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</script></head><body><div id="package-header"><ul class="links" id="page-menu"><li><a href="src/Control-Monad-Trans-Maybe.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.Maybe</p></div><div id="table-of-contents"><p class="caption">Contents</p><ul><li><a href="#g:1">The MaybeT monad transformer
</a></li><li><a href="#g:2">Lifting other operations
</a></li></ul></div><div id="description"><p class="caption">Description</p><div class="doc"><p>The <code><a href="Control-Monad-Trans-Maybe.html#t:MaybeT">MaybeT</a></code> monad transformer adds the ability to fail to a monad.
</p><p>A sequence of actions succeeds, producing a value, only if all the
actions in the sequence are successful. If one fails, the rest of
the sequence is skipped and the composite action fails.
</p><p>For a variant allowing a range of error values, see
<a href="Control-Monad-Trans-Error.html">Control.Monad.Trans.Error</a>.
</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">newtype</span> <a href="#t:MaybeT">MaybeT</a> m a = <a href="#v:MaybeT">MaybeT</a> {<ul class="subs"><li><a href="#v:runMaybeT">runMaybeT</a> :: m (<a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Data-Maybe.html#t:Maybe">Maybe</a> a)</li></ul>}</li><li class="src short"><a href="#v:mapMaybeT">mapMaybeT</a> :: (m (<a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Data-Maybe.html#t:Maybe">Maybe</a> a) -> n (<a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Data-Maybe.html#t:Maybe">Maybe</a> b)) -> <a href="Control-Monad-Trans-Maybe.html#t:MaybeT">MaybeT</a> m a -> <a href="Control-Monad-Trans-Maybe.html#t:MaybeT">MaybeT</a> n b</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-Maybe.html#t:Maybe">Maybe</a> a -> m (<a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Data-Maybe.html#t:Maybe">Maybe</a> b)) -> m (<a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Data-Maybe.html#t:Maybe">Maybe</a> a)) -> m (<a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Data-Maybe.html#t:Maybe">Maybe</a> a)) -> ((a -> <a href="Control-Monad-Trans-Maybe.html#t:MaybeT">MaybeT</a> m b) -> <a href="Control-Monad-Trans-Maybe.html#t:MaybeT">MaybeT</a> m a) -> <a href="Control-Monad-Trans-Maybe.html#t:MaybeT">MaybeT</a> m a</li><li class="src short"><a href="#v:liftCatch">liftCatch</a> :: (m (<a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Data-Maybe.html#t:Maybe">Maybe</a> a) -> (e -> m (<a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Data-Maybe.html#t:Maybe">Maybe</a> a)) -> m (<a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Data-Maybe.html#t:Maybe">Maybe</a> a)) -> <a href="Control-Monad-Trans-Maybe.html#t:MaybeT">MaybeT</a> m a -> (e -> <a href="Control-Monad-Trans-Maybe.html#t:MaybeT">MaybeT</a> m a) -> <a href="Control-Monad-Trans-Maybe.html#t:MaybeT">MaybeT</a> m a</li><li class="src short"><a href="#v:liftListen">liftListen</a> :: <a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Control-Monad.html#t:Monad">Monad</a> m => (m (<a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Data-Maybe.html#t:Maybe">Maybe</a> a) -> m (<a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Data-Maybe.html#t:Maybe">Maybe</a> a, w)) -> <a href="Control-Monad-Trans-Maybe.html#t:MaybeT">MaybeT</a> m a -> <a href="Control-Monad-Trans-Maybe.html#t:MaybeT">MaybeT</a> m (a, w)</li><li class="src short"><a href="#v:liftPass">liftPass</a> :: <a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Control-Monad.html#t:Monad">Monad</a> m => (m (<a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Data-Maybe.html#t:Maybe">Maybe</a> a, w -> w) -> m (<a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Data-Maybe.html#t:Maybe">Maybe</a> a)) -> <a href="Control-Monad-Trans-Maybe.html#t:MaybeT">MaybeT</a> m (a, w -> w) -> <a href="Control-Monad-Trans-Maybe.html#t:MaybeT">MaybeT</a> m a</li></ul></div><div id="interface"><h1 id="g:1">The MaybeT monad transformer
</h1><div class="top"><p class="src"><span class="keyword">newtype</span> <a name="t:MaybeT" class="def">MaybeT</a> m a <a href="src/Control-Monad-Trans-Maybe.html#MaybeT" class="link">Source</a></p><div class="doc"><p>The parameterizable maybe monad, obtained by composing an arbitrary
monad with the <code><a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Data-Maybe.html#t:Maybe">Maybe</a></code> monad.
</p><p>Computations are actions that may produce a value or fail.
</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 yields a successful computation, while <code>>>=</code>
sequences two subcomputations, failing on the first error.
</p></div><div class="subs constructors"><p class="caption">Constructors</p><table><tr><td class="src"><a name="v:MaybeT" class="def">MaybeT</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:runMaybeT" class="def">runMaybeT</a> :: m (<a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Data-Maybe.html#t:Maybe">Maybe</a> a)</dt><dd class="doc empty"> </dd></dl><div class="clear"></div></div></td></tr></table></div><div class="subs instances"><p id="control.i:MaybeT" class="caption collapser" onclick="toggleSection('i:MaybeT')">Instances</p><div id="section.i:MaybeT" class="show"><table><tr><td class="src"><a href="Control-Monad-Trans-Class.html#t:MonadTrans">MonadTrans</a> <a href="Control-Monad-Trans-Maybe.html#t:MaybeT">MaybeT</a></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-Maybe.html#t:MaybeT">MaybeT</a> 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-Maybe.html#t:MaybeT">MaybeT</a> 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/Control-Monad.html#t:MonadPlus">MonadPlus</a> (<a href="Control-Monad-Trans-Maybe.html#t:MaybeT">MaybeT</a> 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:Monad">Monad</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-Maybe.html#t:MaybeT">MaybeT</a> 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:Monad">Monad</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-Maybe.html#t:MaybeT">MaybeT</a> 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-Maybe.html#t:MaybeT">MaybeT</a> m)</td><td class="doc empty"> </td></tr></table></div></div></div><div class="top"><p class="src"><a name="v:mapMaybeT" class="def">mapMaybeT</a> :: (m (<a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Data-Maybe.html#t:Maybe">Maybe</a> a) -> n (<a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Data-Maybe.html#t:Maybe">Maybe</a> b)) -> <a href="Control-Monad-Trans-Maybe.html#t:MaybeT">MaybeT</a> m a -> <a href="Control-Monad-Trans-Maybe.html#t:MaybeT">MaybeT</a> n b<a href="src/Control-Monad-Trans-Maybe.html#mapMaybeT" class="link">Source</a></p><div class="doc"><p>Transform the computation inside a <code>MaybeT</code>.
</p></div></div><h1 id="g:2">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-Maybe.html#t:Maybe">Maybe</a> a -> m (<a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Data-Maybe.html#t:Maybe">Maybe</a> b)) -> m (<a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Data-Maybe.html#t:Maybe">Maybe</a> a)) -> m (<a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Data-Maybe.html#t:Maybe">Maybe</a> a)) -> ((a -> <a href="Control-Monad-Trans-Maybe.html#t:MaybeT">MaybeT</a> m b) -> <a href="Control-Monad-Trans-Maybe.html#t:MaybeT">MaybeT</a> m a) -> <a href="Control-Monad-Trans-Maybe.html#t:MaybeT">MaybeT</a> m a<a href="src/Control-Monad-Trans-Maybe.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 href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Data-Maybe.html#t:Maybe">Maybe</a> a) -> (e -> m (<a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Data-Maybe.html#t:Maybe">Maybe</a> a)) -> m (<a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Data-Maybe.html#t:Maybe">Maybe</a> a)) -> <a href="Control-Monad-Trans-Maybe.html#t:MaybeT">MaybeT</a> m a -> (e -> <a href="Control-Monad-Trans-Maybe.html#t:MaybeT">MaybeT</a> m a) -> <a href="Control-Monad-Trans-Maybe.html#t:MaybeT">MaybeT</a> m a<a href="src/Control-Monad-Trans-Maybe.html#liftCatch" class="link">Source</a></p><div class="doc"><p>Lift a <code>catchError</code> operation to the new monad.
</p></div></div><div class="top"><p class="src"><a name="v:liftListen" class="def">liftListen</a> :: <a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Control-Monad.html#t:Monad">Monad</a> m => (m (<a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Data-Maybe.html#t:Maybe">Maybe</a> a) -> m (<a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Data-Maybe.html#t:Maybe">Maybe</a> a, w)) -> <a href="Control-Monad-Trans-Maybe.html#t:MaybeT">MaybeT</a> m a -> <a href="Control-Monad-Trans-Maybe.html#t:MaybeT">MaybeT</a> m (a, w)<a href="src/Control-Monad-Trans-Maybe.html#liftListen" class="link">Source</a></p><div class="doc"><p>Lift a <code>listen</code> operation to the new monad.
</p></div></div><div class="top"><p class="src"><a name="v:liftPass" class="def">liftPass</a> :: <a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Control-Monad.html#t:Monad">Monad</a> m => (m (<a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Data-Maybe.html#t:Maybe">Maybe</a> a, w -> w) -> m (<a href="/usr/share/doc/ghc/html/libraries/base-4.3.1.0/Data-Maybe.html#t:Maybe">Maybe</a> a)) -> <a href="Control-Monad-Trans-Maybe.html#t:MaybeT">MaybeT</a> m (a, w -> w) -> <a href="Control-Monad-Trans-Maybe.html#t:MaybeT">MaybeT</a> m a<a href="src/Control-Monad-Trans-Maybe.html#liftPass" class="link">Source</a></p><div class="doc"><p>Lift a <code>pass</code> operation to the new monad.
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