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</script></head><body><div id="package-header"><ul class="links" id="page-menu"><li><a href="src/Control-Monad-Trans-Reader.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.Reader</p></div><div id="table-of-contents"><p class="caption">Contents</p><ul><li><a href="#g:1">The Reader monad
</a></li><li><a href="#g:2">The ReaderT monad transformer
</a></li><li><a href="#g:3">Reader 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>Declaration of the <code><a href="Control-Monad-Trans-Reader.html#t:ReaderT">ReaderT</a></code> monad transformer, which adds a static
environment to a given monad.
</p><p>If the computation is to modify the stored information, 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:Reader">Reader</a> r = <a href="Control-Monad-Trans-Reader.html#t:ReaderT">ReaderT</a> r <a href="Data-Functor-Identity.html#t:Identity">Identity</a></li><li class="src short"><a href="#v:reader">reader</a> :: (r -> a) -> <a href="Control-Monad-Trans-Reader.html#t:Reader">Reader</a> r a</li><li class="src short"><a href="#v:runReader">runReader</a> :: <a href="Control-Monad-Trans-Reader.html#t:Reader">Reader</a> r a -> r -> a</li><li class="src short"><a href="#v:mapReader">mapReader</a> :: (a -> b) -> <a href="Control-Monad-Trans-Reader.html#t:Reader">Reader</a> r a -> <a href="Control-Monad-Trans-Reader.html#t:Reader">Reader</a> r b</li><li class="src short"><a href="#v:withReader">withReader</a> :: (r' -> r) -> <a href="Control-Monad-Trans-Reader.html#t:Reader">Reader</a> r a -> <a href="Control-Monad-Trans-Reader.html#t:Reader">Reader</a> r' a</li><li class="src short"><span class="keyword">newtype</span> <a href="#t:ReaderT">ReaderT</a> r m a = <a href="#v:ReaderT">ReaderT</a> {<ul class="subs"><li><a href="#v:runReaderT">runReaderT</a> :: r -> m a</li></ul>}</li><li class="src short"><a href="#v:mapReaderT">mapReaderT</a> :: (m a -> n b) -> <a href="Control-Monad-Trans-Reader.html#t:ReaderT">ReaderT</a> r m a -> <a href="Control-Monad-Trans-Reader.html#t:ReaderT">ReaderT</a> r n b</li><li class="src short"><a href="#v:withReaderT">withReaderT</a> :: (r' -> r) -> <a href="Control-Monad-Trans-Reader.html#t:ReaderT">ReaderT</a> r m a -> <a href="Control-Monad-Trans-Reader.html#t:ReaderT">ReaderT</a> r' m a</li><li class="src short"><a href="#v:ask">ask</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-Reader.html#t:ReaderT">ReaderT</a> r m r</li><li class="src short"><a href="#v:local">local</a> :: <a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Control-Monad.html#t:Monad">Monad</a> m => (r -> r) -> <a href="Control-Monad-Trans-Reader.html#t:ReaderT">ReaderT</a> r m a -> <a href="Control-Monad-Trans-Reader.html#t:ReaderT">ReaderT</a> r m a</li><li class="src short"><a href="#v:asks">asks</a> :: <a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Control-Monad.html#t:Monad">Monad</a> m => (r -> a) -> <a href="Control-Monad-Trans-Reader.html#t:ReaderT">ReaderT</a> r m a</li><li class="src short"><a href="#v:liftCallCC">liftCallCC</a> :: (((a -> m b) -> m a) -> m a) -> ((a -> <a href="Control-Monad-Trans-Reader.html#t:ReaderT">ReaderT</a> r m b) -> <a href="Control-Monad-Trans-Reader.html#t:ReaderT">ReaderT</a> r m a) -> <a href="Control-Monad-Trans-Reader.html#t:ReaderT">ReaderT</a> r m a</li><li class="src short"><a href="#v:liftCatch">liftCatch</a> :: (m a -> (e -> m a) -> m a) -> <a href="Control-Monad-Trans-Reader.html#t:ReaderT">ReaderT</a> r m a -> (e -> <a href="Control-Monad-Trans-Reader.html#t:ReaderT">ReaderT</a> r m a) -> <a href="Control-Monad-Trans-Reader.html#t:ReaderT">ReaderT</a> r m a</li></ul></div><div id="interface"><h1 id="g:1">The Reader monad
</h1><div class="top"><p class="src"><span class="keyword">type</span> <a name="t:Reader" class="def">Reader</a> r = <a href="Control-Monad-Trans-Reader.html#t:ReaderT">ReaderT</a> r <a href="Data-Functor-Identity.html#t:Identity">Identity</a><a href="src/Control-Monad-Trans-Reader.html#Reader" class="link">Source</a></p><div class="doc"><p>The parameterizable reader monad.
</p><p>Computations are functions of a shared environment.
</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 ignores the environment, while <code>>>=</code> passes
the inherited environment to both subcomputations.
</p></div></div><div class="top"><p class="src"><a name="v:reader" class="def">reader</a> :: (r -> a) -> <a href="Control-Monad-Trans-Reader.html#t:Reader">Reader</a> r a<a href="src/Control-Monad-Trans-Reader.html#reader" class="link">Source</a></p><div class="doc"><p>Constructor for computations in the reader monad.
</p></div></div><div class="top"><p class="src"><a name="v:runReader" class="def">runReader</a><a href="src/Control-Monad-Trans-Reader.html#runReader" class="link">Source</a></p><div class="subs arguments"><p class="caption">Arguments</p><table><tr><td class="src">:: <a href="Control-Monad-Trans-Reader.html#t:Reader">Reader</a> r a</td><td class="doc"><p>A <code>Reader</code> to run.
</p></td></tr><tr><td class="src">-> r</td><td class="doc"><p>An initial environment.
</p></td></tr><tr><td class="src">-> a</td><td class="doc empty"> </td></tr></table></div><div class="doc"><p>Runs a <code>Reader</code> and extracts the final value from it.
</p></div></div><div class="top"><p class="src"><a name="v:mapReader" class="def">mapReader</a> :: (a -> b) -> <a href="Control-Monad-Trans-Reader.html#t:Reader">Reader</a> r a -> <a href="Control-Monad-Trans-Reader.html#t:Reader">Reader</a> r b<a href="src/Control-Monad-Trans-Reader.html#mapReader" class="link">Source</a></p><div class="doc"><p>Transform the value returned by a <code>Reader</code>.
</p></div></div><div class="top"><p class="src"><a name="v:withReader" class="def">withReader</a><a href="src/Control-Monad-Trans-Reader.html#withReader" class="link">Source</a></p><div class="subs arguments"><p class="caption">Arguments</p><table><tr><td class="src">:: (r' -> r)</td><td class="doc"><p>The function to modify the environment.
</p></td></tr><tr><td class="src">-> <a href="Control-Monad-Trans-Reader.html#t:Reader">Reader</a> r a</td><td class="doc"><p>Computation to run in the modified environment.
</p></td></tr><tr><td class="src">-> <a href="Control-Monad-Trans-Reader.html#t:Reader">Reader</a> r' a</td><td class="doc empty"> </td></tr></table></div><div class="doc"><p>Execute a computation in a modified environment
(a specialization of <code><a href="Control-Monad-Trans-Reader.html#v:withReaderT">withReaderT</a></code>).
</p></div></div><h1 id="g:2">The ReaderT monad transformer
</h1><div class="top"><p class="src"><span class="keyword">newtype</span> <a name="t:ReaderT" class="def">ReaderT</a> r m a <a href="src/Control-Monad-Trans-Reader.html#ReaderT" class="link">Source</a></p><div class="doc"><p>The reader monad transformer,
which adds a read-only environment to the given monad.
</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 ignores the environment, while <code>>>=</code> passes
the inherited environment to both subcomputations.
</p></div><div class="subs constructors"><p class="caption">Constructors</p><table><tr><td class="src"><a name="v:ReaderT" class="def">ReaderT</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:runReaderT" class="def">runReaderT</a> :: r -> m a</dt><dd class="doc"><p>The underlying computation, as a function of the environment.
</p></dd></dl><div class="clear"></div></div></td></tr></table></div><div class="subs instances"><p id="control.i:ReaderT" class="caption collapser" onclick="toggleSection('i:ReaderT')">Instances</p><div id="section.i:ReaderT" class="show"><table><tr><td class="src"><a href="Control-Monad-Trans-Class.html#t:MonadTrans">MonadTrans</a> (<a href="Control-Monad-Trans-Reader.html#t:ReaderT">ReaderT</a> r)</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: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-Reader.html#t:ReaderT">ReaderT</a> r 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-Reader.html#t:ReaderT">ReaderT</a> r 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-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-Reader.html#t:ReaderT">ReaderT</a> r 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: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-Reader.html#t:ReaderT">ReaderT</a> r 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-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-Reader.html#t:ReaderT">ReaderT</a> r 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-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-Reader.html#t:ReaderT">ReaderT</a> r m)</td><td class="doc empty"> </td></tr><tr><td class="src"><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-Reader.html#t:ReaderT">ReaderT</a> r m)</td><td class="doc empty"> </td></tr></table></div></div></div><div class="top"><p class="src"><a name="v:mapReaderT" class="def">mapReaderT</a> :: (m a -> n b) -> <a href="Control-Monad-Trans-Reader.html#t:ReaderT">ReaderT</a> r m a -> <a href="Control-Monad-Trans-Reader.html#t:ReaderT">ReaderT</a> r n b<a href="src/Control-Monad-Trans-Reader.html#mapReaderT" class="link">Source</a></p><div class="doc"><p>Transform the computation inside a <code>ReaderT</code>.
</p></div></div><div class="top"><p class="src"><a name="v:withReaderT" class="def">withReaderT</a><a href="src/Control-Monad-Trans-Reader.html#withReaderT" class="link">Source</a></p><div class="subs arguments"><p class="caption">Arguments</p><table><tr><td class="src">:: (r' -> r)</td><td class="doc"><p>The function to modify the environment.
</p></td></tr><tr><td class="src">-> <a href="Control-Monad-Trans-Reader.html#t:ReaderT">ReaderT</a> r m a</td><td class="doc"><p>Computation to run in the modified environment.
</p></td></tr><tr><td class="src">-> <a href="Control-Monad-Trans-Reader.html#t:ReaderT">ReaderT</a> r' m a</td><td class="doc empty"> </td></tr></table></div><div class="doc"><p>Execute a computation in a modified environment
(a more general version of <code><a href="Control-Monad-Trans-Reader.html#v:local">local</a></code>).
</p></div></div><h1 id="g:3">Reader operations
</h1><div class="top"><p class="src"><a name="v:ask" class="def">ask</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-Reader.html#t:ReaderT">ReaderT</a> r m r<a href="src/Control-Monad-Trans-Reader.html#ask" class="link">Source</a></p><div class="doc"><p>Fetch the value of the environment.
</p></div></div><div class="top"><p class="src"><a name="v:local" class="def">local</a><a href="src/Control-Monad-Trans-Reader.html#local" class="link">Source</a></p><div class="subs arguments"><p class="caption">Arguments</p><table><tr><td class="src">:: <a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Control-Monad.html#t:Monad">Monad</a> m</td><td class="doc empty"> </td></tr><tr><td class="src">=> (r -> r)</td><td class="doc"><p>The function to modify the environment.
</p></td></tr><tr><td class="src">-> <a href="Control-Monad-Trans-Reader.html#t:ReaderT">ReaderT</a> r m a</td><td class="doc"><p>Computation to run in the modified environment.
</p></td></tr><tr><td class="src">-> <a href="Control-Monad-Trans-Reader.html#t:ReaderT">ReaderT</a> r m a</td><td class="doc empty"> </td></tr></table></div><div class="doc"><p>Execute a computation in a modified environment
(a specialization of <code><a href="Control-Monad-Trans-Reader.html#v:withReaderT">withReaderT</a></code>).
</p></div></div><div class="top"><p class="src"><a name="v:asks" class="def">asks</a><a href="src/Control-Monad-Trans-Reader.html#asks" class="link">Source</a></p><div class="subs arguments"><p class="caption">Arguments</p><table><tr><td class="src">:: <a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Control-Monad.html#t:Monad">Monad</a> m</td><td class="doc empty"> </td></tr><tr><td class="src">=> (r -> a)</td><td class="doc"><p>The selector function to apply to the environment.
</p></td></tr><tr><td class="src">-> <a href="Control-Monad-Trans-Reader.html#t:ReaderT">ReaderT</a> r m a</td><td class="doc empty"> </td></tr></table></div><div class="doc"><p>Retrieve a function of the current environment.
</p></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="src/Control-Monad-Trans-Reader.html#liftCallCC" class="link">Source</a></p><div class="subs arguments"><p class="caption">Arguments</p><table><tr><td class="src">:: (((a -> m b) -> m a) -> m a)</td><td class="doc"><p><code>callCC</code> on the argument monad.
</p></td></tr><tr><td class="src">-> ((a -> <a href="Control-Monad-Trans-Reader.html#t:ReaderT">ReaderT</a> r m b) -> <a href="Control-Monad-Trans-Reader.html#t:ReaderT">ReaderT</a> r m a)</td><td class="doc empty"> </td></tr><tr><td class="src">-> <a href="Control-Monad-Trans-Reader.html#t:ReaderT">ReaderT</a> r m a</td><td class="doc empty"> </td></tr></table></div><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><a href="src/Control-Monad-Trans-Reader.html#liftCatch" class="link">Source</a></p><div class="subs arguments"><p class="caption">Arguments</p><table><tr><td class="src">:: (m a -> (e -> m a) -> m a)</td><td class="doc"><p><code>catch</code> on the argument monad.
</p></td></tr><tr><td class="src">-> <a href="Control-Monad-Trans-Reader.html#t:ReaderT">ReaderT</a> r m a</td><td class="doc"><p>Computation to attempt.
</p></td></tr><tr><td class="src">-> (e -> <a href="Control-Monad-Trans-Reader.html#t:ReaderT">ReaderT</a> r m a)</td><td class="doc"><p>Exception handler.
</p></td></tr><tr><td class="src">-> <a href="Control-Monad-Trans-Reader.html#t:ReaderT">ReaderT</a> r m a</td><td class="doc empty"> </td></tr></table></div><div class="doc"><p>Lift a <code>catchError</code> operation to the new monad.
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