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</script></head><body><div id="package-header"><ul class="links" id="page-menu"><li><a href="src/Control-Monad-Writer-Lazy.html">Source</a></li><li><a href="index.html">Contents</a></li><li><a href="doc-index.html">Index</a></li></ul><p class="caption">mtl-2.0.1.0: Monad classes, using functional dependencies</p></div><div id="content"><div id="module-header"><table class="info"><tr><th>Portability</th><td>non-portable (multi-param classes, functional dependencies)</td></tr><tr><th>Stability</th><td>experimental</td></tr><tr><th>Maintainer</th><td>libraries@haskell.org</td></tr></table><p class="caption">Control.Monad.Writer.Lazy</p></div><div id="table-of-contents"><p class="caption">Contents</p><ul><li><a href="#g:1">MonadWriter class
</a></li><li><a href="#g:2">The Writer monad
</a></li><li><a href="#g:3">The WriterT monad transformer
</a></li></ul></div><div id="description"><p class="caption">Description</p><div class="doc"><p>Lazy writer monads.
</p><p>Inspired by the paper
<em>Functional Programming with Overloading and Higher-Order Polymorphism</em>,
Mark P Jones (<a href="http://web.cecs.pdx.edu/~mpj/pubs/springschool.html">http://web.cecs.pdx.edu/~mpj/pubs/springschool.html</a>)
Advanced School of Functional Programming, 1995.
</p></div></div><div id="synopsis"><p id="control.syn" class="caption expander" onclick="toggleSection('syn')">Synopsis</p><ul id="section.syn" class="hide" onclick="toggleSection('syn')"><li class="src short"><span class="keyword">class</span> (<a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Data-Monoid.html#t:Monoid">Monoid</a> w, <a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Control-Monad.html#t:Monad">Monad</a> m) => <a href="#t:MonadWriter">MonadWriter</a> w m | m -> w <span class="keyword">where</span><ul class="subs"><li><a href="#v:tell">tell</a> :: w -> m <a href="/usr/share/doc/ghc/html/libraries/ghc-prim-0.2.0.0/GHC-Unit.html#t:-40--41-">()</a></li><li><a href="#v:listen">listen</a> :: m a -> m (a, w)</li><li><a href="#v:pass">pass</a> :: m (a, w -> w) -> m a</li></ul></li><li class="src short"><a href="#v:listens">listens</a> :: <a href="Control-Monad-Writer-Lazy.html#t:MonadWriter">MonadWriter</a> w m => (w -> b) -> m a -> m (a, b)</li><li class="src short"><a href="#v:censor">censor</a> :: <a href="Control-Monad-Writer-Lazy.html#t:MonadWriter">MonadWriter</a> w m => (w -> w) -> m a -> m a</li><li class="src short"><span class="keyword">type</span> <a href="#t:Writer">Writer</a> w = <a href="Control-Monad-Writer-Lazy.html#t:WriterT">WriterT</a> w <a href="/usr/share/doc/ghc/html/libraries/transformers-0.2.2.0/Data-Functor-Identity.html#t:Identity">Identity</a></li><li class="src short"><a href="#v:writer">writer</a> :: (a, w) -> <a href="Control-Monad-Writer-Lazy.html#t:Writer">Writer</a> w a</li><li class="src short"><a href="#v:runWriter">runWriter</a> :: <a href="Control-Monad-Writer-Lazy.html#t:Writer">Writer</a> w a -> (a, w)</li><li class="src short"><a href="#v:execWriter">execWriter</a> :: <a href="Control-Monad-Writer-Lazy.html#t:Writer">Writer</a> w a -> w</li><li class="src short"><a href="#v:mapWriter">mapWriter</a> :: ((a, w) -> (b, w')) -> <a href="Control-Monad-Writer-Lazy.html#t:Writer">Writer</a> w a -> <a href="Control-Monad-Writer-Lazy.html#t:Writer">Writer</a> w' b</li><li class="src short"><span class="keyword">newtype</span> <a href="#t:WriterT">WriterT</a> w m a = <a href="#v:WriterT">WriterT</a> {<ul class="subs"><li><a href="#v:runWriterT">runWriterT</a> :: m (a, w)</li></ul>}</li><li class="src short"><a href="#v:execWriterT">execWriterT</a> :: <a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Control-Monad.html#t:Monad">Monad</a> m => <a href="Control-Monad-Writer-Lazy.html#t:WriterT">WriterT</a> w m a -> m w</li><li class="src short"><a href="#v:mapWriterT">mapWriterT</a> :: (m (a, w) -> n (b, w')) -> <a href="Control-Monad-Writer-Lazy.html#t:WriterT">WriterT</a> w m a -> <a href="Control-Monad-Writer-Lazy.html#t:WriterT">WriterT</a> w' n b</li><li class="src short">module <a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Control-Monad.html">Control.Monad</a></li><li class="src short">module <a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Control-Monad-Fix.html">Control.Monad.Fix</a></li><li class="src short">module <a href="Control-Monad-Trans.html">Control.Monad.Trans</a></li><li class="src short">module <a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Data-Monoid.html">Data.Monoid</a></li></ul></div><div id="interface"><h1 id="g:1">MonadWriter class
</h1><div class="top"><p class="src"><span class="keyword">class</span> (<a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Data-Monoid.html#t:Monoid">Monoid</a> w, <a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Control-Monad.html#t:Monad">Monad</a> m) => <a name="t:MonadWriter" class="def">MonadWriter</a> w m | m -> w <span class="keyword">where</span><a href="src/Control-Monad-Writer-Class.html#MonadWriter" class="link">Source</a></p><div class="subs methods"><p class="caption">Methods</p><p class="src"><a name="v:tell" class="def">tell</a> :: w -> m <a href="/usr/share/doc/ghc/html/libraries/ghc-prim-0.2.0.0/GHC-Unit.html#t:-40--41-">()</a><a href="src/Control-Monad-Writer-Class.html#tell" class="link">Source</a></p><div class="doc"><p><code><code><a href="Control-Monad-Writer-Lazy.html#v:tell">tell</a></code> w</code> is an action that produces the output <code>w</code>.
</p></div><p class="src"><a name="v:listen" class="def">listen</a> :: m a -> m (a, w)<a href="src/Control-Monad-Writer-Class.html#listen" class="link">Source</a></p><div class="doc"><p><code><code><a href="Control-Monad-Writer-Lazy.html#v:listen">listen</a></code> m</code> is an action that executes the action <code>m</code> and adds
its output to the value of the computation.
</p></div><p class="src"><a name="v:pass" class="def">pass</a> :: m (a, w -> w) -> m a<a href="src/Control-Monad-Writer-Class.html#pass" class="link">Source</a></p><div class="doc"><p><code><code><a href="Control-Monad-Writer-Lazy.html#v:pass">pass</a></code> m</code> is an action that executes the action <code>m</code>, which
returns a value and a function, and returns the value, applying
the function to the output.
</p></div></div><div class="subs instances"><p id="control.i:MonadWriter" class="caption collapser" onclick="toggleSection('i:MonadWriter')">Instances</p><div id="section.i:MonadWriter" class="show"><table><tr><td class="src"><a href="Control-Monad-Writer-Lazy.html#t:MonadWriter">MonadWriter</a> w m => <a href="Control-Monad-Writer-Lazy.html#t:MonadWriter">MonadWriter</a> w (<a href="/usr/share/doc/ghc/html/libraries/transformers-0.2.2.0/Control-Monad-Trans-Maybe.html#t:MaybeT">MaybeT</a> m)</td><td class="doc empty"> </td></tr><tr><td class="src"><a href="Control-Monad-Writer-Lazy.html#t:MonadWriter">MonadWriter</a> w m => <a href="Control-Monad-Writer-Lazy.html#t:MonadWriter">MonadWriter</a> w (<a href="/usr/share/doc/ghc/html/libraries/transformers-0.2.2.0/Control-Monad-Trans-Identity.html#t:IdentityT">IdentityT</a> m)</td><td class="doc empty"> </td></tr><tr><td class="src"><a href="Control-Monad-Writer-Lazy.html#t:MonadWriter">MonadWriter</a> w m => <a href="Control-Monad-Writer-Lazy.html#t:MonadWriter">MonadWriter</a> w (<a href="Control-Monad-State-Strict.html#t:StateT">StateT</a> s m)</td><td class="doc empty"> </td></tr><tr><td class="src"><a href="Control-Monad-Writer-Lazy.html#t:MonadWriter">MonadWriter</a> w m => <a href="Control-Monad-Writer-Lazy.html#t:MonadWriter">MonadWriter</a> w (<a href="Control-Monad-State-Lazy.html#t:StateT">StateT</a> s m)</td><td class="doc empty"> </td></tr><tr><td class="src"><a href="Control-Monad-Writer-Lazy.html#t:MonadWriter">MonadWriter</a> w m => <a href="Control-Monad-Writer-Lazy.html#t:MonadWriter">MonadWriter</a> w (<a href="Control-Monad-Reader.html#t:ReaderT">ReaderT</a> r m)</td><td class="doc empty"> </td></tr><tr><td class="src">(<a href="Control-Monad-Error-Class.html#t:Error">Error</a> e, <a href="Control-Monad-Writer-Lazy.html#t:MonadWriter">MonadWriter</a> w m) => <a href="Control-Monad-Writer-Lazy.html#t:MonadWriter">MonadWriter</a> w (<a href="Control-Monad-Error.html#t:ErrorT">ErrorT</a> e m)</td><td class="doc empty"> </td></tr><tr><td class="src">(<a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Data-Monoid.html#t:Monoid">Monoid</a> w, <a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Control-Monad.html#t:Monad">Monad</a> m) => <a href="Control-Monad-Writer-Lazy.html#t:MonadWriter">MonadWriter</a> w (<a href="Control-Monad-Writer-Strict.html#t:WriterT">WriterT</a> w m)</td><td class="doc empty"> </td></tr><tr><td class="src">(<a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Data-Monoid.html#t:Monoid">Monoid</a> w, <a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Control-Monad.html#t:Monad">Monad</a> m) => <a href="Control-Monad-Writer-Lazy.html#t:MonadWriter">MonadWriter</a> w (<a href="Control-Monad-Writer-Lazy.html#t:WriterT">WriterT</a> w m)</td><td class="doc empty"> </td></tr><tr><td class="src">(<a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Data-Monoid.html#t:Monoid">Monoid</a> w, <a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Control-Monad.html#t:Monad">Monad</a> m) => <a href="Control-Monad-Writer-Lazy.html#t:MonadWriter">MonadWriter</a> w (<a href="Control-Monad-RWS-Strict.html#t:RWST">RWST</a> r w s m)</td><td class="doc empty"> </td></tr><tr><td class="src">(<a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Data-Monoid.html#t:Monoid">Monoid</a> w, <a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Control-Monad.html#t:Monad">Monad</a> m) => <a href="Control-Monad-Writer-Lazy.html#t:MonadWriter">MonadWriter</a> w (<a href="Control-Monad-RWS-Lazy.html#t:RWST">RWST</a> r w s m)</td><td class="doc empty"> </td></tr></table></div></div></div><div class="top"><p class="src"><a name="v:listens" class="def">listens</a> :: <a href="Control-Monad-Writer-Lazy.html#t:MonadWriter">MonadWriter</a> w m => (w -> b) -> m a -> m (a, b)<a href="src/Control-Monad-Writer-Class.html#listens" class="link">Source</a></p><div class="doc"><p><code><code><a href="Control-Monad-Writer-Lazy.html#v:listens">listens</a></code> f m</code> is an action that executes the action <code>m</code> and adds
the result of applying <code>f</code> to the output to the value of the computation.
</p><ul><li><pre><code><a href="Control-Monad-Writer-Lazy.html#v:listens">listens</a></code> f m = <code><a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Control-Monad.html#v:liftM">liftM</a></code> (id *** f) (<code><a href="Control-Monad-Writer-Lazy.html#v:listen">listen</a></code> m)</pre></li></ul></div></div><div class="top"><p class="src"><a name="v:censor" class="def">censor</a> :: <a href="Control-Monad-Writer-Lazy.html#t:MonadWriter">MonadWriter</a> w m => (w -> w) -> m a -> m a<a href="src/Control-Monad-Writer-Class.html#censor" class="link">Source</a></p><div class="doc"><p><code><code><a href="Control-Monad-Writer-Lazy.html#v:censor">censor</a></code> f m</code> is an action that executes the action <code>m</code> and
applies the function <code>f</code> to its output, leaving the return value
unchanged.
</p><ul><li><pre><code><a href="Control-Monad-Writer-Lazy.html#v:censor">censor</a></code> f m = <code><a href="Control-Monad-Writer-Lazy.html#v:pass">pass</a></code> (<code><a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Control-Monad.html#v:liftM">liftM</a></code> (\x -> (x,f)) m)</pre></li></ul></div></div><h1 id="g:2">The Writer monad
</h1><div class="top"><p class="src"><span class="keyword">type</span> <a name="t:Writer" class="def">Writer</a> w = <a href="Control-Monad-Writer-Lazy.html#t:WriterT">WriterT</a> w <a href="/usr/share/doc/ghc/html/libraries/transformers-0.2.2.0/Data-Functor-Identity.html#t:Identity">Identity</a></p><div class="doc"><p>A writer monad parameterized by the type <code>w</code> of output to accumulate.
</p><p>The <code><a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Control-Monad.html#v:return">return</a></code> function produces the output <code><a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Data-Monoid.html#v:mempty">mempty</a></code>, while <code>>>=</code>
combines the outputs of the subcomputations using <code><a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Data-Monoid.html#v:mappend">mappend</a></code>.
</p></div></div><div class="top"><p class="src"><a name="v:writer" class="def">writer</a> :: (a, w) -> <a href="Control-Monad-Writer-Lazy.html#t:Writer">Writer</a> w a</p><div class="doc"><p>Construct a writer computation from a (result, output) pair.
(The inverse of <code><a href="Control-Monad-Writer-Lazy.html#v:runWriter">runWriter</a></code>.)
</p></div></div><div class="top"><p class="src"><a name="v:runWriter" class="def">runWriter</a> :: <a href="Control-Monad-Writer-Lazy.html#t:Writer">Writer</a> w a -> (a, w)</p><div class="doc"><p>Unwrap a writer computation as a (result, output) pair.
(The inverse of <code><a href="Control-Monad-Writer-Lazy.html#v:writer">writer</a></code>.)
</p></div></div><div class="top"><p class="src"><a name="v:execWriter" class="def">execWriter</a> :: <a href="Control-Monad-Writer-Lazy.html#t:Writer">Writer</a> w a -> w</p><div class="doc"><p>Extract the output from a writer computation.
</p><ul><li><pre><code><a href="Control-Monad-Writer-Lazy.html#v:execWriter">execWriter</a></code> m = <code><a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Data-Tuple.html#v:snd">snd</a></code> (<code><a href="Control-Monad-Writer-Lazy.html#v:runWriter">runWriter</a></code> m)</pre></li></ul></div></div><div class="top"><p class="src"><a name="v:mapWriter" class="def">mapWriter</a> :: ((a, w) -> (b, w')) -> <a href="Control-Monad-Writer-Lazy.html#t:Writer">Writer</a> w a -> <a href="Control-Monad-Writer-Lazy.html#t:Writer">Writer</a> w' b</p><div class="doc"><p>Map both the return value and output of a computation using
the given function.
</p><ul><li> <code><code><a href="Control-Monad-Writer-Lazy.html#v:runWriter">runWriter</a></code> (<code><a href="Control-Monad-Writer-Lazy.html#v:mapWriter">mapWriter</a></code> f m) = f (<code><a href="Control-Monad-Writer-Lazy.html#v:runWriter">runWriter</a></code> m</code>)
</li></ul></div></div><h1 id="g:3">The WriterT monad transformer
</h1><div class="top"><p class="src"><span class="keyword">newtype</span> <a name="t:WriterT" class="def">WriterT</a> w m a </p><div class="doc"><p>A writer monad parameterized by:
</p><ul><li> <code>w</code> - the output to accumulate.
</li><li> <code>m</code> - The inner monad.
</li></ul><p>The <code><a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Control-Monad.html#v:return">return</a></code> function produces the output <code><a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Data-Monoid.html#v:mempty">mempty</a></code>, while <code>>>=</code>
combines the outputs of the subcomputations using <code><a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Data-Monoid.html#v:mappend">mappend</a></code>.
</p></div><div class="subs constructors"><p class="caption">Constructors</p><table><tr><td class="src"><a name="v:WriterT" class="def">WriterT</a></td><td class="doc empty"> </td></tr><tr><td colspan="2"><div class="subs fields"><p class="caption">Fields</p><dl><dt class="src"><a name="v:runWriterT" class="def">runWriterT</a> :: m (a, w)</dt><dd class="doc empty"> </dd></dl><div class="clear"></div></div></td></tr></table></div><div class="subs instances"><p id="control.i:WriterT" class="caption collapser" onclick="toggleSection('i:WriterT')">Instances</p><div id="section.i:WriterT" class="show"><table><tr><td class="src">(<a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Data-Monoid.html#t:Monoid">Monoid</a> w, <a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Control-Monad.html#t:Monad">Monad</a> m) => <a href="Control-Monad-Writer-Lazy.html#t:MonadWriter">MonadWriter</a> w (<a href="Control-Monad-Writer-Lazy.html#t:WriterT">WriterT</a> w m)</td><td class="doc empty"> </td></tr><tr><td class="src">(<a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Data-Monoid.html#t:Monoid">Monoid</a> w, <a href="Control-Monad-State-Class.html#t:MonadState">MonadState</a> s m) => <a href="Control-Monad-State-Class.html#t:MonadState">MonadState</a> s (<a href="Control-Monad-Writer-Lazy.html#t:WriterT">WriterT</a> w m)</td><td class="doc empty"> </td></tr><tr><td class="src">(<a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Data-Monoid.html#t:Monoid">Monoid</a> w, <a href="Control-Monad-Reader-Class.html#t:MonadReader">MonadReader</a> r m) => <a href="Control-Monad-Reader-Class.html#t:MonadReader">MonadReader</a> r (<a href="Control-Monad-Writer-Lazy.html#t:WriterT">WriterT</a> w m)</td><td class="doc empty"> </td></tr><tr><td class="src">(<a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Data-Monoid.html#t:Monoid">Monoid</a> w, <a href="Control-Monad-Error-Class.html#t:MonadError">MonadError</a> e m) => <a href="Control-Monad-Error-Class.html#t:MonadError">MonadError</a> e (<a href="Control-Monad-Writer-Lazy.html#t:WriterT">WriterT</a> w m)</td><td class="doc empty"> </td></tr><tr><td class="src"><a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Data-Monoid.html#t:Monoid">Monoid</a> w => <a href="/usr/share/doc/ghc/html/libraries/transformers-0.2.2.0/Control-Monad-Trans-Class.html#t:MonadTrans">MonadTrans</a> (<a href="Control-Monad-Writer-Lazy.html#t:WriterT">WriterT</a> w)</td><td class="doc empty"> </td></tr><tr><td class="src">(<a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Data-Monoid.html#t:Monoid">Monoid</a> w, <a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Control-Monad.html#t:Monad">Monad</a> m) => <a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Control-Monad.html#t:Monad">Monad</a> (<a href="Control-Monad-Writer-Lazy.html#t:WriterT">WriterT</a> w m)</td><td class="doc empty"> </td></tr><tr><td class="src"><a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Control-Monad.html#t:Functor">Functor</a> m => <a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Control-Monad.html#t:Functor">Functor</a> (<a href="Control-Monad-Writer-Lazy.html#t:WriterT">WriterT</a> w m)</td><td class="doc empty"> </td></tr><tr><td class="src">(<a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Data-Monoid.html#t:Monoid">Monoid</a> w, <a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Control-Monad-Fix.html#t:MonadFix">MonadFix</a> m) => <a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Control-Monad-Fix.html#t:MonadFix">MonadFix</a> (<a href="Control-Monad-Writer-Lazy.html#t:WriterT">WriterT</a> w m)</td><td class="doc empty"> </td></tr><tr><td class="src">(<a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Data-Monoid.html#t:Monoid">Monoid</a> w, <a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Control-Monad.html#t:MonadPlus">MonadPlus</a> m) => <a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Control-Monad.html#t:MonadPlus">MonadPlus</a> (<a href="Control-Monad-Writer-Lazy.html#t:WriterT">WriterT</a> w m)</td><td class="doc empty"> </td></tr><tr><td class="src">(<a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Data-Monoid.html#t:Monoid">Monoid</a> w, <a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Control-Applicative.html#t:Applicative">Applicative</a> m) => <a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Control-Applicative.html#t:Applicative">Applicative</a> (<a href="Control-Monad-Writer-Lazy.html#t:WriterT">WriterT</a> w m)</td><td class="doc empty"> </td></tr><tr><td class="src">(<a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Data-Monoid.html#t:Monoid">Monoid</a> w, <a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Control-Applicative.html#t:Alternative">Alternative</a> m) => <a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Control-Applicative.html#t:Alternative">Alternative</a> (<a href="Control-Monad-Writer-Lazy.html#t:WriterT">WriterT</a> w m)</td><td class="doc empty"> </td></tr><tr><td class="src">(<a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Data-Monoid.html#t:Monoid">Monoid</a> w, <a href="/usr/share/doc/ghc/html/libraries/transformers-0.2.2.0/Control-Monad-IO-Class.html#t:MonadIO">MonadIO</a> m) => <a href="/usr/share/doc/ghc/html/libraries/transformers-0.2.2.0/Control-Monad-IO-Class.html#t:MonadIO">MonadIO</a> (<a href="Control-Monad-Writer-Lazy.html#t:WriterT">WriterT</a> w m)</td><td class="doc empty"> </td></tr><tr><td class="src">(<a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Data-Monoid.html#t:Monoid">Monoid</a> w, <a href="Control-Monad-Cont-Class.html#t:MonadCont">MonadCont</a> m) => <a href="Control-Monad-Cont-Class.html#t:MonadCont">MonadCont</a> (<a href="Control-Monad-Writer-Lazy.html#t:WriterT">WriterT</a> w m)</td><td class="doc empty"> </td></tr></table></div></div></div><div class="top"><p class="src"><a name="v:execWriterT" class="def">execWriterT</a> :: <a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Control-Monad.html#t:Monad">Monad</a> m => <a href="Control-Monad-Writer-Lazy.html#t:WriterT">WriterT</a> w m a -> m w</p><div class="doc"><p>Extract the output from a writer computation.
</p><ul><li><pre><code><a href="Control-Monad-Writer-Lazy.html#v:execWriterT">execWriterT</a></code> m = <code><a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Control-Monad.html#v:liftM">liftM</a></code> <code><a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Data-Tuple.html#v:snd">snd</a></code> (<code><a href="Control-Monad-Writer-Lazy.html#v:runWriterT">runWriterT</a></code> m)</pre></li></ul></div></div><div class="top"><p class="src"><a name="v:mapWriterT" class="def">mapWriterT</a> :: (m (a, w) -> n (b, w')) -> <a href="Control-Monad-Writer-Lazy.html#t:WriterT">WriterT</a> w m a -> <a href="Control-Monad-Writer-Lazy.html#t:WriterT">WriterT</a> w' n b</p><div class="doc"><p>Map both the return value and output of a computation using
the given function.
</p><ul><li> <code><code><a href="Control-Monad-Writer-Lazy.html#v:runWriterT">runWriterT</a></code> (<code><a href="Control-Monad-Writer-Lazy.html#v:mapWriterT">mapWriterT</a></code> f m) = f (<code><a href="Control-Monad-Writer-Lazy.html#v:runWriterT">runWriterT</a></code> m</code>)
</li></ul></div></div><div class="top"><p class="src">module <a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Control-Monad.html">Control.Monad</a></p></div><div class="top"><p class="src">module <a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Control-Monad-Fix.html">Control.Monad.Fix</a></p></div><div class="top"><p class="src">module <a href="Control-Monad-Trans.html">Control.Monad.Trans</a></p></div><div class="top"><p class="src">module <a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Data-Monoid.html">Data.Monoid</a></p></div></div></div><div id="footer"><p>Produced by <a href="http://www.haskell.org/haddock/">Haddock</a> version 2.9.2</p></div></body></html>
