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- </div>
- <div class="section">
- <div class="titlepage"><div><div><h3 class="title">
- <a name="spirit_x3.tutorials.rexpr"></a><a class="link" href="rexpr.html" title="RExpressions - Recursive ASTs!">RExpressions - Recursive ASTs!</a>
- </h3></div></div></div>
- <p>
- In this example, we'll explore more on how to create heierarchical ASTs.
- We will parse a minimalistic JSON-like language and compile the results into
- our data structures in the form of a tree.
- </p>
- <p>
- <span class="emphasis"><em>rexpr</em></span> is a parser for RExpressions, a language resembling
- a minimal subset of json, limited to a dictionary (composed of key=value
- pairs) where the value can itself be a string or a recursive dictionary.
- </p>
- <p>
- Here's an Example:
- </p>
- <pre class="programlisting"><span class="special">{</span>
- <span class="string">"color"</span> <span class="special">=</span> <span class="string">"blue"</span>
- <span class="string">"size"</span> <span class="special">=</span> <span class="string">"29 cm."</span>
- <span class="string">"position"</span> <span class="special">=</span> <span class="special">{</span>
- <span class="string">"x"</span> <span class="special">=</span> <span class="string">"123"</span>
- <span class="string">"y"</span> <span class="special">=</span> <span class="string">"456"</span>
- <span class="special">}</span>
- <span class="special">}</span>
- </pre>
- <p>
- This simple parser for X3 is intended as a minimal starting point. It is
- minimal, yet complete with all things necessary parts that make up rules
- (and grammars), except error handling and reporting.
- </p>
- <p>
- The example can be found here: <a href="../../../../../example/x3/rexpr/rexpr_min/rexpr.cpp" target="_top">rexpr.cpp</a>
- </p>
- <p>
- The same parser, but complete with error handling and reporting, better file
- organization, separate compilation of grammars, and a full test harness,
- can be found in this directory:
- </p>
- <p>
- <a href="../../../../../example/x3/rexpr/rexpr_full/" target="_top">rexpr/rexpr_full/</a>
- </p>
- <div class="note"><table border="0" summary="Note">
- <tr>
- <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../images/note.png"></td>
- <th align="left">Note</th>
- </tr>
- <tr><td align="left" valign="top"><p>
- rexpr_full is the canonical structure proposed as best practice on how
- parsers are written using Spirit X3.
- </p></td></tr>
- </table></div>
- <h5>
- <a name="spirit_x3.tutorials.rexpr.h0"></a>
- <span class="phrase"><a name="spirit_x3.tutorials.rexpr.the_ast"></a></span><a class="link" href="rexpr.html#spirit_x3.tutorials.rexpr.the_ast">The
- AST</a>
- </h5>
- <p>
- Here's the AST:
- </p>
- <pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">client</span> <span class="special">{</span> <span class="keyword">namespace</span> <span class="identifier">ast</span>
- <span class="special">{</span>
- <span class="keyword">namespace</span> <span class="identifier">x3</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">x3</span><span class="special">;</span>
- <span class="keyword">struct</span> <span class="identifier">rexpr</span><span class="special">;</span>
- <span class="keyword">struct</span> <span class="identifier">rexpr_value</span> <span class="special">:</span> <span class="identifier">x3</span><span class="special">::</span><span class="identifier">variant</span><span class="special"><</span>
- <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span>
- <span class="special">,</span> <span class="identifier">x3</span><span class="special">::</span><span class="identifier">forward_ast</span><span class="special"><</span><span class="identifier">rexpr</span><span class="special">></span>
- <span class="special">></span>
- <span class="special">{</span>
- <span class="keyword">using</span> <span class="identifier">base_type</span><span class="special">::</span><span class="identifier">base_type</span><span class="special">;</span>
- <span class="keyword">using</span> <span class="identifier">base_type</span><span class="special">::</span><span class="keyword">operator</span><span class="special">=;</span>
- <span class="special">};</span>
- <span class="keyword">typedef</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">map</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">,</span> <span class="identifier">rexpr_value</span><span class="special">></span> <span class="identifier">rexpr_map</span><span class="special">;</span>
- <span class="keyword">typedef</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">,</span> <span class="identifier">rexpr_value</span><span class="special">></span> <span class="identifier">rexpr_key_value</span><span class="special">;</span>
- <span class="keyword">struct</span> <span class="identifier">rexpr</span>
- <span class="special">{</span>
- <span class="identifier">rexpr_map</span> <span class="identifier">entries</span><span class="special">;</span>
- <span class="special">};</span>
- <span class="special">}}</span>
- </pre>
- <p>
- <code class="computeroutput"><span class="identifier">x3</span><span class="special">::</span><span class="identifier">variant</span></code> is a support utility in Spirit
- X3 that extends <a href="http://www.boost.org/doc/html/variant.html" target="_top">Boost.Variant</a>.
- Typically, you use <a href="http://www.boost.org/doc/html/variant.html" target="_top">Boost.Variant</a>
- right out of the box and refer to a particular template instantiation using
- a typedef. For example:
- </p>
- <pre class="programlisting"><span class="keyword">typedef</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">variant</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">,</span> <span class="keyword">int</span><span class="special">></span> <span class="identifier">my_variant</span><span class="special">;</span>
- </pre>
- <p>
- Instead of doing that, we create a <code class="computeroutput"><span class="keyword">class</span></code>
- (or <code class="computeroutput"><span class="keyword">struct</span></code> in the case) that
- subclasses from <code class="computeroutput"><span class="identifier">x3</span><span class="special">::</span><span class="identifier">variant</span></code>. By making the variant a subclass,
- you have a distinct type in your namespace. You also have control of the
- constructors and assignment operators, as well as giving you the freedom
- to add more functionality.
- </p>
- <p>
- <code class="computeroutput"><span class="identifier">rexpr_value</span></code> has two variant
- elements. It can either be a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span></code>
- or a <code class="computeroutput"><span class="identifier">rexpr</span></code>. Since <code class="computeroutput"><span class="identifier">rexpr</span></code> recursively contains a <code class="computeroutput"><span class="identifier">rexpr_value</span></code>, it has to be forward declared.
- This recursive data structure requires <code class="computeroutput"><span class="identifier">rexpr</span></code>
- to be wrapped in a <code class="computeroutput"><span class="identifier">x3</span><span class="special">::</span><span class="identifier">forward_ast</span></code> as shown in the declaration.
- </p>
- <p>
- We need to tell fusion about our <code class="computeroutput"><span class="identifier">rexpr</span></code>
- struct to make it a first-class fusion citizen:
- </p>
- <pre class="programlisting"><span class="identifier">BOOST_FUSION_ADAPT_STRUCT</span><span class="special">(</span>
- <span class="identifier">client</span><span class="special">::</span><span class="identifier">ast</span><span class="special">::</span><span class="identifier">rexpr</span><span class="special">,</span>
- <span class="identifier">entries</span>
- <span class="special">)</span>
- </pre>
- <p>
- So essentially, a <code class="computeroutput"><span class="identifier">rexpr_value</span></code>
- value is either a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span></code> or a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">map</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">,</span> <span class="identifier">rexpr_value</span><span class="special">></span></code>.
- </p>
- <h5>
- <a name="spirit_x3.tutorials.rexpr.h1"></a>
- <span class="phrase"><a name="spirit_x3.tutorials.rexpr.walking_the_ast"></a></span><a class="link" href="rexpr.html#spirit_x3.tutorials.rexpr.walking_the_ast">Walking
- the AST</a>
- </h5>
- <p>
- We add a utility to print out the AST:
- </p>
- <pre class="programlisting"><span class="keyword">int</span> <span class="keyword">const</span> <span class="identifier">tabsize</span> <span class="special">=</span> <span class="number">4</span><span class="special">;</span>
- <span class="keyword">struct</span> <span class="identifier">rexpr_printer</span>
- <span class="special">{</span>
- <span class="keyword">typedef</span> <span class="keyword">void</span> <span class="identifier">result_type</span><span class="special">;</span>
- <span class="identifier">rexpr_printer</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">indent</span> <span class="special">=</span> <span class="number">0</span><span class="special">)</span>
- <span class="special">:</span> <span class="identifier">indent</span><span class="special">(</span><span class="identifier">indent</span><span class="special">)</span> <span class="special">{}</span>
- <span class="keyword">void</span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">rexpr</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">ast</span><span class="special">)</span> <span class="keyword">const</span>
- <span class="special">{</span>
- <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="char">'{'</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
- <span class="keyword">for</span> <span class="special">(</span><span class="keyword">auto</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">entry</span> <span class="special">:</span> <span class="identifier">ast</span><span class="special">.</span><span class="identifier">entries</span><span class="special">)</span>
- <span class="special">{</span>
- <span class="identifier">tab</span><span class="special">(</span><span class="identifier">indent</span><span class="special">+</span><span class="identifier">tabsize</span><span class="special">);</span>
- <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="char">'"'</span> <span class="special"><<</span> <span class="identifier">entry</span><span class="special">.</span><span class="identifier">first</span> <span class="special"><<</span> <span class="string">"\" = "</span><span class="special">;</span>
- <span class="identifier">boost</span><span class="special">::</span><span class="identifier">apply_visitor</span><span class="special">(</span><span class="identifier">rexpr_printer</span><span class="special">(</span><span class="identifier">indent</span><span class="special">+</span><span class="identifier">tabsize</span><span class="special">),</span> <span class="identifier">entry</span><span class="special">.</span><span class="identifier">second</span><span class="special">);</span>
- <span class="special">}</span>
- <span class="identifier">tab</span><span class="special">(</span><span class="identifier">indent</span><span class="special">);</span>
- <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="char">'}'</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
- <span class="special">}</span>
- <span class="keyword">void</span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">text</span><span class="special">)</span> <span class="keyword">const</span>
- <span class="special">{</span>
- <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="char">'"'</span> <span class="special"><<</span> <span class="identifier">text</span> <span class="special"><<</span> <span class="char">'"'</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
- <span class="special">}</span>
- <span class="keyword">void</span> <span class="identifier">tab</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">spaces</span><span class="special">)</span> <span class="keyword">const</span>
- <span class="special">{</span>
- <span class="keyword">for</span> <span class="special">(</span><span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> <span class="identifier">i</span> <span class="special"><</span> <span class="identifier">spaces</span><span class="special">;</span> <span class="special">++</span><span class="identifier">i</span><span class="special">)</span>
- <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="char">' '</span><span class="special">;</span>
- <span class="special">}</span>
- <span class="keyword">int</span> <span class="identifier">indent</span><span class="special">;</span>
- <span class="special">};</span>
- </pre>
- <p>
- Traversing the AST is a recursive excercise. <code class="computeroutput"><span class="identifier">rexpr_printer</span></code>
- is a function object and the main entry point is <code class="computeroutput"><span class="keyword">void</span>
- <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">rexpr</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">ast</span><span class="special">)</span></code>. Notice how it recursively calls an instance
- of itself for each of the key value pairs, using <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">apply_visitor</span></code>.
- Before and after iterating through all the elements in the map, the braces
- are printed to surround the entire body, taking care of correct indentation
- at each point in the recursive traversal.
- </p>
- <p>
- The <code class="computeroutput"><span class="keyword">operator</span><span class="special">()</span></code>
- for <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span></code> should be self explanatory. It simply
- prints the text inside double quotes.
- </p>
- <h5>
- <a name="spirit_x3.tutorials.rexpr.h2"></a>
- <span class="phrase"><a name="spirit_x3.tutorials.rexpr.the_grammar"></a></span><a class="link" href="rexpr.html#spirit_x3.tutorials.rexpr.the_grammar">The
- Grammar</a>
- </h5>
- <pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">client</span> <span class="special">{</span> <span class="keyword">namespace</span> <span class="identifier">parser</span>
- <span class="special">{</span>
- <span class="keyword">namespace</span> <span class="identifier">x3</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">x3</span><span class="special">;</span>
- <span class="keyword">namespace</span> <span class="identifier">ascii</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">x3</span><span class="special">::</span><span class="identifier">ascii</span><span class="special">;</span>
- <span class="keyword">using</span> <span class="identifier">x3</span><span class="special">::</span><span class="identifier">lit</span><span class="special">;</span>
- <span class="keyword">using</span> <span class="identifier">x3</span><span class="special">::</span><span class="identifier">lexeme</span><span class="special">;</span>
- <span class="keyword">using</span> <span class="identifier">ascii</span><span class="special">::</span><span class="identifier">char_</span><span class="special">;</span>
- <span class="keyword">using</span> <span class="identifier">ascii</span><span class="special">::</span><span class="identifier">string</span><span class="special">;</span>
- <span class="identifier">x3</span><span class="special">::</span><span class="identifier">rule</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">rexpr_value</span><span class="special">,</span> <span class="identifier">ast</span><span class="special">::</span><span class="identifier">rexpr_value</span><span class="special">></span>
- <span class="identifier">rexpr_value</span> <span class="special">=</span> <span class="string">"rexpr_value"</span><span class="special">;</span>
- <span class="identifier">x3</span><span class="special">::</span><span class="identifier">rule</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">rexpr</span><span class="special">,</span> <span class="identifier">ast</span><span class="special">::</span><span class="identifier">rexpr</span><span class="special">></span>
- <span class="identifier">rexpr</span> <span class="special">=</span> <span class="string">"rexpr"</span><span class="special">;</span>
- <span class="identifier">x3</span><span class="special">::</span><span class="identifier">rule</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">rexpr_key_value</span><span class="special">,</span> <span class="identifier">ast</span><span class="special">::</span><span class="identifier">rexpr_key_value</span><span class="special">></span>
- <span class="identifier">rexpr_key_value</span> <span class="special">=</span> <span class="string">"rexpr_key_value"</span><span class="special">;</span>
- <span class="keyword">auto</span> <span class="keyword">const</span> <span class="identifier">quoted_string</span> <span class="special">=</span>
- <span class="identifier">lexeme</span><span class="special">[</span><span class="char">'"'</span> <span class="special">>></span> <span class="special">*(</span><span class="identifier">char_</span> <span class="special">-</span> <span class="char">'"'</span><span class="special">)</span> <span class="special">>></span> <span class="char">'"'</span><span class="special">];</span>
- <span class="keyword">auto</span> <span class="keyword">const</span> <span class="identifier">rexpr_value_def</span> <span class="special">=</span>
- <span class="identifier">quoted_string</span> <span class="special">|</span> <span class="identifier">rexpr</span><span class="special">;</span>
- <span class="keyword">auto</span> <span class="keyword">const</span> <span class="identifier">rexpr_key_value_def</span> <span class="special">=</span>
- <span class="identifier">quoted_string</span> <span class="special">>></span> <span class="char">'='</span> <span class="special">>></span> <span class="identifier">rexpr_value</span><span class="special">;</span>
- <span class="keyword">auto</span> <span class="keyword">const</span> <span class="identifier">rexpr_def</span> <span class="special">=</span>
- <span class="char">'{'</span> <span class="special">>></span> <span class="special">*</span><span class="identifier">rexpr_key_value</span> <span class="special">>></span> <span class="char">'}'</span><span class="special">;</span>
- <span class="identifier">BOOST_SPIRIT_DEFINE</span><span class="special">(</span><span class="identifier">rexpr_value</span><span class="special">,</span> <span class="identifier">rexpr</span><span class="special">,</span> <span class="identifier">rexpr_key_value</span><span class="special">);</span>
- <span class="special">}}</span>
- </pre>
- <p>
- We declare three rules <code class="computeroutput"><span class="identifier">rexpr_value</span></code>,
- <code class="computeroutput"><span class="identifier">rexpr</span></code> and <code class="computeroutput"><span class="identifier">rexpr_key_value</span></code>.
- Same deal as before. The attributes declared for each rule are the same structures
- we declared in our AST above. These are the structures our rules will produce.
- </p>
- <p>
- So, going top-down (from the start rule), <code class="computeroutput"><span class="identifier">rexpr</span></code>
- is defined as zero or more <code class="computeroutput"><span class="identifier">rexpr_key_value</span></code>s
- enclosed inside the curly braces. That's the kleene star in action there:
- <code class="computeroutput"><span class="special">*</span><span class="identifier">rexpr_key_value</span></code>.
- </p>
- <div class="note"><table border="0" summary="Note">
- <tr>
- <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../images/note.png"></td>
- <th align="left">Note</th>
- </tr>
- <tr><td align="left" valign="top"><p>
- Take note of the convention: we use <code class="computeroutput"><span class="identifier">rexpr</span></code>
- name as the rule name and <code class="computeroutput"><span class="identifier">rexpr_def</span></code>
- as the rule definition name.
- </p></td></tr>
- </table></div>
- <p>
- To make the grammar clean, we capture the <code class="computeroutput"><span class="identifier">quoted_string</span></code>
- production using C++ <code class="computeroutput"><span class="keyword">auto</span></code>.
- </p>
- <p>
- <code class="computeroutput"><span class="identifier">rexpr_value</span></code> is a <code class="computeroutput"><span class="identifier">quoted_string</span></code> followed by the assignment
- operator <code class="computeroutput"><span class="char">'='</span></code>, followed by a <code class="computeroutput"><span class="identifier">rexpr_value</span></code>. <code class="computeroutput"><span class="identifier">rexpr_value</span></code>
- is a <code class="computeroutput"><span class="identifier">quoted_string</span></code> <span class="emphasis"><em>OR</em></span>
- a <code class="computeroutput"><span class="identifier">rexpr</span></code>. This uses the alternative
- operator <code class="computeroutput"><span class="special">|</span></code>, and maps nicely
- to the variant AST the rule is associated with.
- </p>
- <p>
- Finally, we associate the rules and their definitions:
- </p>
- <pre class="programlisting"><span class="identifier">BOOST_SPIRIT_DEFINE</span><span class="special">(</span><span class="identifier">rexpr_value</span><span class="special">,</span> <span class="identifier">rexpr</span><span class="special">,</span> <span class="identifier">rexpr_key_value</span><span class="special">);</span>
- </pre>
- </div>
- <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
- <td align="left"></td>
- <td align="right"><div class="copyright-footer">Copyright © 2001-2018 Joel de Guzman,
- Hartmut Kaiser<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>)
- </p>
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