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- <div class="titlepage"><div><div><h3 class="title">
- <a name="spirit_x3.abstracts.compound_attributes"></a><a class="link" href="compound_attributes.html" title="Attributes of Compound Components">Attributes of
- Compound Components</a>
- </h3></div></div></div>
- <p>
- <span class="emphasis"><em>Spirit.X3</em></span> implement well defined attribute type propagation
- rules for all compound parsers, such as sequences, alternatives, Kleene star,
- etc. The main attribute propagation rule for a sequences is for instance:
- </p>
- <pre class="programlisting"><span class="identifier">a</span><span class="special">:</span> <span class="identifier">A</span><span class="special">,</span> <span class="identifier">b</span><span class="special">:</span> <span class="identifier">B</span> <span class="special">--></span> <span class="special">(</span><span class="identifier">a</span> <span class="special">>></span> <span class="identifier">b</span><span class="special">):</span> <span class="identifier">tuple</span><span class="special"><</span><span class="identifier">A</span><span class="special">,</span> <span class="identifier">B</span><span class="special">></span>
- </pre>
- <p>
- which reads as:
- </p>
- <div class="blockquote"><blockquote class="blockquote"><p>
- Given <code class="computeroutput"><span class="identifier">a</span></code> and <code class="computeroutput"><span class="identifier">b</span></code> are parsers, and <code class="computeroutput"><span class="identifier">A</span></code>
- is the attribute type of <code class="computeroutput"><span class="identifier">a</span></code>,
- and <code class="computeroutput"><span class="identifier">B</span></code> is the attribute
- type of <code class="computeroutput"><span class="identifier">b</span></code>, then the attribute
- type of <code class="computeroutput"><span class="identifier">a</span> <span class="special">>></span>
- <span class="identifier">b</span></code> (<code class="computeroutput"><span class="identifier">a</span>
- <span class="special"><<</span> <span class="identifier">b</span></code>)
- will be <code class="computeroutput"><span class="identifier">tuple</span><span class="special"><</span><span class="identifier">A</span><span class="special">,</span> <span class="identifier">B</span><span class="special">></span></code>.
- </p></blockquote></div>
- <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>
- The notation <code class="computeroutput"><span class="identifier">tuple</span><span class="special"><</span><span class="identifier">A</span><span class="special">,</span> <span class="identifier">B</span><span class="special">></span></code> is used as a placeholder expression
- for any fusion sequence holding the types A and B, such as <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">fusion</span><span class="special">::</span><span class="identifier">tuple</span><span class="special"><</span><span class="identifier">A</span><span class="special">,</span> <span class="identifier">B</span><span class="special">></span></code> or <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">A</span><span class="special">,</span> <span class="identifier">B</span><span class="special">></span></code>
- (for more information see <a href="../../../../../../../libs/fusion/doc/html/index.html" target="_top">Boost.Fusion</a>).
- </p></td></tr>
- </table></div>
- <p>
- As you can see, in order for a type to be compatible with the attribute type
- of a compound expression it has to
- </p>
- <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
- <li class="listitem">
- either be convertible to the attribute type,
- </li>
- <li class="listitem">
- or it has to expose certain functionalities, i.e. it needs to conform
- to a concept compatible with the component.
- </li>
- </ul></div>
- <p>
- Each compound component implements its own set of attribute propagation rules.
- For a full list of how the different compound parsers consume attributes
- see the sections Parser Compound Attribute Rules.
- </p>
- <h5>
- <a name="spirit_x3.abstracts.compound_attributes.h0"></a>
- <span class="phrase"><a name="spirit_x3.abstracts.compound_attributes.the_attribute_of_sequence_parsers"></a></span><a class="link" href="compound_attributes.html#spirit_x3.abstracts.compound_attributes.the_attribute_of_sequence_parsers">The
- Attribute of Sequence Parsers</a>
- </h5>
- <p>
- Sequences require an attribute type to expose the concept of a fusion sequence,
- where all elements of that fusion sequence have to be compatible with the
- corresponding element of the component sequence. For example, the expression:
- </p>
- <pre class="programlisting"><span class="identifier">double_</span> <span class="special">>></span> <span class="identifier">double_</span>
- </pre>
- <p>
- is compatible with any fusion sequence holding two types, where both types
- have to be compatible with <code class="computeroutput"><span class="keyword">double</span></code>.
- The first element of the fusion sequence has to be compatible with the attribute
- of the first <code class="computeroutput"><span class="identifier">double_</span></code>, and
- the second element of the fusion sequence has to be compatible with the attribute
- of the second <code class="computeroutput"><span class="identifier">double_</span></code>. If
- we assume to have an instance of a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="keyword">double</span><span class="special">,</span> <span class="keyword">double</span><span class="special">></span></code>,
- we can directly use the expressions above to do both, parse input to fill
- the attribute:
- </p>
- <pre class="programlisting"><span class="comment">// the following parses "1.0 2.0" into a pair of double</span>
- <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">input</span><span class="special">(</span><span class="string">"1.0 2.0"</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">iterator</span> <span class="identifier">strbegin</span> <span class="special">=</span> <span class="identifier">input</span><span class="special">.</span><span class="identifier">begin</span><span class="special">();</span>
- <span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="keyword">double</span><span class="special">,</span> <span class="keyword">double</span><span class="special">></span> <span class="identifier">p</span><span class="special">;</span>
- <span class="identifier">x3</span><span class="special">::</span><span class="identifier">phrase_parse</span><span class="special">(</span><span class="identifier">strbegin</span><span class="special">,</span> <span class="identifier">input</span><span class="special">.</span><span class="identifier">end</span><span class="special">(),</span>
- <span class="identifier">x3</span><span class="special">::</span><span class="identifier">double_</span> <span class="special">>></span> <span class="identifier">x3</span><span class="special">::</span><span class="identifier">double_</span><span class="special">,</span> <span class="comment">// parser grammar</span>
- <span class="identifier">x3</span><span class="special">::</span><span class="identifier">space</span><span class="special">,</span> <span class="comment">// delimiter grammar</span>
- <span class="identifier">p</span><span class="special">);</span> <span class="comment">// attribute to fill while parsing</span>
- </pre>
- <div class="tip"><table border="0" summary="Tip">
- <tr>
- <td rowspan="2" align="center" valign="top" width="25"><img alt="[Tip]" src="../../images/tip.png"></td>
- <th align="left">Tip</th>
- </tr>
- <tr><td align="left" valign="top"><p>
- <span class="bold"><strong>For sequences only:</strong></span> To keep it simple,
- unlike __Spirit.qi__, <span class="emphasis"><em>Spirit.X3</em></span> does not support more
- than one attribute anymore in the <code class="computeroutput"><span class="identifier">parse</span></code>
- and <code class="computeroutput"><span class="identifier">phrase_parse</span></code> function.
- Just use <code class="computeroutput"><span class="identifier">std</span><span class="special">:</span><span class="identifier">tuple</span><span class="error">'</span><span class="special">.</span> <span class="identifier">Be</span> <span class="identifier">sure</span> <span class="identifier">to</span>
- <span class="identifier">include</span> </code>boost/fusion/adapted/std_tuple.hpp'
- in this case.
- </p></td></tr>
- </table></div>
- <h5>
- <a name="spirit_x3.abstracts.compound_attributes.h1"></a>
- <span class="phrase"><a name="spirit_x3.abstracts.compound_attributes.the_attribute_of_alternative_parsers"></a></span><a class="link" href="compound_attributes.html#spirit_x3.abstracts.compound_attributes.the_attribute_of_alternative_parsers">The
- Attribute of Alternative Parsers</a>
- </h5>
- <p>
- Alternative parsers are all about - well - alternatives. In order to store
- possibly different result (attribute) types from the different alternatives
- we use the data type <a href="http://www.boost.org/doc/html/variant.html" target="_top">Boost.Variant</a>.
- The main attribute propagation rule of these components is:
- </p>
- <pre class="programlisting"><span class="identifier">a</span><span class="special">:</span> <span class="identifier">A</span><span class="special">,</span> <span class="identifier">b</span><span class="special">:</span> <span class="identifier">B</span> <span class="special">--></span> <span class="special">(</span><span class="identifier">a</span> <span class="special">|</span> <span class="identifier">b</span><span class="special">):</span> <span class="identifier">variant</span><span class="special"><</span><span class="identifier">A</span><span class="special">,</span> <span class="identifier">B</span><span class="special">></span>
- </pre>
- <p>
- Alternatives have a second very important attribute propagation rule:
- </p>
- <pre class="programlisting"><span class="identifier">a</span><span class="special">:</span> <span class="identifier">A</span><span class="special">,</span> <span class="identifier">b</span><span class="special">:</span> <span class="identifier">A</span> <span class="special">--></span> <span class="special">(</span><span class="identifier">a</span> <span class="special">|</span> <span class="identifier">b</span><span class="special">):</span> <span class="identifier">A</span>
- </pre>
- <p>
- often simplifying things significantly. If all sub expressions of an alternative
- expose the same attribute type, the overall alternative will expose exactly
- the same attribute type as well.
- </p>
- </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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