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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" alt="simple_ast"></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"> |
| <p class="simpara"> |
| evaluate left: |
| </p> |
| <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"> |
| <p class="simpara"> |
| evaluate right: |
| </p> |
| <div class="itemizedlist"><ul class="itemizedlist" type="circle"><li class="listitem"> |
| <p class="simpara"> |
| <code class="computeroutput"><span class="identifier">rule</span><span class="special">::</span><span class="identifier">multiplies</span></code> is matched |
| </p> |
| <div class="itemizedlist"><ul class="itemizedlist" type="square"> |
| <li class="listitem"> |
| <p class="simpara"> |
| evaluate left: |
| </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">value</span></code> is matched |
| </li></ul></div> |
| </li> |
| <li class="listitem"> |
| <p class="simpara"> |
| evaluate right: |
| </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">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> |
| <a name="phoenix.inside.actions.writing_an_action"></a><h5> |
| <a name="id848903"></a> |
| <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> |
| <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> |
| <td align="left"></td> |
| <td align="right"><div class="copyright-footer">Copyright © 2002-2005, 2010 Joel de Guzman, Dan Marsden, Thomas Heller<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> |
| </div></td> |
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