<!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.Reader.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-Reader-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.Reader.Class</p></div><div id="description"><p class="caption">Description</p><div class="doc"><dl><dt>Computation type:</dt><dd> Computations which read values from a shared environment.
</dd><dt>Binding strategy:</dt><dd> Monad values are functions from the environment to a value.
The bound function is applied to the bound value, and both have access
to the shared environment.
</dd><dt>Useful for:</dt><dd> Maintaining variable bindings, or other shared environment.
</dd><dt>Zero and plus:</dt><dd> None.
</dd><dt>Example type:</dt><dd> <code><code>Reader</code> [(String,Value)] a</code>
</dd></dl><p>The <code>Reader</code> monad (also called the Environment monad).
Represents a computation, which can read values from
a shared environment, pass values from function to function,
and execute sub-computations in a modified environment.
Using <code>Reader</code> monad for such computations is often clearer and easier
than using the <code>Control.Monad.State.State</code> monad.
</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/">http://web.cecs.pdx.edu/~mpj/</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/Control-Monad.html#t:Monad">Monad</a> m => <a href="#t:MonadReader">MonadReader</a> r m | m -> r <span class="keyword">where</span><ul class="subs"><li><a href="#v:ask">ask</a> :: m r</li><li><a href="#v:local">local</a> :: (r -> r) -> m a -> m a</li></ul></li><li class="src short"><a href="#v:asks">asks</a> :: <a href="Control-Monad-Reader-Class.html#t:MonadReader">MonadReader</a> r m => (r -> 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/Control-Monad.html#t:Monad">Monad</a> m => <a name="t:MonadReader" class="def">MonadReader</a> r m | m -> r <span class="keyword">where</span><a href="src/Control-Monad-Reader-Class.html#MonadReader" class="link">Source</a></p><div class="doc"><p>See examples in <a href="Control-Monad-Reader.html">Control.Monad.Reader</a>.
Note, the partially applied function type <code>(->) r</code> is a simple reader monad.
See the <code>instance</code> declaration below.
</p></div><div class="subs methods"><p class="caption">Methods</p><p class="src"><a name="v:ask" class="def">ask</a> :: m r<a href="src/Control-Monad-Reader-Class.html#ask" class="link">Source</a></p><div class="doc"><p>Retrieves the monad environment.
</p></div><p class="src"><a name="v:local" class="def">local</a><a href="src/Control-Monad-Reader-Class.html#local" 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">-> m a</td><td class="doc"><p><code>Reader</code> to run in the modified environment.
</p></td></tr><tr><td class="src">-> m a</td><td class="doc empty"> </td></tr></table></div><div class="doc"><p>Executes a computation in a modified environment.
</p></div></div><div class="subs instances"><p id="control.i:MonadReader" class="caption collapser" onclick="toggleSection('i:MonadReader')">Instances</p><div id="section.i:MonadReader" class="show"><table><tr><td class="src"><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="/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-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-List.html#t:ListT">ListT</a> m)</td><td class="doc empty"> </td></tr><tr><td class="src"><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="/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-Reader-Class.html#t:MonadReader">MonadReader</a> r ((->) 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/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-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="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="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-State-Strict.html#t:StateT">StateT</a> s m)</td><td class="doc empty"> </td></tr><tr><td class="src"><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-State-Lazy.html#t:StateT">StateT</a> s 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-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-Error.html#t:ErrorT">ErrorT</a> e m)</td><td class="doc empty"> </td></tr><tr><td class="src"><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-Cont.html#t:ContT">ContT</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:Monad">Monad</a> m => <a href="Control-Monad-Reader-Class.html#t:MonadReader">MonadReader</a> r (<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="/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/Data-Monoid.html#t:Monoid">Monoid</a> w) => <a href="Control-Monad-Reader-Class.html#t:MonadReader">MonadReader</a> r (<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/Control-Monad.html#t:Monad">Monad</a> m, <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 (<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:asks" class="def">asks</a><a href="src/Control-Monad-Reader-Class.html#asks" class="link">Source</a></p><div class="subs arguments"><p class="caption">Arguments</p><table><tr><td class="src">:: <a href="Control-Monad-Reader-Class.html#t:MonadReader">MonadReader</a> r 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">-> m a</td><td class="doc empty"> </td></tr></table></div><div class="doc"><p>Retrieves a function of the current environment.
</p></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>
