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<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="phoenix.inside.actions"></a><a class="link" href="actions.html" title="More on Actions">More on Actions</a>
</h3></div></div></div>
<p>
As you know from the <a class="link" href="actor.html" title="Actors in Detail">Actors in Detail</a>
section, Actions are what brings life to a Phoenix expression tree.
</p>
<p>
When dealing with a Phoenix expression tree, it gets evaluated top-down.
Example:
</p>
<pre class="programlisting"><span class="identifier">_1</span> <span class="special">+</span> <span class="number">3</span> <span class="special">*</span> <span class="identifier">_2</span>
</pre>
<p>
Can be visualized as an AST in the following way:
</p>
<p>
<span class="inlinemediaobject"><img src="../../images/simple_ast.png"></span>
</p>
<p>
In terms of actions this means:
</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
<code class="computeroutput"><span class="identifier">rule</span><span class="special">::</span><span class="identifier">plus</span></code> is matched
</li>
<li class="listitem">
evaluate left:
<div class="itemizedlist"><ul class="itemizedlist" type="circle"><li class="listitem">
<code class="computeroutput"><span class="identifier">rule</span><span class="special">::</span><span class="identifier">placeholder</span></code> is matched
</li></ul></div>
</li>
<li class="listitem">
evaluate right:
<div class="itemizedlist"><ul class="itemizedlist" type="circle"><li class="listitem">
<code class="computeroutput"><span class="identifier">rule</span><span class="special">::</span><span class="identifier">multiplies</span></code> is matched
<div class="itemizedlist"><ul class="itemizedlist" type="square">
<li class="listitem">
evaluate left:
<div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem">
<code class="computeroutput"><span class="identifier">rule</span><span class="special">::</span><span class="identifier">value</span></code>
is matched
</li></ul></div>
</li>
<li class="listitem">
evaluate right:
<div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem">
<code class="computeroutput"><span class="identifier">rule</span><span class="special">::</span><span class="identifier">placeholder</span></code>
is matched
</li></ul></div>
</li>
</ul></div>
</li></ul></div>
</li>
</ul></div>
<p>
Every time a rule is matched, an action will be called. The action determines
how the Phoenix AST will be traversed.
</p>
<h5>
<a name="phoenix.inside.actions.h0"></a>
<span><a name="phoenix.inside.actions.writing_an_action"></a></span><a class="link" href="actions.html#phoenix.inside.actions.writing_an_action">Writing
an Action</a>
</h5>
<p>
As mentioned in <a class="link" href="actor.html" title="Actors in Detail">Actors in Detail</a>
actions are <a href="http://www.boost.org/doc/libs/release/doc/html/PrimitiveTransform.html" target="_top">Proto
Primitive Transforms</a> for convenience Phoenix provides an abstraction
to this:
</p>
<pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Fun</span><span class="special">></span>
<span class="keyword">struct</span> <span class="identifier">call</span><span class="special">;</span>
</pre>
<p>
This is similar to <a href="http://www.boost.org/doc/libs/release/doc/html/boost/proto/call.html" target="_top"><code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">call</span></code></a> but does more. It calls the
<code class="computeroutput"><span class="identifier">Fun</span></code> function object passed
as template parameter with the <code class="computeroutput"><span class="identifier">Context</span></code>
and the children of the expression associated with the rule.
</p>
<p>
Lets have an (simplified) example on how to write an evaluation action for
<code class="computeroutput"><span class="identifier">rule</span><span class="special">::</span><span class="identifier">plus</span></code>:
</p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">plus_eval</span>
<span class="special">{</span>
<span class="keyword">typedef</span> <span class="keyword">int</span> <span class="identifier">result_type</span><span class="special">;</span>
<span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Lhs</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Rhs</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Context</span><span class="special">></span>
<span class="identifier">result_type</span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">Lhs</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">lhs</span><span class="special">,</span> <span class="identifier">Rhs</span> <span class="keyword">const</span> <span class="special">&</span><span class="identifier">rhs</span><span class="special">,</span> <span class="identifier">Context</span> <span class="special">&</span> <span class="identifier">ctx</span><span class="special">)</span>
<span class="special">{</span>
<span class="keyword">return</span> <span class="identifier">eval</span><span class="special">(</span><span class="identifier">lhs</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">)</span> <span class="special">+</span> <span class="identifier">eval</span><span class="special">(</span><span class="identifier">rhs</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">);</span>
<span class="special">}</span>
<span class="special">};</span>
<span class="keyword">template</span> <span class="special"><></span>
<span class="keyword">struct</span> <span class="identifier">default_actions</span><span class="special">::</span><span class="identifier">when</span><span class="special"><</span><span class="identifier">rule</span><span class="special">::</span><span class="identifier">plus</span><span class="special">></span>
<span class="special">:</span> <span class="identifier">call</span><span class="special"><</span><span class="identifier">plus_eval</span><span class="special">></span>
<span class="special">{};</span>
</pre>
<p>
That's it. When evaluating a <code class="computeroutput"><span class="identifier">plus</span></code>
expression, the <code class="computeroutput"><span class="identifier">plus_eval</span></code>
callable gets called with the left hand side and right hand side expression
and the associated Context.
</p>
<p>
<span class="bold"><strong>But there is more:</strong></span> As Actions <span class="emphasis"><em>can</em></span>
be full fletched <a href="http://www.boost.org/doc/libs/release/doc/html/Transform.html" target="_top">Proto
Transforms</a>, you can in fact use any proto expression you can imagine
as the action. Phoenix predifines a set of callables and transform to deal
with the Context information passed along and of course every Phoenix expression
can be used as a Phoenix grammar or <a href="http://www.boost.org/doc/libs/release/doc/html/boost/proto/pass_through.html" target="_top">Proto
Pass Through Transform</a>.
</p>
<div class="variablelist">
<p class="title"><b></b></p>
<dl>
<dt><span class="term"><code class="computeroutput"><span class="identifier">functional</span><span class="special">::</span><span class="identifier">context</span><span class="special">(</span><span class="identifier">Env</span><span class="special">,</span> <span class="identifier">Actions</span><span class="special">)</span></code></span></dt>
<dd><p>
A <a href="http://www.boost.org/doc/libs/release/doc/html/Callable.html" target="_top">Proto
Callable Transform</a> that creates a new context out of the <code class="computeroutput"><span class="identifier">Env</span></code> and <code class="computeroutput"><span class="identifier">Actions</span></code>
parameter
</p></dd>
<dt><span class="term"><code class="computeroutput"><span class="identifier">functional</span><span class="special">::</span><span class="identifier">env</span><span class="special">(</span><span class="identifier">Context</span><span class="special">)</span></code></span></dt>
<dd><p>
A <a href="http://www.boost.org/doc/libs/release/doc/html/Callable.html" target="_top">Proto
Callable Transform</a> that returns the environment out of the
<code class="computeroutput"><span class="identifier">Context</span></code> parameter
</p></dd>
<dt><span class="term"><code class="computeroutput"><span class="identifier">functional</span><span class="special">::</span><span class="identifier">actions</span><span class="special">(</span><span class="identifier">Context</span><span class="special">)</span></code></span></dt>
<dd><p>
A <a href="http://www.boost.org/doc/libs/release/doc/html/Callable.html" target="_top">Proto
Callable Transform</a> that returns the actions out of the <code class="computeroutput"><span class="identifier">Context</span></code> parameter
</p></dd>
<dt><span class="term"><code class="computeroutput"><span class="identifier">_context</span></code></span></dt>
<dd><p>
A <a href="http://www.boost.org/doc/libs/release/doc/html/PrimitiveTransform.html" target="_top">Proto
Primitive Transform</a> that returns the current context
</p></dd>
<dt><span class="term"><code class="computeroutput"><span class="identifier">_env</span></code></span></dt>
<dd><p>
A <a href="http://www.boost.org/doc/libs/release/doc/html/PrimitiveTransform.html" target="_top">Proto
Primitive Transform</a> that returns the current environment
</p></dd>
<dt><span class="term"><code class="computeroutput"><span class="identifier">_actions</span></code></span></dt>
<dd><p>
A <a href="http://www.boost.org/doc/libs/release/doc/html/PrimitiveTransform.html" target="_top">Proto
Primitive Transform</a> that returns the current actions
</p></dd>
<dt><span class="term"><code class="computeroutput"><span class="identifier">context</span><span class="special">(</span><span class="identifier">env</span><span class="special">,</span> <span class="identifier">actions</span><span class="special">)</span></code></span></dt>
<dd><p>
A regular function that creates a context
</p></dd>
<dt><span class="term"><code class="computeroutput"><span class="identifier">env</span><span class="special">(</span><span class="identifier">ctx</span><span class="special">)</span></code></span></dt>
<dd><p>
A regular function that returns the environment from the given context
</p></dd>
<dt><span class="term"><code class="computeroutput"><span class="identifier">actions</span><span class="special">(</span><span class="identifier">ctx</span><span class="special">)</span></code></span></dt>
<dd><p>
A regular function that returns the actions from the given context
</p></dd>
</dl>
</div>
<p>
Phoenix is equipped with a predefined set of expressions, rules and actions
to make all the stuff work you learned in the <a class="link" href="../starter_kit.html" title="Starter Kit">Starter
Kit</a> and <a class="link" href="../modules.html" title="Modules">Modules</a> sections. See
the <a class="link" href="rules.html" title="Predefined Expressions and Rules">next section</a> for more details!
</p>
</div>
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<td align="right"><div class="copyright-footer">Copyright © 2002-2005, 2010, 2014, 2015 Joel de Guzman, Dan Marsden, Thomas
Heller, John Fletcher<p>
Distributed under the Boost Software License, Version 1.0. (See accompanying
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
</p>
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