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| <div class="titlepage"><div><div><h4 class="title"> |
| <a name="spirit.abstracts.attributes.nonterminal_attributes"></a><a class="link" href="nonterminal_attributes.html" title="Attributes of Rules and Grammars">Attributes |
| of Rules and Grammars</a> |
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| <p> |
| Nonterminals are well known from parsers where they are used as the main |
| means of constructing more complex parsers out of simpler ones. The nonterminals |
| in the parser world are very similar to functions in an imperative programming |
| language. They can be used to encapsulate parser expressions for a particular |
| input sequence. After being defined, the nonterminals can be used as 'normal' |
| parsers in more complex expressions whenever the encapsulated input needs |
| to be recognized. Parser nonterminals in <span class="emphasis"><em>Spirit.Qi</em></span> |
| may accept <span class="emphasis"><em>parameters</em></span> (inherited attributes) and usually |
| return a value (the synthesized attribute). |
| </p> |
| <p> |
| Both, the types of the inherited and the synthesized attributes have to |
| be explicitly specified while defining the particular <code class="computeroutput"><span class="identifier">grammar</span></code> |
| or the <code class="computeroutput"><span class="identifier">rule</span></code> (the Spirit |
| <a href="../../../../../repository/doc/html/index.html" target="_top">Repository</a> additionally |
| has <code class="computeroutput"><span class="identifier">subrules</span></code> which conform |
| to a similar interface). As an example, the following code declares a |
| <span class="emphasis"><em>Spirit.Qi</em></span> <code class="computeroutput"><span class="identifier">rule</span></code> |
| exposing an <code class="computeroutput"><span class="keyword">int</span></code> as its synthesized |
| attribute, while expecting a single <code class="computeroutput"><span class="keyword">double</span></code> |
| as its inherited attribute (see the section about the <span class="emphasis"><em>Spirit.Qi</em></span> |
| Rule for more information): |
| </p> |
| <pre class="programlisting"><span class="identifier">qi</span><span class="special">::</span><span class="identifier">rule</span><span class="special"><</span><span class="identifier">Iterator</span><span class="special">,</span> <span class="keyword">int</span><span class="special">(</span><span class="keyword">double</span><span class="special">)></span> <span class="identifier">r</span><span class="special">;</span> |
| </pre> |
| <p> |
| In the world of generators, nonterminals are just as useful as in the parser |
| world. Generator nonterminals encapsulate a format description for a particular |
| data type, and, whenever we need to emit output for this data type, the |
| corresponding nonterminal is invoked in a similar way as the predefined |
| <span class="emphasis"><em>Spirit.Karma</em></span> generator primitives. The <span class="emphasis"><em>Spirit.Karma</em></span> |
| nonterminals are very similar to the <span class="emphasis"><em>Spirit.Qi</em></span> nonterminals. |
| Generator nonterminals may accept <span class="emphasis"><em>parameters</em></span> as well, |
| and we call those inherited attributes too. The main difference is that |
| they do not expose a synthesized attribute (as parsers do), but they require |
| a special <span class="emphasis"><em>consumed attribute</em></span>. Usually the consumed |
| attribute is the value the generator creates its output from. Even if the |
| consumed attribute is not 'returned' from the generator we chose to use |
| the same function style declaration syntax as used in <span class="emphasis"><em>Spirit.Qi</em></span>. |
| The example below declares a <span class="emphasis"><em>Spirit.Karma</em></span> <code class="computeroutput"><span class="identifier">rule</span></code> consuming a <code class="computeroutput"><span class="keyword">double</span></code> |
| while not expecting any additional inherited attributes. |
| </p> |
| <pre class="programlisting"><span class="identifier">karma</span><span class="special">::</span><span class="identifier">rule</span><span class="special"><</span><span class="identifier">OutputIterator</span><span class="special">,</span> <span class="keyword">double</span><span class="special">()></span> <span class="identifier">r</span><span class="special">;</span> |
| </pre> |
| <p> |
| The inherited attributes of nonterminal parsers and generators are normally |
| passed to the component during its invocation. These are the <span class="emphasis"><em>parameters</em></span> |
| the parser or generator may accept and they can be used to parameterize |
| the component depending on the context they are invoked from. |
| </p> |
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| <td align="right"><div class="copyright-footer">Copyright © 2001-2010 Joel de Guzman, Hartmut Kaiser<p> |
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| 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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