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24</div>
25<div class="section">
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>
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