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</script></head><body><div id="package-header"><ul class="links" id="page-menu"><li><a href="src/Control-Monad-Trans-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">transformers-0.2.2.0: Concrete functor and monad transformers</p></div><div id="content"><div id="module-header"><table class="info"><tr><th>Portability</th><td>portable</td></tr><tr><th>Stability</th><td>experimental</td></tr><tr><th>Maintainer</th><td>ross@soi.city.ac.uk</td></tr></table><p class="caption">Control.Monad.Trans.Writer.Lazy</p></div><div id="table-of-contents"><p class="caption">Contents</p><ul><li><a href="#g:1">The Writer monad
</a></li><li><a href="#g:2">The WriterT monad transformer
</a></li><li><a href="#g:3">Writer operations
</a></li><li><a href="#g:4">Lifting other operations
</a></li></ul></div><div id="description"><p class="caption">Description</p><div class="doc"><p>The lazy <code><a href="Control-Monad-Trans-Writer-Lazy.html#t:WriterT">WriterT</a></code> monad transformer, which adds collection of
outputs (such as a count or string output) to a given monad.
</p><p>This version builds its output lazily; for a strict version, see
<a href="Control-Monad-Trans-Writer-Strict.html">Control.Monad.Trans.Writer.Strict</a>, which has the same interface.
</p><p>This monad transformer provides only limited access to the output
during the computation. For more general access, use
<a href="Control-Monad-Trans-State.html">Control.Monad.Trans.State</a> instead.
</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">type</span> <a href="#t:Writer">Writer</a> w = <a href="Control-Monad-Trans-Writer-Lazy.html#t:WriterT">WriterT</a> w <a href="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-Trans-Writer-Lazy.html#t:Writer">Writer</a> w a</li><li class="src short"><a href="#v:runWriter">runWriter</a> :: <a href="Control-Monad-Trans-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-Trans-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-Trans-Writer-Lazy.html#t:Writer">Writer</a> w a -> <a href="Control-Monad-Trans-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-Trans-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-Trans-Writer-Lazy.html#t:WriterT">WriterT</a> w m a -> <a href="Control-Monad-Trans-Writer-Lazy.html#t:WriterT">WriterT</a> w' n b</li><li class="src short"><a href="#v:tell">tell</a> :: (<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) => w -> <a href="Control-Monad-Trans-Writer-Lazy.html#t:WriterT">WriterT</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 class="src short"><a href="#v:listen">listen</a> :: (<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-Trans-Writer-Lazy.html#t:WriterT">WriterT</a> w m a -> <a href="Control-Monad-Trans-Writer-Lazy.html#t:WriterT">WriterT</a> w m (a, w)</li><li class="src short"><a href="#v:listens">listens</a> :: (<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) => (w -> b) -> <a href="Control-Monad-Trans-Writer-Lazy.html#t:WriterT">WriterT</a> w m a -> <a href="Control-Monad-Trans-Writer-Lazy.html#t:WriterT">WriterT</a> w m (a, b)</li><li class="src short"><a href="#v:pass">pass</a> :: (<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-Trans-Writer-Lazy.html#t:WriterT">WriterT</a> w m (a, w -> w) -> <a href="Control-Monad-Trans-Writer-Lazy.html#t:WriterT">WriterT</a> w m a</li><li class="src short"><a href="#v:censor">censor</a> :: (<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) => (w -> w) -> <a href="Control-Monad-Trans-Writer-Lazy.html#t:WriterT">WriterT</a> w m a -> <a href="Control-Monad-Trans-Writer-Lazy.html#t:WriterT">WriterT</a> w m a</li><li class="src short"><a href="#v:liftCallCC">liftCallCC</a> :: <a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Data-Monoid.html#t:Monoid">Monoid</a> w => ((((a, w) -> m (b, w)) -> m (a, w)) -> m (a, w)) -> ((a -> <a href="Control-Monad-Trans-Writer-Lazy.html#t:WriterT">WriterT</a> w m b) -> <a href="Control-Monad-Trans-Writer-Lazy.html#t:WriterT">WriterT</a> w m a) -> <a href="Control-Monad-Trans-Writer-Lazy.html#t:WriterT">WriterT</a> w m a</li><li class="src short"><a href="#v:liftCatch">liftCatch</a> :: (m (a, w) -> (e -> m (a, w)) -> m (a, w)) -> <a href="Control-Monad-Trans-Writer-Lazy.html#t:WriterT">WriterT</a> w m a -> (e -> <a href="Control-Monad-Trans-Writer-Lazy.html#t:WriterT">WriterT</a> w m a) -> <a href="Control-Monad-Trans-Writer-Lazy.html#t:WriterT">WriterT</a> w m a</li></ul></div><div id="interface"><h1 id="g:1">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-Trans-Writer-Lazy.html#t:WriterT">WriterT</a> w <a href="Data-Functor-Identity.html#t:Identity">Identity</a><a href="src/Control-Monad-Trans-Writer-Lazy.html#Writer" class="link">Source</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-Trans-Writer-Lazy.html#t:Writer">Writer</a> w a<a href="src/Control-Monad-Trans-Writer-Lazy.html#writer" class="link">Source</a></p><div class="doc"><p>Construct a writer computation from a (result, output) pair.
(The inverse of <code><a href="Control-Monad-Trans-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-Trans-Writer-Lazy.html#t:Writer">Writer</a> w a -> (a, w)<a href="src/Control-Monad-Trans-Writer-Lazy.html#runWriter" class="link">Source</a></p><div class="doc"><p>Unwrap a writer computation as a (result, output) pair.
(The inverse of <code><a href="Control-Monad-Trans-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-Trans-Writer-Lazy.html#t:Writer">Writer</a> w a -> w<a href="src/Control-Monad-Trans-Writer-Lazy.html#execWriter" class="link">Source</a></p><div class="doc"><p>Extract the output from a writer computation.
</p><ul><li><pre><code><a href="Control-Monad-Trans-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-Trans-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-Trans-Writer-Lazy.html#t:Writer">Writer</a> w a -> <a href="Control-Monad-Trans-Writer-Lazy.html#t:Writer">Writer</a> w' b<a href="src/Control-Monad-Trans-Writer-Lazy.html#mapWriter" class="link">Source</a></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-Trans-Writer-Lazy.html#v:runWriter">runWriter</a></code> (<code><a href="Control-Monad-Trans-Writer-Lazy.html#v:mapWriter">mapWriter</a></code> f m) = f (<code><a href="Control-Monad-Trans-Writer-Lazy.html#v:runWriter">runWriter</a></code> m</code>)
</li></ul></div></div><h1 id="g:2">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 <a href="src/Control-Monad-Trans-Writer-Lazy.html#WriterT" class="link">Source</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="Control-Monad-Trans-Class.html#t:MonadTrans">MonadTrans</a> (<a href="Control-Monad-Trans-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-Trans-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-Trans-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-Trans-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-Trans-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-Trans-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-Trans-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-IO-Class.html#t:MonadIO">MonadIO</a> m) => <a href="Control-Monad-IO-Class.html#t:MonadIO">MonadIO</a> (<a href="Control-Monad-Trans-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-Trans-Writer-Lazy.html#t:WriterT">WriterT</a> w m a -> m w<a href="src/Control-Monad-Trans-Writer-Lazy.html#execWriterT" class="link">Source</a></p><div class="doc"><p>Extract the output from a writer computation.
</p><ul><li><pre><code><a href="Control-Monad-Trans-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-Trans-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-Trans-Writer-Lazy.html#t:WriterT">WriterT</a> w m a -> <a href="Control-Monad-Trans-Writer-Lazy.html#t:WriterT">WriterT</a> w' n b<a href="src/Control-Monad-Trans-Writer-Lazy.html#mapWriterT" class="link">Source</a></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-Trans-Writer-Lazy.html#v:runWriterT">runWriterT</a></code> (<code><a href="Control-Monad-Trans-Writer-Lazy.html#v:mapWriterT">mapWriterT</a></code> f m) = f (<code><a href="Control-Monad-Trans-Writer-Lazy.html#v:runWriterT">runWriterT</a></code> m</code>)
</li></ul></div></div><h1 id="g:3">Writer operations
</h1><div class="top"><p class="src"><a name="v:tell" class="def">tell</a> :: (<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) => w -> <a href="Control-Monad-Trans-Writer-Lazy.html#t:WriterT">WriterT</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-Trans-Writer-Lazy.html#tell" class="link">Source</a></p><div class="doc"><p><code><code><a href="Control-Monad-Trans-Writer-Lazy.html#v:tell">tell</a></code> w</code> is an action that produces the output <code>w</code>.
</p></div></div><div class="top"><p class="src"><a name="v:listen" class="def">listen</a> :: (<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-Trans-Writer-Lazy.html#t:WriterT">WriterT</a> w m a -> <a href="Control-Monad-Trans-Writer-Lazy.html#t:WriterT">WriterT</a> w m (a, w)<a href="src/Control-Monad-Trans-Writer-Lazy.html#listen" class="link">Source</a></p><div class="doc"><p><code><code><a href="Control-Monad-Trans-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><ul><li><pre><code><a href="Control-Monad-Trans-Writer-Lazy.html#v:runWriterT">runWriterT</a></code> (<code><a href="Control-Monad-Trans-Writer-Lazy.html#v:listen">listen</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> (\(a, w) -> ((a, w), w)) (<code><a href="Control-Monad-Trans-Writer-Lazy.html#v:runWriterT">runWriterT</a></code> m)</pre></li></ul></div></div><div class="top"><p class="src"><a name="v:listens" class="def">listens</a> :: (<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) => (w -> b) -> <a href="Control-Monad-Trans-Writer-Lazy.html#t:WriterT">WriterT</a> w m a -> <a href="Control-Monad-Trans-Writer-Lazy.html#t:WriterT">WriterT</a> w m (a, b)<a href="src/Control-Monad-Trans-Writer-Lazy.html#listens" class="link">Source</a></p><div class="doc"><p><code><code><a href="Control-Monad-Trans-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-Trans-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-Trans-Writer-Lazy.html#v:listen">listen</a></code> m)</pre></li><li><pre><code><a href="Control-Monad-Trans-Writer-Lazy.html#v:runWriterT">runWriterT</a></code> (<code><a href="Control-Monad-Trans-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> (\(a, w) -> ((a, f w), w)) (<code><a href="Control-Monad-Trans-Writer-Lazy.html#v:runWriterT">runWriterT</a></code> m)</pre></li></ul></div></div><div class="top"><p class="src"><a name="v:pass" class="def">pass</a> :: (<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-Trans-Writer-Lazy.html#t:WriterT">WriterT</a> w m (a, w -> w) -> <a href="Control-Monad-Trans-Writer-Lazy.html#t:WriterT">WriterT</a> w m a<a href="src/Control-Monad-Trans-Writer-Lazy.html#pass" class="link">Source</a></p><div class="doc"><p><code><code><a href="Control-Monad-Trans-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><ul><li><pre><code><a href="Control-Monad-Trans-Writer-Lazy.html#v:runWriterT">runWriterT</a></code> (<code><a href="Control-Monad-Trans-Writer-Lazy.html#v:pass">pass</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> (\((a, f), w) -> (a, f w)) (<code><a href="Control-Monad-Trans-Writer-Lazy.html#v:runWriterT">runWriterT</a></code> m)</pre></li></ul></div></div><div class="top"><p class="src"><a name="v:censor" class="def">censor</a> :: (<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) => (w -> w) -> <a href="Control-Monad-Trans-Writer-Lazy.html#t:WriterT">WriterT</a> w m a -> <a href="Control-Monad-Trans-Writer-Lazy.html#t:WriterT">WriterT</a> w m a<a href="src/Control-Monad-Trans-Writer-Lazy.html#censor" class="link">Source</a></p><div class="doc"><p><code><code><a href="Control-Monad-Trans-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-Trans-Writer-Lazy.html#v:censor">censor</a></code> f m = <code><a href="Control-Monad-Trans-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><li><pre><code><a href="Control-Monad-Trans-Writer-Lazy.html#v:runWriterT">runWriterT</a></code> (<code><a href="Control-Monad-Trans-Writer-Lazy.html#v:censor">censor</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> (\(a, w) -> (a, f w)) (<code><a href="Control-Monad-Trans-Writer-Lazy.html#v:runWriterT">runWriterT</a></code> m)</pre></li></ul></div></div><h1 id="g:4">Lifting other operations
</h1><div class="top"><p class="src"><a name="v:liftCallCC" class="def">liftCallCC</a> :: <a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Data-Monoid.html#t:Monoid">Monoid</a> w => ((((a, w) -> m (b, w)) -> m (a, w)) -> m (a, w)) -> ((a -> <a href="Control-Monad-Trans-Writer-Lazy.html#t:WriterT">WriterT</a> w m b) -> <a href="Control-Monad-Trans-Writer-Lazy.html#t:WriterT">WriterT</a> w m a) -> <a href="Control-Monad-Trans-Writer-Lazy.html#t:WriterT">WriterT</a> w m a<a href="src/Control-Monad-Trans-Writer-Lazy.html#liftCallCC" class="link">Source</a></p><div class="doc"><p>Lift a <code>callCC</code> operation to the new monad.
</p></div></div><div class="top"><p class="src"><a name="v:liftCatch" class="def">liftCatch</a> :: (m (a, w) -> (e -> m (a, w)) -> m (a, w)) -> <a href="Control-Monad-Trans-Writer-Lazy.html#t:WriterT">WriterT</a> w m a -> (e -> <a href="Control-Monad-Trans-Writer-Lazy.html#t:WriterT">WriterT</a> w m a) -> <a href="Control-Monad-Trans-Writer-Lazy.html#t:WriterT">WriterT</a> w m a<a href="src/Control-Monad-Trans-Writer-Lazy.html#liftCatch" class="link">Source</a></p><div class="doc"><p>Lift a <code>catchError</code> operation to the new monad.
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