1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 4<title>More on Actions</title> 5<link rel="stylesheet" href="../../../../../../doc/src/boostbook.css" type="text/css"> 6<meta name="generator" content="DocBook XSL Stylesheets V1.79.1"> 7<link rel="home" href="../../index.html" title="Chapter 1. Phoenix 3.2.0"> 8<link rel="up" href="../inside.html" title="Inside Phoenix"> 9<link rel="prev" href="expression/boilerplate_macros.html" title="Boilerplate Macros"> 10<link rel="next" href="rules.html" title="Predefined Expressions and Rules"> 11</head> 12<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> 13<table cellpadding="2" width="100%"><tr> 14<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../../boost.png"></td> 15<td align="center"><a href="../../../../../../index.html">Home</a></td> 16<td align="center"><a href="../../../../../../libs/libraries.htm">Libraries</a></td> 17<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> 18<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> 19<td align="center"><a href="../../../../../../more/index.htm">More</a></td> 20</tr></table> 21<hr> 22<div class="spirit-nav"> 23<a accesskey="p" href="expression/boilerplate_macros.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../inside.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="rules.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a> 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" style="list-style-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" style="list-style-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" style="list-style-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" style="list-style-type: square; "> 63<li class="listitem"> 64 evaluate left: 65 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-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" style="list-style-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 class="phrase"><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"><</span><span class="keyword">typename</span> <span class="identifier">Fun</span><span class="special">></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"><</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> 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">&</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> 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"><></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"><</span><span class="identifier">rule</span><span class="special">::</span><span class="identifier">plus</span><span class="special">></span> 122 <span class="special">:</span> <span class="identifier">call</span><span class="special"><</span><span class="identifier">plus_eval</span><span class="special">></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 class="variablelist"> 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 © 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> 204</tr></table> 205<hr> 206<div class="spirit-nav"> 207<a accesskey="p" href="expression/boilerplate_macros.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../inside.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="rules.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a> 208</div> 209</body> 210</html> 211
