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24 | </div> | |
25 | <div class="section"> | |
26 | <div class="titlepage"><div><div><h3 class="title"> | |
27 | <a name="phoenix.examples.transforming_the_expression_tree"></a><a class="link" href="transforming_the_expression_tree.html" title="Transforming the Expression Tree">Transforming | |
28 | the Expression Tree</a> | |
29 | </h3></div></div></div> | |
30 | <p> | |
31 | This example will show how to write <a class="link" href="../inside/actions.html" title="More on Actions">Actions</a> | |
32 | that transform the Phoenix AST. | |
33 | </p> | |
34 | <div class="blockquote"><blockquote class="blockquote"> | |
35 | <p> | |
36 | "<span class="emphasis"><em>Lisp macros transform the program structure itself, with | |
37 | the full language available to express such transformations.</em></span>" | |
38 | </p> | |
39 | <p> | |
40 | <a href="http://en.wikipedia.org/wiki/Lisp_macro#Lisp_macros" target="_top">Wikipedia</a> | |
41 | </p> | |
42 | </blockquote></div> | |
43 | <p> | |
44 | What we want to do is to invert some arithmetic operators, i.e. plus will | |
45 | be transformed to minus, minus to plus, multiplication to division and division | |
46 | to multiplication. | |
47 | </p> | |
48 | <p> | |
49 | Let's start with defining our default action: | |
50 | </p> | |
51 | <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">invert_actions</span> | |
52 | <span class="special">{</span> | |
53 | <span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Rule</span><span class="special">></span> | |
54 | <span class="keyword">struct</span> <span class="identifier">when</span> | |
55 | <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span> <span class="comment">// the default is proto::_</span> | |
56 | <span class="special">{};</span> | |
57 | <span class="special">};</span> | |
58 | </pre> | |
59 | <p> | |
60 | By default, we don't want to do anything, well, not exactly nothing, but | |
61 | just return the expression. This is done by <a href="http://www.boost.org/doc/libs/release/doc/html/boost/proto/_.html" target="_top">proto::_</a> | |
62 | which, used as a transform, just passes the current expression along. Making | |
63 | this action an identity transform. | |
64 | </p> | |
65 | <p> | |
66 | So, after the basics are set up, we can start by writing the transformations | |
67 | we want to have on our tree: | |
68 | </p> | |
69 | <pre class="programlisting"><span class="comment">// Transform plus to minus</span> | |
70 | <span class="keyword">template</span> <span class="special"><></span> | |
71 | <span class="keyword">struct</span> <span class="identifier">invert_actions</span><span class="special">::</span><span class="identifier">when</span><span class="special"><</span><span class="identifier">phoenix</span><span class="special">::</span><span class="identifier">rule</span><span class="special">::</span><span class="identifier">plus</span><span class="special">></span> | |
72 | <span class="special">:</span> <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><span class="special"><</span> | |
73 | <a href="http://www.boost.org/doc/libs/release/doc/html/boost/proto/functional/make_expr.html" target="_top"><code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">functional</span><span class="special">::</span><span class="identifier">make_expr</span></code></a><span class="special"><</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">minus</span><span class="special">>(</span> | |
74 | <span class="identifier">phoenix</span><span class="special">::</span><span class="identifier">evaluator</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_left</span><span class="special">,</span> <span class="identifier">phoenix</span><span class="special">::</span><span class="identifier">_context</span><span class="special">)</span> | |
75 | <span class="special">,</span> <span class="identifier">phoenix</span><span class="special">::</span><span class="identifier">evaluator</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_right</span><span class="special">,</span> <span class="identifier">phoenix</span><span class="special">::</span><span class="identifier">_context</span><span class="special">)</span> | |
76 | <span class="special">)</span> | |
77 | <span class="special">></span> | |
78 | <span class="special">{};</span> | |
79 | </pre> | |
80 | <p> | |
81 | Wow, this looks complicated! Granted you need to know a little bit about | |
82 | <a href="http://www.boost.org/doc/libs/release/libs/proto/index.html" target="_top">Boost.Proto</a> | |
83 | (For a good introduction read through the <a href="http://cpp-next.com/archive/2010/08/expressive-c-introduction/" target="_top">Expressive | |
84 | C++</a> series). | |
85 | </p> | |
86 | <p> | |
87 | What is done is the following: | |
88 | </p> | |
89 | <div class="itemizedlist"><ul class="itemizedlist" type="disc"> | |
90 | <li class="listitem"> | |
91 | The left expression is passed to evaluator (with the current context, | |
92 | that contains our invert_actions) | |
93 | </li> | |
94 | <li class="listitem"> | |
95 | The right expression is passed to evaluator (with the current context, | |
96 | that contains our invert_actions) | |
97 | </li> | |
98 | <li class="listitem"> | |
99 | The result of these two <a href="http://www.boost.org/doc/libs/release/doc/html/Transform.html" target="_top">Proto | |
100 | Transforms</a> is passed to <a href="http://www.boost.org/doc/libs/release/doc/html/boost/proto/functional/make_expr.html" target="_top"><code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">functional</span><span class="special">::</span><span class="identifier">make_expr</span></code></a> which returns the | |
101 | freshly created expression | |
102 | </li> | |
103 | </ul></div> | |
104 | <p> | |
105 | After you know what is going on, maybe the rest doesn't look so scary anymore: | |
106 | </p> | |
107 | <pre class="programlisting"><span class="comment">// Transform minus to plus</span> | |
108 | <span class="keyword">template</span> <span class="special"><></span> | |
109 | <span class="keyword">struct</span> <span class="identifier">invert_actions</span><span class="special">::</span><span class="identifier">when</span><span class="special"><</span><span class="identifier">phoenix</span><span class="special">::</span><span class="identifier">rule</span><span class="special">::</span><span class="identifier">minus</span><span class="special">></span> | |
110 | <span class="special">:</span> <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><span class="special"><</span> | |
111 | <a href="http://www.boost.org/doc/libs/release/doc/html/boost/proto/functional/make_expr.html" target="_top"><code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">functional</span><span class="special">::</span><span class="identifier">make_expr</span></code></a><span class="special"><</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">plus</span><span class="special">>(</span> | |
112 | <span class="identifier">phoenix</span><span class="special">::</span><span class="identifier">evaluator</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_left</span><span class="special">,</span> <span class="identifier">phoenix</span><span class="special">::</span><span class="identifier">_context</span><span class="special">)</span> | |
113 | <span class="special">,</span> <span class="identifier">phoenix</span><span class="special">::</span><span class="identifier">evaluator</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_right</span><span class="special">,</span> <span class="identifier">phoenix</span><span class="special">::</span><span class="identifier">_context</span><span class="special">)</span> | |
114 | <span class="special">)</span> | |
115 | <span class="special">></span> | |
116 | <span class="special">{};</span> | |
117 | ||
118 | <span class="comment">// Transform multiplies to divides</span> | |
119 | <span class="keyword">template</span> <span class="special"><></span> | |
120 | <span class="keyword">struct</span> <span class="identifier">invert_actions</span><span class="special">::</span><span class="identifier">when</span><span class="special"><</span><span class="identifier">phoenix</span><span class="special">::</span><span class="identifier">rule</span><span class="special">::</span><span class="identifier">multiplies</span><span class="special">></span> | |
121 | <span class="special">:</span> <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><span class="special"><</span> | |
122 | <a href="http://www.boost.org/doc/libs/release/doc/html/boost/proto/functional/make_expr.html" target="_top"><code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">functional</span><span class="special">::</span><span class="identifier">make_expr</span></code></a><span class="special"><</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">divides</span><span class="special">>(</span> | |
123 | <span class="identifier">phoenix</span><span class="special">::</span><span class="identifier">evaluator</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_left</span><span class="special">,</span> <span class="identifier">phoenix</span><span class="special">::</span><span class="identifier">_context</span><span class="special">)</span> | |
124 | <span class="special">,</span> <span class="identifier">phoenix</span><span class="special">::</span><span class="identifier">evaluator</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_right</span><span class="special">,</span> <span class="identifier">phoenix</span><span class="special">::</span><span class="identifier">_context</span><span class="special">)</span> | |
125 | <span class="special">)</span> | |
126 | <span class="special">></span> | |
127 | <span class="special">{};</span> | |
128 | ||
129 | <span class="comment">// Transform divides to multiplies</span> | |
130 | <span class="keyword">template</span> <span class="special"><></span> | |
131 | <span class="keyword">struct</span> <span class="identifier">invert_actions</span><span class="special">::</span><span class="identifier">when</span><span class="special"><</span><span class="identifier">phoenix</span><span class="special">::</span><span class="identifier">rule</span><span class="special">::</span><span class="identifier">divides</span><span class="special">></span> | |
132 | <span class="special">:</span> <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><span class="special"><</span> | |
133 | <a href="http://www.boost.org/doc/libs/release/doc/html/boost/proto/functional/make_expr.html" target="_top"><code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">functional</span><span class="special">::</span><span class="identifier">make_expr</span></code></a><span class="special"><</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">multiplies</span><span class="special">>(</span> | |
134 | <span class="identifier">phoenix</span><span class="special">::</span><span class="identifier">evaluator</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_left</span><span class="special">,</span> <span class="identifier">phoenix</span><span class="special">::</span><span class="identifier">_context</span><span class="special">)</span> | |
135 | <span class="special">,</span> <span class="identifier">phoenix</span><span class="special">::</span><span class="identifier">evaluator</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_right</span><span class="special">,</span> <span class="identifier">phoenix</span><span class="special">::</span><span class="identifier">_context</span><span class="special">)</span> | |
136 | <span class="special">)</span> | |
137 | <span class="special">></span> | |
138 | <span class="special">{};</span> | |
139 | </pre> | |
140 | <p> | |
141 | That's it! Now that we have our actions defined, we want to evaluate some | |
142 | of our expressions with them: | |
143 | </p> | |
144 | <pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">></span> | |
145 | <span class="comment">// Calculate the result type: our transformed AST</span> | |
146 | <span class="keyword">typename</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">result_of</span><span class="special"><</span> | |
147 | <span class="identifier">phoenix</span><span class="special">::</span><span class="identifier">evaluator</span><span class="special">(</span> | |
148 | <span class="identifier">Expr</span> <span class="keyword">const</span><span class="special">&</span> | |
149 | <span class="special">,</span> <span class="identifier">phoenix</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">context</span><span class="special"><</span><span class="keyword">int</span><span class="special">,</span> <span class="identifier">invert_actions</span><span class="special">>::</span><span class="identifier">type</span> | |
150 | <span class="special">)</span> | |
151 | <span class="special">>::</span><span class="identifier">type</span> | |
152 | <span class="identifier">invert</span><span class="special">(</span><span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&</span> <span class="identifier">expr</span><span class="special">)</span> | |
153 | <span class="special">{</span> | |
154 | <span class="keyword">return</span> | |
155 | <span class="comment">// Evaluate it with our actions</span> | |
156 | <span class="identifier">phoenix</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span> | |
157 | <span class="identifier">expr</span> | |
158 | <span class="special">,</span> <span class="identifier">phoenix</span><span class="special">::</span><span class="identifier">context</span><span class="special">(</span> | |
159 | <span class="keyword">int</span><span class="special">()</span> | |
160 | <span class="special">,</span> <span class="identifier">invert_actions</span><span class="special">()</span> | |
161 | <span class="special">)</span> | |
162 | <span class="special">);</span> | |
163 | <span class="special">}</span> | |
164 | </pre> | |
165 | <p> | |
166 | Run some tests to see if it is working: | |
167 | </p> | |
168 | <pre class="programlisting"><span class="identifier">invert</span><span class="special">(</span><span class="identifier">_1</span><span class="special">);</span> <span class="comment">// --> _1</span> | |
169 | <span class="identifier">invert</span><span class="special">(</span><span class="identifier">_1</span> <span class="special">+</span> <span class="identifier">_2</span><span class="special">);</span> <span class="comment">// --> _1 - _2</span> | |
170 | <span class="identifier">invert</span><span class="special">(</span><span class="identifier">_1</span> <span class="special">+</span> <span class="identifier">_2</span> <span class="special">-</span> <span class="identifier">_3</span><span class="special">);</span> <span class="comment">// --> _1 - _2 + _3</span> | |
171 | <span class="identifier">invert</span><span class="special">(</span><span class="identifier">_1</span> <span class="special">*</span> <span class="identifier">_2</span><span class="special">);</span> <span class="comment">// --> _1 / _2</span> | |
172 | <span class="identifier">invert</span><span class="special">(</span><span class="identifier">_1</span> <span class="special">*</span> <span class="identifier">_2</span> <span class="special">/</span> <span class="identifier">_3</span><span class="special">);</span> <span class="comment">// --> _1 / _2 * _3</span> | |
173 | <span class="identifier">invert</span><span class="special">(</span><span class="identifier">_1</span> <span class="special">*</span> <span class="identifier">_2</span> <span class="special">+</span> <span class="identifier">_3</span><span class="special">);</span> <span class="comment">// --> _1 / _2 - _3</span> | |
174 | <span class="identifier">invert</span><span class="special">(</span><span class="identifier">_1</span> <span class="special">*</span> <span class="identifier">_2</span> <span class="special">-</span> <span class="identifier">_3</span><span class="special">);</span> <span class="comment">// --> _1 / _2 + _2</span> | |
175 | <span class="identifier">invert</span><span class="special">(</span><span class="identifier">if_</span><span class="special">(</span><span class="identifier">_1</span> <span class="special">*</span> <span class="identifier">_4</span><span class="special">)[</span><span class="identifier">_2</span> <span class="special">-</span> <span class="identifier">_3</span><span class="special">]);</span> <span class="comment">// --> if_(_1 / _4)[_2 + _3]</span> | |
176 | <span class="identifier">_1</span> <span class="special">*</span> <span class="identifier">invert</span><span class="special">(</span><span class="identifier">_2</span> <span class="special">-</span> <span class="identifier">_3</span><span class="special">));</span> <span class="comment">// --> _1 * _2 + _3</span> | |
177 | </pre> | |
178 | <p> | |
179 | <span class="inlinemediaobject"><img src="../../images/note.png"></span> | |
180 | The complete example can be found here: <a href="../../../../example/invert.cpp" target="_top">example/invert.cpp</a> | |
181 | </p> | |
182 | <p> | |
183 | <span class="emphasis"><em>Pretty simple ...</em></span> | |
184 | </p> | |
185 | </div> | |
186 | <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> | |
187 | <td align="left"></td> | |
188 | <td align="right"><div class="copyright-footer">Copyright © 2002-2005, 2010, 2014, 2015 Joel de Guzman, Dan Marsden, Thomas | |
189 | Heller, John Fletcher<p> | |
190 | Distributed under the Boost Software License, Version 1.0. (See accompanying | |
191 | 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>) | |
192 | </p> | |
193 | </div></td> | |
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