<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>Control.Monad.Writer.Class</title><link href="ocean.css" rel="stylesheet" type="text/css" title="Ocean" /><script src="haddock-util.js" type="text/javascript"></script><script type="text/javascript">//<![CDATA[
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</script></head><body><div id="package-header"><ul class="links" id="page-menu"><li><a href="src/Control-Monad-Writer-Class.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.Class</p></div><div id="description"><p class="caption">Description</p><div class="doc"><p>The MonadWriter class.
</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-Class.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-Class.html#t:MonadWriter">MonadWriter</a> w m => (w -> w) -> m a -> m a</li></ul></div><div id="interface"><h1>Documentation</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-Class.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-Class.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-Class.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-Class.html#t:MonadWriter">MonadWriter</a> w m => <a href="Control-Monad-Writer-Class.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-Class.html#t:MonadWriter">MonadWriter</a> w m => <a href="Control-Monad-Writer-Class.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-Class.html#t:MonadWriter">MonadWriter</a> w m => <a href="Control-Monad-Writer-Class.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-Class.html#t:MonadWriter">MonadWriter</a> w m => <a href="Control-Monad-Writer-Class.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-Class.html#t:MonadWriter">MonadWriter</a> w m => <a href="Control-Monad-Writer-Class.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-Class.html#t:MonadWriter">MonadWriter</a> w m) => <a href="Control-Monad-Writer-Class.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-Class.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-Class.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-Class.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-Class.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-Class.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-Class.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-Class.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-Class.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-Class.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-Class.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-Class.html#v:censor">censor</a></code> f m = <code><a href="Control-Monad-Writer-Class.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></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>
