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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.Writer.Lazy</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 lazy 'WriterT' monad transformer, which adds collection of</span>
<a name="line-13"></a><span class='hs-comment'>-- outputs (such as a count or string output) to a given monad.</span>
<a name="line-14"></a><span class='hs-comment'>--</span>
<a name="line-15"></a><span class='hs-comment'>-- This version builds its output lazily; for a strict version, see</span>
<a name="line-16"></a><span class='hs-comment'>-- "Control.Monad.Trans.Writer.Strict", which has the same interface.</span>
<a name="line-17"></a><span class='hs-comment'>--</span>
<a name="line-18"></a><span class='hs-comment'>-- This monad transformer provides only limited access to the output</span>
<a name="line-19"></a><span class='hs-comment'>-- during the computation. For more general access, use</span>
<a name="line-20"></a><span class='hs-comment'>-- "Control.Monad.Trans.State" instead.</span>
<a name="line-21"></a><span class='hs-comment'>-----------------------------------------------------------------------------</span>
<a name="line-22"></a>
<a name="line-23"></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'>Writer</span><span class='hs-varop'>.</span><span class='hs-conid'>Lazy</span> <span class='hs-layout'>(</span>
<a name="line-24"></a> <span class='hs-comment'>-- * The Writer monad</span>
<a name="line-25"></a> <span class='hs-conid'>Writer</span><span class='hs-layout'>,</span>
<a name="line-26"></a> <span class='hs-varid'>writer</span><span class='hs-layout'>,</span>
<a name="line-27"></a> <span class='hs-varid'>runWriter</span><span class='hs-layout'>,</span>
<a name="line-28"></a> <span class='hs-varid'>execWriter</span><span class='hs-layout'>,</span>
<a name="line-29"></a> <span class='hs-varid'>mapWriter</span><span class='hs-layout'>,</span>
<a name="line-30"></a> <span class='hs-comment'>-- * The WriterT monad transformer</span>
<a name="line-31"></a> <span class='hs-conid'>WriterT</span><span class='hs-layout'>(</span><span class='hs-keyglyph'>..</span><span class='hs-layout'>)</span><span class='hs-layout'>,</span>
<a name="line-32"></a> <span class='hs-varid'>execWriterT</span><span class='hs-layout'>,</span>
<a name="line-33"></a> <span class='hs-varid'>mapWriterT</span><span class='hs-layout'>,</span>
<a name="line-34"></a> <span class='hs-comment'>-- * Writer operations</span>
<a name="line-35"></a> <span class='hs-varid'>tell</span><span class='hs-layout'>,</span>
<a name="line-36"></a> <span class='hs-varid'>listen</span><span class='hs-layout'>,</span>
<a name="line-37"></a> <span class='hs-varid'>listens</span><span class='hs-layout'>,</span>
<a name="line-38"></a> <span class='hs-varid'>pass</span><span class='hs-layout'>,</span>
<a name="line-39"></a> <span class='hs-varid'>censor</span><span class='hs-layout'>,</span>
<a name="line-40"></a> <span class='hs-comment'>-- * Lifting other operations</span>
<a name="line-41"></a> <span class='hs-varid'>liftCallCC</span><span class='hs-layout'>,</span>
<a name="line-42"></a> <span class='hs-varid'>liftCatch</span><span class='hs-layout'>,</span>
<a name="line-43"></a> <span class='hs-layout'>)</span> <span class='hs-keyword'>where</span>
<a name="line-44"></a>
<a name="line-45"></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-46"></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-47"></a><span class='hs-keyword'>import</span> <span class='hs-conid'>Data</span><span class='hs-varop'>.</span><span class='hs-conid'>Functor</span><span class='hs-varop'>.</span><span class='hs-conid'>Identity</span>
<a name="line-48"></a>
<a name="line-49"></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-50"></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-51"></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'>Fix</span>
<a name="line-52"></a><span class='hs-keyword'>import</span> <span class='hs-conid'>Data</span><span class='hs-varop'>.</span><span class='hs-conid'>Monoid</span>
<a name="line-53"></a>
<a name="line-54"></a><a name="Writer"></a><span class='hs-comment'>-- ---------------------------------------------------------------------------</span>
<a name="line-55"></a><a name="Writer"></a><span class='hs-comment'>-- | A writer monad parameterized by the type @w@ of output to accumulate.</span>
<a name="line-56"></a><a name="Writer"></a><span class='hs-comment'>--</span>
<a name="line-57"></a><a name="Writer"></a><span class='hs-comment'>-- The 'return' function produces the output 'mempty', while @>>=@</span>
<a name="line-58"></a><a name="Writer"></a><span class='hs-comment'>-- combines the outputs of the subcomputations using 'mappend'.</span>
<a name="line-59"></a><a name="Writer"></a><span class='hs-keyword'>type</span> <span class='hs-conid'>Writer</span> <span class='hs-varid'>w</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>WriterT</span> <span class='hs-varid'>w</span> <span class='hs-conid'>Identity</span>
<a name="line-60"></a>
<a name="line-61"></a><a name="writer"></a><span class='hs-comment'>-- | Construct a writer computation from a (result, output) pair.</span>
<a name="line-62"></a><span class='hs-comment'>-- (The inverse of 'runWriter'.)</span>
<a name="line-63"></a><span class='hs-definition'>writer</span> <span class='hs-keyglyph'>::</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> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>Writer</span> <span class='hs-varid'>w</span> <span class='hs-varid'>a</span>
<a name="line-64"></a><span class='hs-definition'>writer</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>WriterT</span> <span class='hs-varop'>.</span> <span class='hs-conid'>Identity</span>
<a name="line-65"></a>
<a name="line-66"></a><a name="runWriter"></a><span class='hs-comment'>-- | Unwrap a writer computation as a (result, output) pair.</span>
<a name="line-67"></a><span class='hs-comment'>-- (The inverse of 'writer'.)</span>
<a name="line-68"></a><span class='hs-definition'>runWriter</span> <span class='hs-keyglyph'>::</span> <span class='hs-conid'>Writer</span> <span class='hs-varid'>w</span> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>-></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-69"></a><span class='hs-definition'>runWriter</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>runIdentity</span> <span class='hs-varop'>.</span> <span class='hs-varid'>runWriterT</span>
<a name="line-70"></a>
<a name="line-71"></a><a name="execWriter"></a><span class='hs-comment'>-- | Extract the output from a writer computation.</span>
<a name="line-72"></a><span class='hs-comment'>--</span>
<a name="line-73"></a><span class='hs-comment'>-- * @'execWriter' m = 'snd' ('runWriter' m)@</span>
<a name="line-74"></a><span class='hs-definition'>execWriter</span> <span class='hs-keyglyph'>::</span> <span class='hs-conid'>Writer</span> <span class='hs-varid'>w</span> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>w</span>
<a name="line-75"></a><span class='hs-definition'>execWriter</span> <span class='hs-varid'>m</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>snd</span> <span class='hs-layout'>(</span><span class='hs-varid'>runWriter</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span>
<a name="line-76"></a>
<a name="line-77"></a><a name="mapWriter"></a><span class='hs-comment'>-- | Map both the return value and output of a computation using</span>
<a name="line-78"></a><span class='hs-comment'>-- the given function.</span>
<a name="line-79"></a><span class='hs-comment'>--</span>
<a name="line-80"></a><span class='hs-comment'>-- * @'runWriter' ('mapWriter' f m) = f ('runWriter' m@)</span>
<a name="line-81"></a><span class='hs-definition'>mapWriter</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-layout'>,</span> <span class='hs-varid'>w</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-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'>Writer</span> <span class='hs-varid'>w</span> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>Writer</span> <span class='hs-varid'>w'</span> <span class='hs-varid'>b</span>
<a name="line-82"></a><span class='hs-definition'>mapWriter</span> <span class='hs-varid'>f</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>mapWriterT</span> <span class='hs-layout'>(</span><span class='hs-conid'>Identity</span> <span class='hs-varop'>.</span> <span class='hs-varid'>f</span> <span class='hs-varop'>.</span> <span class='hs-varid'>runIdentity</span><span class='hs-layout'>)</span>
<a name="line-83"></a>
<a name="line-84"></a><a name="WriterT"></a><span class='hs-comment'>-- ---------------------------------------------------------------------------</span>
<a name="line-85"></a><a name="WriterT"></a><span class='hs-comment'>-- | A writer monad parameterized by:</span>
<a name="line-86"></a><a name="WriterT"></a><span class='hs-comment'>--</span>
<a name="line-87"></a><a name="WriterT"></a><span class='hs-comment'>-- * @w@ - the output to accumulate.</span>
<a name="line-88"></a><a name="WriterT"></a><span class='hs-comment'>--</span>
<a name="line-89"></a><a name="WriterT"></a><span class='hs-comment'>-- * @m@ - The inner monad.</span>
<a name="line-90"></a><a name="WriterT"></a><span class='hs-comment'>--</span>
<a name="line-91"></a><a name="WriterT"></a><span class='hs-comment'>-- The 'return' function produces the output 'mempty', while @>>=@</span>
<a name="line-92"></a><a name="WriterT"></a><span class='hs-comment'>-- combines the outputs of the subcomputations using 'mappend'.</span>
<a name="line-93"></a><a name="WriterT"></a><span class='hs-keyword'>newtype</span> <span class='hs-conid'>WriterT</span> <span class='hs-varid'>w</span> <span class='hs-varid'>m</span> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>WriterT</span> <span class='hs-layout'>{</span> <span class='hs-varid'>runWriterT</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-layout'>,</span> <span class='hs-varid'>w</span><span class='hs-layout'>)</span> <span class='hs-layout'>}</span>
<a name="line-94"></a>
<a name="line-95"></a><a name="execWriterT"></a><span class='hs-comment'>-- | Extract the output from a writer computation.</span>
<a name="line-96"></a><span class='hs-comment'>--</span>
<a name="line-97"></a><span class='hs-comment'>-- * @'execWriterT' m = 'liftM' 'snd' ('runWriterT' m)@</span>
<a name="line-98"></a><span class='hs-definition'>execWriterT</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-conid'>WriterT</span> <span class='hs-varid'>w</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'>w</span>
<a name="line-99"></a><span class='hs-definition'>execWriterT</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-keyglyph'>~</span><span class='hs-layout'>(</span><span class='hs-keyword'>_</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'>runWriterT</span> <span class='hs-varid'>m</span>
<a name="line-101"></a> <span class='hs-varid'>return</span> <span class='hs-varid'>w</span>
<a name="line-102"></a>
<a name="line-103"></a><a name="mapWriterT"></a><span class='hs-comment'>-- | Map both the return value and output of a computation using</span>
<a name="line-104"></a><span class='hs-comment'>-- the given function.</span>
<a name="line-105"></a><span class='hs-comment'>--</span>
<a name="line-106"></a><span class='hs-comment'>-- * @'runWriterT' ('mapWriterT' f m) = f ('runWriterT' m@)</span>
<a name="line-107"></a><span class='hs-definition'>mapWriterT</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-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'>n</span> <span class='hs-layout'>(</span><span class='hs-varid'>b</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'>WriterT</span> <span class='hs-varid'>w</span> <span class='hs-varid'>m</span> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>WriterT</span> <span class='hs-varid'>w'</span> <span class='hs-varid'>n</span> <span class='hs-varid'>b</span>
<a name="line-108"></a><span class='hs-definition'>mapWriterT</span> <span class='hs-varid'>f</span> <span class='hs-varid'>m</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>WriterT</span> <span class='hs-varop'>$</span> <span class='hs-varid'>f</span> <span class='hs-layout'>(</span><span class='hs-varid'>runWriterT</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span>
<a name="line-109"></a>
<a name="line-110"></a><span class='hs-keyword'>instance</span> <span class='hs-layout'>(</span><span class='hs-conid'>Functor</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=></span> <span class='hs-conid'>Functor</span> <span class='hs-layout'>(</span><span class='hs-conid'>WriterT</span> <span class='hs-varid'>w</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyword'>where</span>
<a name="line-111"></a> <span class='hs-varid'>fmap</span> <span class='hs-varid'>f</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>mapWriterT</span> <span class='hs-varop'>$</span> <span class='hs-varid'>fmap</span> <span class='hs-varop'>$</span> <span class='hs-keyglyph'>\</span> <span class='hs-keyglyph'>~</span><span class='hs-layout'>(</span><span class='hs-varid'>a</span><span class='hs-layout'>,</span> <span class='hs-varid'>w</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>-></span> <span class='hs-layout'>(</span><span class='hs-varid'>f</span> <span class='hs-varid'>a</span><span class='hs-layout'>,</span> <span class='hs-varid'>w</span><span class='hs-layout'>)</span>
<a name="line-112"></a>
<a name="line-113"></a><span class='hs-keyword'>instance</span> <span class='hs-layout'>(</span><span class='hs-conid'>Monoid</span> <span class='hs-varid'>w</span><span class='hs-layout'>,</span> <span class='hs-conid'>Applicative</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=></span> <span class='hs-conid'>Applicative</span> <span class='hs-layout'>(</span><span class='hs-conid'>WriterT</span> <span class='hs-varid'>w</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyword'>where</span>
<a name="line-114"></a> <span class='hs-varid'>pure</span> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>WriterT</span> <span class='hs-varop'>$</span> <span class='hs-varid'>pure</span> <span class='hs-layout'>(</span><span class='hs-varid'>a</span><span class='hs-layout'>,</span> <span class='hs-varid'>mempty</span><span class='hs-layout'>)</span>
<a name="line-115"></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'>WriterT</span> <span class='hs-varop'>$</span> <span class='hs-varid'>liftA2</span> <span class='hs-varid'>k</span> <span class='hs-layout'>(</span><span class='hs-varid'>runWriterT</span> <span class='hs-varid'>f</span><span class='hs-layout'>)</span> <span class='hs-layout'>(</span><span class='hs-varid'>runWriterT</span> <span class='hs-varid'>v</span><span class='hs-layout'>)</span>
<a name="line-116"></a> <span class='hs-keyword'>where</span> <span class='hs-varid'>k</span> <span class='hs-keyglyph'>~</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> <span class='hs-keyglyph'>~</span><span class='hs-layout'>(</span><span class='hs-varid'>b</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-layout'>(</span><span class='hs-varid'>a</span> <span class='hs-varid'>b</span><span class='hs-layout'>,</span> <span class='hs-varid'>w</span> <span class='hs-varop'>`mappend`</span> <span class='hs-varid'>w'</span><span class='hs-layout'>)</span>
<a name="line-117"></a>
<a name="line-118"></a><span class='hs-keyword'>instance</span> <span class='hs-layout'>(</span><span class='hs-conid'>Monoid</span> <span class='hs-varid'>w</span><span class='hs-layout'>,</span> <span class='hs-conid'>Alternative</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'>WriterT</span> <span class='hs-varid'>w</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyword'>where</span>
<a name="line-119"></a> <span class='hs-varid'>empty</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>WriterT</span> <span class='hs-varid'>empty</span>
<a name="line-120"></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'>WriterT</span> <span class='hs-varop'>$</span> <span class='hs-varid'>runWriterT</span> <span class='hs-varid'>m</span> <span class='hs-varop'><|></span> <span class='hs-varid'>runWriterT</span> <span class='hs-varid'>n</span>
<a name="line-121"></a>
<a name="line-122"></a><span class='hs-keyword'>instance</span> <span class='hs-layout'>(</span><span class='hs-conid'>Monoid</span> <span class='hs-varid'>w</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'>WriterT</span> <span class='hs-varid'>w</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyword'>where</span>
<a name="line-123"></a> <span class='hs-varid'>return</span> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>WriterT</span> <span class='hs-varop'>$</span> <span class='hs-varid'>return</span> <span class='hs-layout'>(</span><span class='hs-varid'>a</span><span class='hs-layout'>,</span> <span class='hs-varid'>mempty</span><span class='hs-layout'>)</span>
<a name="line-124"></a> <span class='hs-varid'>m</span> <span class='hs-varop'>>>=</span> <span class='hs-varid'>k</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>WriterT</span> <span class='hs-varop'>$</span> <span class='hs-keyword'>do</span>
<a name="line-125"></a> <span class='hs-keyglyph'>~</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> <span class='hs-keyglyph'><-</span> <span class='hs-varid'>runWriterT</span> <span class='hs-varid'>m</span>
<a name="line-126"></a> <span class='hs-keyglyph'>~</span><span class='hs-layout'>(</span><span class='hs-varid'>b</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'>runWriterT</span> <span class='hs-layout'>(</span><span class='hs-varid'>k</span> <span class='hs-varid'>a</span><span class='hs-layout'>)</span>
<a name="line-127"></a> <span class='hs-varid'>return</span> <span class='hs-layout'>(</span><span class='hs-varid'>b</span><span class='hs-layout'>,</span> <span class='hs-varid'>w</span> <span class='hs-varop'>`mappend`</span> <span class='hs-varid'>w'</span><span class='hs-layout'>)</span>
<a name="line-128"></a> <span class='hs-varid'>fail</span> <span class='hs-varid'>msg</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>WriterT</span> <span class='hs-varop'>$</span> <span class='hs-varid'>fail</span> <span class='hs-varid'>msg</span>
<a name="line-129"></a>
<a name="line-130"></a><span class='hs-keyword'>instance</span> <span class='hs-layout'>(</span><span class='hs-conid'>Monoid</span> <span class='hs-varid'>w</span><span class='hs-layout'>,</span> <span class='hs-conid'>MonadPlus</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=></span> <span class='hs-conid'>MonadPlus</span> <span class='hs-layout'>(</span><span class='hs-conid'>WriterT</span> <span class='hs-varid'>w</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyword'>where</span>
<a name="line-131"></a> <span class='hs-varid'>mzero</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>WriterT</span> <span class='hs-varid'>mzero</span>
<a name="line-132"></a> <span class='hs-varid'>m</span> <span class='hs-varop'>`mplus`</span> <span class='hs-varid'>n</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>WriterT</span> <span class='hs-varop'>$</span> <span class='hs-varid'>runWriterT</span> <span class='hs-varid'>m</span> <span class='hs-varop'>`mplus`</span> <span class='hs-varid'>runWriterT</span> <span class='hs-varid'>n</span>
<a name="line-133"></a>
<a name="line-134"></a><span class='hs-keyword'>instance</span> <span class='hs-layout'>(</span><span class='hs-conid'>Monoid</span> <span class='hs-varid'>w</span><span class='hs-layout'>,</span> <span class='hs-conid'>MonadFix</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=></span> <span class='hs-conid'>MonadFix</span> <span class='hs-layout'>(</span><span class='hs-conid'>WriterT</span> <span class='hs-varid'>w</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyword'>where</span>
<a name="line-135"></a> <span class='hs-varid'>mfix</span> <span class='hs-varid'>m</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>WriterT</span> <span class='hs-varop'>$</span> <span class='hs-varid'>mfix</span> <span class='hs-varop'>$</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-layout'>,</span> <span class='hs-keyword'>_</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>runWriterT</span> <span class='hs-layout'>(</span><span class='hs-varid'>m</span> <span class='hs-varid'>a</span><span class='hs-layout'>)</span>
<a name="line-136"></a>
<a name="line-137"></a><span class='hs-keyword'>instance</span> <span class='hs-layout'>(</span><span class='hs-conid'>Monoid</span> <span class='hs-varid'>w</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=></span> <span class='hs-conid'>MonadTrans</span> <span class='hs-layout'>(</span><span class='hs-conid'>WriterT</span> <span class='hs-varid'>w</span><span class='hs-layout'>)</span> <span class='hs-keyword'>where</span>
<a name="line-138"></a> <span class='hs-varid'>lift</span> <span class='hs-varid'>m</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>WriterT</span> <span class='hs-varop'>$</span> <span class='hs-keyword'>do</span>
<a name="line-139"></a> <span class='hs-varid'>a</span> <span class='hs-keyglyph'><-</span> <span class='hs-varid'>m</span>
<a name="line-140"></a> <span class='hs-varid'>return</span> <span class='hs-layout'>(</span><span class='hs-varid'>a</span><span class='hs-layout'>,</span> <span class='hs-varid'>mempty</span><span class='hs-layout'>)</span>
<a name="line-141"></a>
<a name="line-142"></a><span class='hs-keyword'>instance</span> <span class='hs-layout'>(</span><span class='hs-conid'>Monoid</span> <span class='hs-varid'>w</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'>WriterT</span> <span class='hs-varid'>w</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyword'>where</span>
<a name="line-143"></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-144"></a>
<a name="line-145"></a><a name="tell"></a><span class='hs-comment'>-- | @'tell' w@ is an action that produces the output @w@.</span>
<a name="line-146"></a><span class='hs-definition'>tell</span> <span class='hs-keyglyph'>::</span> <span class='hs-layout'>(</span><span class='hs-conid'>Monoid</span> <span class='hs-varid'>w</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'>w</span> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>WriterT</span> <span class='hs-varid'>w</span> <span class='hs-varid'>m</span> <span class='hs-conid'>()</span>
<a name="line-147"></a><span class='hs-definition'>tell</span> <span class='hs-varid'>w</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>WriterT</span> <span class='hs-varop'>$</span> <span class='hs-varid'>return</span> <span class='hs-layout'>(</span><span class='hs-conid'>()</span><span class='hs-layout'>,</span> <span class='hs-varid'>w</span><span class='hs-layout'>)</span>
<a name="line-148"></a>
<a name="line-149"></a><a name="listen"></a><span class='hs-comment'>-- | @'listen' m@ is an action that executes the action @m@ and adds its</span>
<a name="line-150"></a><span class='hs-comment'>-- output to the value of the computation.</span>
<a name="line-151"></a><span class='hs-comment'>--</span>
<a name="line-152"></a><span class='hs-comment'>-- * @'runWriterT' ('listen' m) = 'liftM' (\\(a, w) -> ((a, w), w)) ('runWriterT' m)@</span>
<a name="line-153"></a><span class='hs-definition'>listen</span> <span class='hs-keyglyph'>::</span> <span class='hs-layout'>(</span><span class='hs-conid'>Monoid</span> <span class='hs-varid'>w</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'>WriterT</span> <span class='hs-varid'>w</span> <span class='hs-varid'>m</span> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>WriterT</span> <span class='hs-varid'>w</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-154"></a><span class='hs-definition'>listen</span> <span class='hs-varid'>m</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>WriterT</span> <span class='hs-varop'>$</span> <span class='hs-keyword'>do</span>
<a name="line-155"></a> <span class='hs-keyglyph'>~</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> <span class='hs-keyglyph'><-</span> <span class='hs-varid'>runWriterT</span> <span class='hs-varid'>m</span>
<a name="line-156"></a> <span class='hs-varid'>return</span> <span class='hs-layout'>(</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><span class='hs-layout'>,</span> <span class='hs-varid'>w</span><span class='hs-layout'>)</span>
<a name="line-157"></a>
<a name="line-158"></a><a name="listens"></a><span class='hs-comment'>-- | @'listens' f m@ is an action that executes the action @m@ and adds</span>
<a name="line-159"></a><span class='hs-comment'>-- the result of applying @f@ to the output to the value of the computation.</span>
<a name="line-160"></a><span class='hs-comment'>--</span>
<a name="line-161"></a><span class='hs-comment'>-- * @'listens' f m = 'liftM' (id *** f) ('listen' m)@</span>
<a name="line-162"></a><span class='hs-comment'>--</span>
<a name="line-163"></a><span class='hs-comment'>-- * @'runWriterT' ('listens' f m) = 'liftM' (\\(a, w) -> ((a, f w), w)) ('runWriterT' m)@</span>
<a name="line-164"></a><span class='hs-definition'>listens</span> <span class='hs-keyglyph'>::</span> <span class='hs-layout'>(</span><span class='hs-conid'>Monoid</span> <span class='hs-varid'>w</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'>w</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-conid'>WriterT</span> <span class='hs-varid'>w</span> <span class='hs-varid'>m</span> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>WriterT</span> <span class='hs-varid'>w</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'>b</span><span class='hs-layout'>)</span>
<a name="line-165"></a><span class='hs-definition'>listens</span> <span class='hs-varid'>f</span> <span class='hs-varid'>m</span> <span class='hs-keyglyph'>=</span> <span class='hs-keyword'>do</span>
<a name="line-166"></a> <span class='hs-keyglyph'>~</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> <span class='hs-keyglyph'><-</span> <span class='hs-varid'>listen</span> <span class='hs-varid'>m</span>
<a name="line-167"></a> <span class='hs-varid'>return</span> <span class='hs-layout'>(</span><span class='hs-varid'>a</span><span class='hs-layout'>,</span> <span class='hs-varid'>f</span> <span class='hs-varid'>w</span><span class='hs-layout'>)</span>
<a name="line-168"></a>
<a name="line-169"></a><a name="pass"></a><span class='hs-comment'>-- | @'pass' m@ is an action that executes the action @m@, which returns</span>
<a name="line-170"></a><span class='hs-comment'>-- a value and a function, and returns the value, applying the function</span>
<a name="line-171"></a><span class='hs-comment'>-- to the output.</span>
<a name="line-172"></a><span class='hs-comment'>--</span>
<a name="line-173"></a><span class='hs-comment'>-- * @'runWriterT' ('pass' m) = 'liftM' (\\((a, f), w) -> (a, f w)) ('runWriterT' m)@</span>
<a name="line-174"></a><span class='hs-definition'>pass</span> <span class='hs-keyglyph'>::</span> <span class='hs-layout'>(</span><span class='hs-conid'>Monoid</span> <span class='hs-varid'>w</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'>WriterT</span> <span class='hs-varid'>w</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'>WriterT</span> <span class='hs-varid'>w</span> <span class='hs-varid'>m</span> <span class='hs-varid'>a</span>
<a name="line-175"></a><span class='hs-definition'>pass</span> <span class='hs-varid'>m</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>WriterT</span> <span class='hs-varop'>$</span> <span class='hs-keyword'>do</span>
<a name="line-176"></a> <span class='hs-keyglyph'>~</span><span class='hs-layout'>(</span><span class='hs-layout'>(</span><span class='hs-varid'>a</span><span class='hs-layout'>,</span> <span class='hs-varid'>f</span><span class='hs-layout'>)</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'>runWriterT</span> <span class='hs-varid'>m</span>
<a name="line-177"></a> <span class='hs-varid'>return</span> <span class='hs-layout'>(</span><span class='hs-varid'>a</span><span class='hs-layout'>,</span> <span class='hs-varid'>f</span> <span class='hs-varid'>w</span><span class='hs-layout'>)</span>
<a name="line-178"></a>
<a name="line-179"></a><a name="censor"></a><span class='hs-comment'>-- | @'censor' f m@ is an action that executes the action @m@ and</span>
<a name="line-180"></a><span class='hs-comment'>-- applies the function @f@ to its output, leaving the return value</span>
<a name="line-181"></a><span class='hs-comment'>-- unchanged.</span>
<a name="line-182"></a><span class='hs-comment'>--</span>
<a name="line-183"></a><span class='hs-comment'>-- * @'censor' f m = 'pass' ('liftM' (\\x -> (x,f)) m)@</span>
<a name="line-184"></a><span class='hs-comment'>--</span>
<a name="line-185"></a><span class='hs-comment'>-- * @'runWriterT' ('censor' f m) = 'liftM' (\\(a, w) -> (a, f w)) ('runWriterT' m)@</span>
<a name="line-186"></a><span class='hs-definition'>censor</span> <span class='hs-keyglyph'>::</span> <span class='hs-layout'>(</span><span class='hs-conid'>Monoid</span> <span class='hs-varid'>w</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'>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'>WriterT</span> <span class='hs-varid'>w</span> <span class='hs-varid'>m</span> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>WriterT</span> <span class='hs-varid'>w</span> <span class='hs-varid'>m</span> <span class='hs-varid'>a</span>
<a name="line-187"></a><span class='hs-definition'>censor</span> <span class='hs-varid'>f</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-188"></a> <span class='hs-varid'>a</span> <span class='hs-keyglyph'><-</span> <span class='hs-varid'>m</span>
<a name="line-189"></a> <span class='hs-varid'>return</span> <span class='hs-layout'>(</span><span class='hs-varid'>a</span><span class='hs-layout'>,</span> <span class='hs-varid'>f</span><span class='hs-layout'>)</span>
<a name="line-190"></a>
<a name="line-191"></a><a name="liftCallCC"></a><span class='hs-comment'>-- | Lift a @callCC@ operation to the new monad.</span>
<a name="line-192"></a><span class='hs-definition'>liftCallCC</span> <span class='hs-keyglyph'>::</span> <span class='hs-layout'>(</span><span class='hs-conid'>Monoid</span> <span class='hs-varid'>w</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=></span> <span class='hs-layout'>(</span><span class='hs-layout'>(</span><span class='hs-layout'>(</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> <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'>w</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-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-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><span class='hs-layout'>)</span> <span class='hs-keyglyph'>-></span>
<a name="line-193"></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'>WriterT</span> <span class='hs-varid'>w</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'>WriterT</span> <span class='hs-varid'>w</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'>WriterT</span> <span class='hs-varid'>w</span> <span class='hs-varid'>m</span> <span class='hs-varid'>a</span>
<a name="line-194"></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'>WriterT</span> <span class='hs-varop'>$</span>
<a name="line-195"></a> <span class='hs-varid'>callCC</span> <span class='hs-varop'>$</span> <span class='hs-keyglyph'>\</span><span class='hs-varid'>c</span> <span class='hs-keyglyph'>-></span>
<a name="line-196"></a> <span class='hs-varid'>runWriterT</span> <span class='hs-layout'>(</span><span class='hs-varid'>f</span> <span class='hs-layout'>(</span><span class='hs-keyglyph'>\</span><span class='hs-varid'>a</span> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>WriterT</span> <span class='hs-varop'>$</span> <span class='hs-varid'>c</span> <span class='hs-layout'>(</span><span class='hs-varid'>a</span><span class='hs-layout'>,</span> <span class='hs-varid'>mempty</span><span class='hs-layout'>)</span><span class='hs-layout'>)</span><span class='hs-layout'>)</span>
<a name="line-197"></a>
<a name="line-198"></a><a name="liftCatch"></a><span class='hs-comment'>-- | Lift a @catchError@ operation to the new monad.</span>
<a name="line-199"></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-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-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-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-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><span class='hs-layout'>)</span> <span class='hs-keyglyph'>-></span>
<a name="line-200"></a> <span class='hs-conid'>WriterT</span> <span class='hs-varid'>w</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'>WriterT</span> <span class='hs-varid'>w</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'>WriterT</span> <span class='hs-varid'>w</span> <span class='hs-varid'>m</span> <span class='hs-varid'>a</span>
<a name="line-201"></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>
<a name="line-202"></a> <span class='hs-conid'>WriterT</span> <span class='hs-varop'>$</span> <span class='hs-varid'>runWriterT</span> <span class='hs-varid'>m</span> <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'>runWriterT</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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