varid'>return</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>lift</span> <span class='hs-varop'>.</span> <span class='hs-varid'>return</span>
<a name="line-65"></a> <span class='hs-varid'>x</span> <span class='hs-varop'>>>=</span> <span class='hs-varid'>f</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>MaybeT</span> <span class='hs-varop'>$</span> <span class='hs-keyword'>do</span>
<a name="line-66"></a> <span class='hs-varid'>v</span> <span class='hs-keyglyph'><-</span> <span class='hs-varid'>runMaybeT</span> <span class='hs-varid'>x</span>
<a name="line-67"></a> <span class='hs-keyword'>case</span> <span class='hs-varid'>v</span> <span class='hs-keyword'>of</span>
<a name="line-68"></a> <span class='hs-conid'>Nothing</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>return</span> <span class='hs-conid'>Nothing</span>
<a name="line-69"></a> <span class='hs-conid'>Just</span> <span class='hs-varid'>y</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>runMaybeT</span> <span class='hs-layout'>(</span><span class='hs-varid'>f</span> <span class='hs-varid'>y</span><span class='hs-layout'>)</span>
<a name="line-70"></a>
<a name="line-71"></a><span class='hs-keyword'>instance</span> <span class='hs-layout'>(</span><span class='hs-conid'>Monad</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=></span> <span class='hs-conid'>MonadPlus</span> <span class='hs-layout'>(</span><span class='hs-conid'>MaybeT</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyword'>where</span>
<a name="line-72"></a> <span class='hs-varid'>mzero</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>MaybeT</span> <span class='hs-layout'>(</span><span class='hs-varid'>return</span> <span class='hs-conid'>Nothing</span><span class='hs-layout'>)</span>
<a name="line-73"></a> <span class='hs-varid'>mplus</span> <span class='hs-varid'>x</span> <span class='hs-varid'>y</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>MaybeT</span> <span class='hs-varop'>$</span> <span class='hs-keyword'>do</span>
<a name="line-74"></a> <span class='hs-varid'>v</span> <span class='hs-keyglyph'><-</span> <span class='hs-varid'>runMaybeT</span> <span class='hs-varid'>x</span>
<a name="line-75"></a> <span class='hs-keyword'>case</span> <span class='hs-varid'>v</span> <span class='hs-keyword'>of</span>
<a name="line-76"></a> <span class='hs-conid'>Nothing</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>runMaybeT</span> <span class='hs-varid'>y</span>
<a name="line-77"></a> <span class='hs-conid'>Just</span> <span class='hs-keyword'>_</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>return</span> <span class='hs-varid'>v</span>
<a name="line-78"></a>
<a name="line-79"></a><span class='hs-keyword'>instance</span> <span class='hs-conid'>MonadTrans</span> <span class='hs-conid'>MaybeT</span> <span class='hs-keyword'>where</span>
<a name="line-80"></a> <span class='hs-varid'>lift</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>MaybeT</span> <span class='hs-varop'>.</span> <span class='hs-varid'>liftM</span> <span class='hs-conid'>Just</span>
<a name="line-81"></a>
<a name="line-82"></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'>MaybeT</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyword'>where</span>
<a name="line-83"></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-84"></a>
<a name="line-85"></a><a name="liftCallCC"></a><span class='hs-comment'>-- | Lift a @callCC@ operation to the new monad.</span>
<a name="line-86"></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-conid'>Maybe</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-conid'>Maybe</span> <span class='hs-varid'>b</span><span class='hs-layout'>)</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-conid'>Maybe</span> <span class='hs-varid'>a</span><span class='hs-layout'>)</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>-></span>
<a name="line-87"></a> <span class='hs-varid'>m</span> <span class='hs-layout'>(</span><span class='hs-conid'>Maybe</span> <span class='hs-varid'>a</span><span class='hs-layout'>)</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>-></span> <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'>MaybeT</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'>MaybeT</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'>MaybeT</span> <span class='hs-varid'>m</span> <span class='hs-varid'>a</span>
<a name="line-88"></a><span class='hs-definition'>liftCallCC</span> <span class='hs-varid'>callCC</span> <span class='hs-varid'>f</span> <span class='hs-keyglyph'>=</span>
<a name="line-89"></a> <span class='hs-conid'>MaybeT</span> <span class='hs-varop'>$</span> <span class='hs-varid'>callCC</span> <span class='hs-varop'>$</span> <span class='hs-keyglyph'>\</span> <span class='hs-varid'>c</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>runMaybeT</span> <span class='hs-layout'>(</span><span class='hs-varid'>f</span> <span class='hs-layout'>(</span><span class='hs-conid'>MaybeT</span> <span class='hs-varop'>.</span> <span class='hs-varid'>c</span> <span class='hs-varop'>.</span> <span class='hs-conid'>Just</span><span class='hs-layout'>)</span><span class='hs-layout'>)</span>
<a name="line-90"></a>
<a name="line-91"></a><a name="liftCatch"></a><span class='hs-comment'>-- | Lift a @catchError@ operation to the new monad.</span>
<a name="line-92"></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-layout'>(</span><span class='hs-conid'>Maybe</span> <span class='hs-varid'>a</span><span class='hs-layout'>)</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-layout'>(</span><span class='hs-conid'>Maybe</span> <span class='hs-varid'>a</span><span class='hs-layout'>)</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-conid'>Maybe</span> <span class='hs-varid'>a</span><span class='hs-layout'>)</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>-></span>
<a name="line-93"></a> <span class='hs-conid'>MaybeT</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'>MaybeT</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'>MaybeT</span> <span class='hs-varid'>m</span> <span class='hs-varid'>a</span>
<a name="line-94"></a><span class='hs-definition'>liftCatch</span> <span class='hs-varid'>f</span> <span class='hs-varid'>m</span> <span class='hs-varid'>h</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>MaybeT</span> <span class='hs-varop'>$</span> <span class='hs-varid'>f</span> <span class='hs-layout'>(</span><span class='hs-varid'>runMaybeT</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-layout'>(</span><span class='hs-varid'>runMaybeT</span> <span class='hs-varop'>.</span> <span class='hs-varid'>h</span><span class='hs-layout'>)</span>
<a name="line-95"></a>
<a name="line-96"></a><a name="liftListen"></a><span class='hs-comment'>-- | Lift a @listen@ operation to the new monad.</span>
<a name="line-97"></a><span class='hs-definition'>liftListen</span> <span class='hs-keyglyph'>::</span> <span class='hs-conid'>Monad</span> <span class='hs-varid'>m</span> <span class='hs-keyglyph'>=></span>
<a name="line-98"></a> <span class='hs-layout'>(</span><span class='hs-varid'>m</span> <span class='hs-layout'>(</span><span class='hs-conid'>Maybe</span> <span class='hs-varid'>a</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-layout'>(</span><span class='hs-conid'>Maybe</span> <span class='hs-varid'>a</span><span class='hs-layout'>,</span><span class='hs-varid'>w</span><span class='hs-layout'>)</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>MaybeT</span> <span class='hs-varid'>m</span> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>MaybeT</span> <span class='hs-varid'>m</span> <span class='hs-layout'>(</span><span class='hs-varid'>a</span><span class='hs-layout'>,</span><span class='hs-varid'>w</span><span class='hs-layout'>)</span>
<a name="line-99"></a><span class='hs-definition'>liftListen</span> <span class='hs-varid'>listen</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>mapMaybeT</span> <span class='hs-varop'>$</span> <span class='hs-keyglyph'>\</span> <span class='hs-varid'>m</span> <span class='hs-keyglyph'>-></span> <span class='hs-keyword'>do</span>
<a name="line-100"></a> <span class='hs-layout'>(</span><span class='hs-varid'>a</span><span class='hs-layout'>,</span> <span class='hs-varid'>w</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'><-</span> <span class='hs-varid'>listen</span> <span class='hs-varid'>m</span>
<a name="line-101"></a> <span class='hs-varid'>return</span> <span class='hs-varop'>$!</span> <span class='hs-varid'>fmap</span> <span class='hs-layout'>(</span><span class='hs-keyglyph'>\</span> <span class='hs-varid'>r</span> <span class='hs-keyglyph'>-></span> <span class='hs-layout'>(</span><span class='hs-varid'>r</span><span class='hs-layout'>,</span> <span class='hs-varid'>w</span><span class='hs-layout'>)</span><span class='hs-layout'>)</span> <span class='hs-varid'>a</span>
<a name="line-102"></a>
<a name="line-103"></a><a name="liftPass"></a><span class='hs-comment'>-- | Lift a @pass@ operation to the new monad.</span>
<a name="line-104"></a><span class='hs-definition'>liftPass</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'>m</span> <span class='hs-layout'>(</span><span class='hs-conid'>Maybe</span> <span class='hs-varid'>a</span><span class='hs-layout'>,</span><span class='hs-varid'>w</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>w</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-conid'>Maybe</span> <span class='hs-varid'>a</span><span class='hs-layout'>)</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>-></span>
<a name="line-105"></a> <span class='hs-conid'>MaybeT</span> <span class='hs-varid'>m</span> <span class='hs-layout'>(</span><span class='hs-varid'>a</span><span class='hs-layout'>,</span><span class='hs-varid'>w</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>w</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>MaybeT</span> <span class='hs-varid'>m</span> <span class='hs-varid'>a</span>
<a name="line-106"></a><span class='hs-definition'>liftPass</span> <span class='hs-varid'>pass</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>mapMaybeT</span> <span class='hs-varop'>$</span> <span class='hs-keyglyph'>\</span> <span class='hs-varid'>m</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>pass</span> <span class='hs-varop'>$</span> <span class='hs-keyword'>do</span>
<a name="line-107"></a> <span class='hs-varid'>a</span> <span class='hs-keyglyph'><-</span> <span class='hs-varid'>m</span>
<a name="line-108"></a> <span class='hs-varid'>return</span> <span class='hs-varop'>$!</span> <span class='hs-keyword'>case</span> <span class='hs-varid'>a</span> <span class='hs-keyword'>of</span>
<a name="line-109"></a> <span class='hs-conid'>Nothing</span> <span class='hs-keyglyph'>-></span> <span class='hs-layout'>(</span><span class='hs-conid'>Nothing</span><span class='hs-layout'>,</span> <span class='hs-varid'>id</span><span class='hs-layout'>)</span>
<a name="line-110"></a> <span class='hs-conid'>Just</span> <span class='hs-layout'>(</span><span class='hs-varid'>v</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-layout'>(</span><span class='hs-conid'>Just</span> <span class='hs-varid'>v</span><span class='hs-layout'>,</span> <span class='hs-varid'>f</span><span class='hs-layout'>)</span>
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