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| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="phoenix.examples.transforming_the_expression_tree"></a><a class="link" href="transforming_the_expression_tree.html" title="Transforming the Expression Tree">Transforming |
| the Expression Tree</a> |
| </h3></div></div></div> |
| <p> |
| This example will show how to write <a class="link" href="../inside/actions.html" title="More on Actions">Actions</a> |
| that transform the Phoenix AST. |
| </p> |
| <div class="blockquote"><blockquote class="blockquote"> |
| <p> |
| "<span class="emphasis"><em>Lisp macros transform the program structure itself, with |
| the full language available to express such transformations.</em></span>" |
| </p> |
| <p> |
| <a href="http://en.wikipedia.org/wiki/Lisp_macro#Lisp_macros" target="_top">Wikipedia</a> |
| </p> |
| </blockquote></div> |
| <p> |
| What we want to do is to invert some arithmetic operators, i.e. plus will |
| be transformed to minus, minus to plus, multiplication to division and division |
| to multiplication. |
| </p> |
| <p> |
| Let's start with defining our default action: |
| </p> |
| <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">invert_actions</span> |
| <span class="special">{</span> |
| <span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Rule</span><span class="special">></span> |
| <span class="keyword">struct</span> <span class="identifier">when</span> |
| <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> <span class="special">{};</span> |
| <span class="special">};</span> |
| </pre> |
| <p> |
| By default, we don't want to do anything, well, not exactly nothing, but |
| 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> |
| which, used as a transform, just passes the current expression along. Making |
| this action an identity transform. |
| </p> |
| <p> |
| So, after the basics are set up, we can start by writing the transformations |
| we want to have on our tree: |
| </p> |
| <pre class="programlisting"><span class="comment">// Transform plus to minus |
| </span><span class="keyword">template</span> <span class="special"><></span> |
| <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> |
| <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> |
| <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> |
| <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> |
| <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> |
| <span class="special">)</span> |
| <span class="special">></span> |
| <span class="special">{};</span> |
| </pre> |
| <p> |
| Wow, this looks complicated! Granted you need to know a little bit about |
| <a href="http://www.boost.org/doc/libs/release/libs/proto/index.html" target="_top">Boost.Proto</a> |
| (For a good introduction read through the <a href="http://cpp-next.com/archive/2010/08/expressive-c-introduction/" target="_top">Expressive |
| C++</a> series). |
| </p> |
| <p> |
| What is done is the following: |
| </p> |
| <div class="itemizedlist"><ul class="itemizedlist" type="disc"> |
| <li class="listitem"> |
| The left expression is passed to evaluator (with the current context, |
| that contains our invert_actions) |
| </li> |
| <li class="listitem"> |
| The right expression is passed to evaluator (with the current context, |
| that contains our invert_actions) |
| </li> |
| <li class="listitem"> |
| The result of these two <a href="http://www.boost.org/doc/libs/release/doc/html/Transform.html" target="_top">Proto |
| 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 |
| freshly created expression |
| </li> |
| </ul></div> |
| <p> |
| After you know what is going on, maybe the rest doesn't look so scary anymore: |
| </p> |
| <pre class="programlisting"><span class="comment">// Transform minus to plus |
| </span><span class="keyword">template</span> <span class="special"><></span> |
| <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> |
| <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> |
| <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> |
| <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> |
| <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> |
| <span class="special">)</span> |
| <span class="special">></span> |
| <span class="special">{};</span> |
| |
| <span class="comment">// Transform multiplies to divides |
| </span><span class="keyword">template</span> <span class="special"><></span> |
| <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> |
| <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> |
| <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> |
| <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> |
| <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> |
| <span class="special">)</span> |
| <span class="special">></span> |
| <span class="special">{};</span> |
| |
| <span class="comment">// Transform divides to multiplies |
| </span><span class="keyword">template</span> <span class="special"><></span> |
| <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> |
| <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> |
| <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> |
| <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> |
| <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> |
| <span class="special">)</span> |
| <span class="special">></span> |
| <span class="special">{};</span> |
| </pre> |
| <p> |
| That's it! Now that we have our actions defined, we want to evaluate some |
| of our expressions with them: |
| </p> |
| <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> |
| <span class="comment">// Calculate the result type: our transformed AST |
| </span><span class="keyword">typename</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">result_of</span><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">Expr</span> <span class="keyword">const</span><span class="special">&</span> |
| <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> |
| <span class="special">)</span> |
| <span class="special">>::</span><span class="identifier">type</span> |
| <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> |
| <span class="special">{</span> |
| <span class="keyword">return</span> |
| <span class="comment">// Evaluate it with our actions |
| </span> <span class="identifier">phoenix</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span> |
| <span class="identifier">expr</span> |
| <span class="special">,</span> <span class="identifier">phoenix</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="special">,</span> <span class="identifier">invert_actions</span><span class="special">()</span> |
| <span class="special">)</span> |
| <span class="special">);</span> |
| <span class="special">}</span> |
| </pre> |
| <p> |
| Run some tests to see if it is working: |
| </p> |
| <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><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><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><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><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><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><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><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><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></pre> |
| <p> |
| <span class="inlinemediaobject"><img src="../../images/note.png" alt="note"></span> The complete example can be found here: <a href="../../../../example/invert.cpp" target="_top">example/invert.cpp</a> |
| </p> |
| <p> |
| <span class="emphasis"><em>Pretty simple ...</em></span> |
| </p> |
| </div> |
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| <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>) |
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