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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 explicitely 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>
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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