<!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.Error.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-Error-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-parameter type classes)</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.Error.Class</p></div><div id="description"><p class="caption">Description</p><div class="doc"><dl><dt>Computation type:</dt><dd> Computations which may fail or throw exceptions.
</dd><dt>Binding strategy:</dt><dd> Failure records information about the cause/location
of the failure. Failure values bypass the bound function,
other values are used as inputs to the bound function.
</dd><dt>Useful for:</dt><dd> Building computations from sequences of functions that may fail
or using exception handling to structure error handling.
</dd><dt>Zero and plus:</dt><dd> Zero is represented by an empty error and the plus operation
executes its second argument if the first fails.
</dd><dt>Example type:</dt><dd> <code><code><a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Data-Either.html#t:Either">Either</a></code> <code><a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Data-Char.html#t:String">String</a></code> a</code>
</dd></dl><p>The Error monad (also called the Exception monad).
</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="#t:Error">Error</a> a <span class="keyword">where</span><ul class="subs"><li><a href="#v:noMsg">noMsg</a> :: a</li><li><a href="#v:strMsg">strMsg</a> :: <a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Data-Char.html#t:String">String</a> -> a</li></ul></li><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:MonadError">MonadError</a> e m | m -> e <span class="keyword">where</span><ul class="subs"><li><a href="#v:throwError">throwError</a> :: e -> m a</li><li><a href="#v:catchError">catchError</a> :: m a -> (e -> m a) -> m a</li></ul></li></ul></div><div id="interface"><h1>Documentation</h1><div class="top"><p class="src"><span class="keyword">class</span> <a name="t:Error" class="def">Error</a> a <span class="keyword">where</span></p><div class="doc"><p>An exception to be thrown.
</p><p>Minimal complete definition: <code><a href="Control-Monad-Error-Class.html#v:noMsg">noMsg</a></code> or <code><a href="Control-Monad-Error-Class.html#v:strMsg">strMsg</a></code>.
</p></div><div class="subs methods"><p class="caption">Methods</p><p class="src"><a name="v:noMsg" class="def">noMsg</a> :: a</p><div class="doc"><p>Creates an exception without a message.
The default implementation is <code><code><a href="Control-Monad-Error-Class.html#v:strMsg">strMsg</a></code> ""</code>.
</p></div><p class="src"><a name="v:strMsg" class="def">strMsg</a> :: <a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Data-Char.html#t:String">String</a> -> a</p><div class="doc"><p>Creates an exception with a message.
The default implementation of <code><code><a href="Control-Monad-Error-Class.html#v:strMsg">strMsg</a></code> s</code> is <code><a href="Control-Monad-Error-Class.html#v:noMsg">noMsg</a></code>.
</p></div></div><div class="subs instances"><p id="control.i:Error" class="caption collapser" onclick="toggleSection('i:Error')">Instances</p><div id="section.i:Error" class="show"><table><tr><td class="src"><a href="Control-Monad-Error-Class.html#t:Error">Error</a> <a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Control-Exception-Base.html#t:IOException">IOException</a></td><td class="doc empty"> </td></tr><tr><td class="src"><a href="/usr/share/doc/ghc/html/libraries/transformers-0.2.2.0/Control-Monad-Trans-Error.html#t:ErrorList">ErrorList</a> a => <a href="Control-Monad-Error-Class.html#t:Error">Error</a> [a]</td><td class="doc"><p>A string can be thrown as an error.
</p></td></tr></table></div></div></div><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:MonadError" class="def">MonadError</a> e m | m -> e <span class="keyword">where</span><a href="src/Control-Monad-Error-Class.html#MonadError" class="link">Source</a></p><div class="doc"><p>The strategy of combining computations that can throw exceptions
by bypassing bound functions
from the point an exception is thrown to the point that it is handled.
</p><p>Is parameterized over the type of error information and
the monad type constructor.
It is common to use <code><code>Data.Either</code> String</code> as the monad type constructor
for an error monad in which error descriptions take the form of strings.
In that case and many other common cases the resulting monad is already defined
as an instance of the <code><a href="Control-Monad-Error-Class.html#t:MonadError">MonadError</a></code> class.
You can also define your own error type and/or use a monad type constructor
other than <code><code><a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Data-Either.html#t:Either">Either</a></code> <code><a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Data-Char.html#t:String">String</a></code></code> or <code><code><a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Data-Either.html#t:Either">Either</a></code> <code><a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/System-IO-Error.html#t:IOError">IOError</a></code></code>.
In these cases you will have to explicitly define instances of the <code><a href="Control-Monad-Error-Class.html#t:Error">Error</a></code>
and/or <code><a href="Control-Monad-Error-Class.html#t:MonadError">MonadError</a></code> classes.
</p></div><div class="subs methods"><p class="caption">Methods</p><p class="src"><a name="v:throwError" class="def">throwError</a> :: e -> m a<a href="src/Control-Monad-Error-Class.html#throwError" class="link">Source</a></p><div class="doc"><p>Is used within a monadic computation to begin exception processing.
</p></div><p class="src"><a name="v:catchError" class="def">catchError</a> :: m a -> (e -> m a) -> m a<a href="src/Control-Monad-Error-Class.html#catchError" class="link">Source</a></p><div class="doc"><p>A handler function to handle previous errors and return to normal execution.
A common idiom is:
</p><pre> do { action1; action2; action3 } `catchError` handler
</pre><p>where the <code>action</code> functions can call <code><a href="Control-Monad-Error-Class.html#v:throwError">throwError</a></code>.
Note that <code>handler</code> and the do-block must have the same return type.
</p></div></div><div class="subs instances"><p id="control.i:MonadError" class="caption collapser" onclick="toggleSection('i:MonadError')">Instances</p><div id="section.i:MonadError" class="show"><table><tr><td class="src"><a href="Control-Monad-Error-Class.html#t:MonadError">MonadError</a> <a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Control-Exception-Base.html#t:IOException">IOException</a> <a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/System-IO.html#t:IO">IO</a></td><td class="doc empty"> </td></tr><tr><td class="src"><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="/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-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-List.html#t:ListT">ListT</a> m)</td><td class="doc empty"> </td></tr><tr><td class="src"><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="/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-Error-Class.html#t:Error">Error</a> e => <a href="Control-Monad-Error-Class.html#t:MonadError">MonadError</a> e (<a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Data-Either.html#t:Either">Either</a> e)</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-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-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="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-State-Strict.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:MonadError">MonadError</a> e m => <a href="Control-Monad-Error-Class.html#t:MonadError">MonadError</a> e (<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:MonadError">MonadError</a> e m => <a href="Control-Monad-Error-Class.html#t:MonadError">MonadError</a> e (<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="Control-Monad-Error-Class.html#t:Error">Error</a> e) => <a href="Control-Monad-Error-Class.html#t:MonadError">MonadError</a> e (<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="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-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="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-RWS-Lazy.html#t:RWST">RWST</a> r w s m)</td><td class="doc empty"> </td></tr></table></div></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>
