ceph/src/boost/libs/phoenix/doc/html/phoenix/inside/actions.html

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  26 <div class="titlepage"><div><div><h3 class="title">
  27 <a name="phoenix.inside.actions"></a><a class="link" href="actions.html" title="More on Actions">More on Actions</a>
  28 </h3></div></div></div>
  29 <p>
  30         As you know from the <a class="link" href="actor.html" title="Actors in Detail">Actors in Detail</a>
  31         section, Actions are what brings life to a Phoenix expression tree.
  32       </p>
  33 <p>
  34         When dealing with a Phoenix expression tree, it gets evaluated top-down.
  35         Example:
  36       </p>
  37 <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>
  38 </pre>
  39 <p>
  40         Can be visualized as an AST in the following way:
  41       </p>
  42 <p>
  43         <span class="inlinemediaobject"><img src="../../images/simple_ast.png"></span>
  44       </p>
  45 <p>
  46         In terms of actions this means:
  47       </p>
  48 <div class="itemizedlist"><ul class="itemizedlist" type="disc">
  49 <li class="listitem">
  50             <code class="computeroutput"><span class="identifier">rule</span><span class="special">::</span><span class="identifier">plus</span></code> is matched
  51           </li>
  52 <li class="listitem">
  53             evaluate left:
  54             <div class="itemizedlist"><ul class="itemizedlist" type="circle"><li class="listitem">
  55                   <code class="computeroutput"><span class="identifier">rule</span><span class="special">::</span><span class="identifier">placeholder</span></code> is matched
  56                 </li></ul></div>
  57           </li>
  58 <li class="listitem">
  59             evaluate right:
  60             <div class="itemizedlist"><ul class="itemizedlist" type="circle"><li class="listitem">
  61                   <code class="computeroutput"><span class="identifier">rule</span><span class="special">::</span><span class="identifier">multiplies</span></code> is matched
  62                   <div class="itemizedlist"><ul class="itemizedlist" type="square">
  63 <li class="listitem">
  64                         evaluate left:
  65                         <div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem">
  66                               <code class="computeroutput"><span class="identifier">rule</span><span class="special">::</span><span class="identifier">value</span></code>
  67                               is matched
  68                             </li></ul></div>
  69                       </li>
  70 <li class="listitem">
  71                         evaluate right:
  72                         <div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem">
  73                               <code class="computeroutput"><span class="identifier">rule</span><span class="special">::</span><span class="identifier">placeholder</span></code>
  74                               is matched
  75                             </li></ul></div>
  76                       </li>
  77 </ul></div>
  78                 </li></ul></div>
  79           </li>
  80 </ul></div>
  81 <p>
  82         Every time a rule is matched, an action will be called. The action determines
  83         how the Phoenix AST will be traversed.
  84       </p>
  85 <h5>
  86 <a name="phoenix.inside.actions.h0"></a>
  87         <span><a name="phoenix.inside.actions.writing_an_action"></a></span><a class="link" href="actions.html#phoenix.inside.actions.writing_an_action">Writing
  88         an Action</a>
  89       </h5>
  90 <p>
  91         As mentioned in <a class="link" href="actor.html" title="Actors in Detail">Actors in Detail</a>
  92         actions are <a href="http://www.boost.org/doc/libs/release/doc/html/PrimitiveTransform.html" target="_top">Proto
  93         Primitive Transforms</a> for convenience Phoenix provides an abstraction
  94         to this:
  95       </p>
  96 <pre class="programlisting"><span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Fun</span><span class="special">&gt;</span>
  97 <span class="keyword">struct</span> <span class="identifier">call</span><span class="special">;</span>
  98 </pre>
  99 <p>
 100         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
 101         <code class="computeroutput"><span class="identifier">Fun</span></code> function object passed
 102         as template parameter with the <code class="computeroutput"><span class="identifier">Context</span></code>
 103         and the children of the expression associated with the rule.
 104       </p>
 105 <p>
 106         Lets have an (simplified) example on how to write an evaluation action for
 107         <code class="computeroutput"><span class="identifier">rule</span><span class="special">::</span><span class="identifier">plus</span></code>:
 108       </p>
 109 <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">plus_eval</span>
 110 <span class="special">{</span>
 111     <span class="keyword">typedef</span> <span class="keyword">int</span> <span class="identifier">result_type</span><span class="special">;</span>
 112
 113     <span class="keyword">template</span> <span class="special">&lt;</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">&gt;</span>
 114     <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">&amp;</span> <span class="identifier">lhs</span><span class="special">,</span> <span class="identifier">Rhs</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">rhs</span><span class="special">,</span> <span class="identifier">Context</span> <span class="special">&amp;</span> <span class="identifier">ctx</span><span class="special">)</span>
 115     <span class="special">{</span>
 116         <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>
 117     <span class="special">}</span>
 118 <span class="special">};</span>
 119
 120 <span class="keyword">template</span> <span class="special">&lt;&gt;</span>
 121 <span class="keyword">struct</span> <span class="identifier">default_actions</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span><span class="identifier">rule</span><span class="special">::</span><span class="identifier">plus</span><span class="special">&gt;</span>
 122     <span class="special">:</span> <span class="identifier">call</span><span class="special">&lt;</span><span class="identifier">plus_eval</span><span class="special">&gt;</span>
 123 <span class="special">{};</span>
 124 </pre>
 125 <p>
 126         That's it. When evaluating a <code class="computeroutput"><span class="identifier">plus</span></code>
 127         expression, the <code class="computeroutput"><span class="identifier">plus_eval</span></code>
 128         callable gets called with the left hand side and right hand side expression
 129         and the associated Context.
 130       </p>
 131 <p>
 132         <span class="bold"><strong>But there is more:</strong></span> As Actions <span class="emphasis"><em>can</em></span>
 133         be full fletched <a href="http://www.boost.org/doc/libs/release/doc/html/Transform.html" target="_top">Proto
 134         Transforms</a>, you can in fact use any proto expression you can imagine
 135         as the action. Phoenix predifines a set of callables and transform to deal
 136         with the Context information passed along and of course every Phoenix expression
 137         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
 138         Pass Through Transform</a>.
 139       </p>
 140 <div class="variablelist">
 141 <p class="title"><b></b></p>
 142 <dl>
 143 <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>
 144 <dd><p>
 145               A <a href="http://www.boost.org/doc/libs/release/doc/html/Callable.html" target="_top">Proto
 146               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>
 147               parameter
 148             </p></dd>
 149 <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>
 150 <dd><p>
 151               A <a href="http://www.boost.org/doc/libs/release/doc/html/Callable.html" target="_top">Proto
 152               Callable Transform</a> that returns the environment out of the
 153               <code class="computeroutput"><span class="identifier">Context</span></code> parameter
 154             </p></dd>
 155 <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>
 156 <dd><p>
 157               A <a href="http://www.boost.org/doc/libs/release/doc/html/Callable.html" target="_top">Proto
 158               Callable Transform</a> that returns the actions out of the <code class="computeroutput"><span class="identifier">Context</span></code> parameter
 159             </p></dd>
 160 <dt><span class="term"><code class="computeroutput"><span class="identifier">_context</span></code></span></dt>
 161 <dd><p>
 162               A <a href="http://www.boost.org/doc/libs/release/doc/html/PrimitiveTransform.html" target="_top">Proto
 163               Primitive Transform</a> that returns the current context
 164             </p></dd>
 165 <dt><span class="term"><code class="computeroutput"><span class="identifier">_env</span></code></span></dt>
 166 <dd><p>
 167               A <a href="http://www.boost.org/doc/libs/release/doc/html/PrimitiveTransform.html" target="_top">Proto
 168               Primitive Transform</a> that returns the current environment
 169             </p></dd>
 170 <dt><span class="term"><code class="computeroutput"><span class="identifier">_actions</span></code></span></dt>
 171 <dd><p>
 172               A <a href="http://www.boost.org/doc/libs/release/doc/html/PrimitiveTransform.html" target="_top">Proto
 173               Primitive Transform</a> that returns the current actions
 174             </p></dd>
 175 <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>
 176 <dd><p>
 177               A regular function that creates a context
 178             </p></dd>
 179 <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>
 180 <dd><p>
 181               A regular function that returns the environment from the given context
 182             </p></dd>
 183 <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>
 184 <dd><p>
 185               A regular function that returns the actions from the given context
 186             </p></dd>
 187 </dl>
 188 </div>
 189 <p>
 190         Phoenix is equipped with a predefined set of expressions, rules and actions
 191         to make all the stuff work you learned in the <a class="link" href="../starter_kit.html" title="Starter Kit">Starter
 192         Kit</a> and <a class="link" href="../modules.html" title="Modules">Modules</a> sections. See
 193         the <a class="link" href="rules.html" title="Predefined Expressions and Rules">next section</a> for more details!
 194       </p>
 195 </div>
 196 <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
 197 <td align="left"></td>
 198 <td align="right"><div class="copyright-footer">Copyright &#169; 2002-2005, 2010, 2014, 2015 Joel de Guzman, Dan Marsden, Thomas
 199       Heller, John Fletcher<p>
 200         Distributed under the Boost Software License, Version 1.0. (See accompanying
 201         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>)
 202       </p>
 203 </div></td>
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