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99 lines
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<title>Dynamic Parsers</title><meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
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<td width="85%"> <font face="Verdana, Arial, Helvetica, sans-serif" size="6"><b>Dynamic
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Parsers </b></font></td>
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<td width="112"><a href="http://spirit.sf.net"><img src="theme/spirit.gif" align="right" border="0" height="48" width="112"></a></td>
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</tr>
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<br>
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<tbody><tr>
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<td width="10"></td>
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</tr>
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</tbody></table>
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<p>We see dynamic parsing everywhere in Spirit. A special group of
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parsers, aptly named dynamic parsers, form the most basic building
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blocks to dynamic parsing. This chapter focuses on these critters.
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You'll notice the similarity of these parsers with C++'s control
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structures. The similarity is not a coincidence. These parsers give an
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imperative flavor to parsing, and, since imperative constructs are not
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native to declarative EBNF, mimicking the host language, C++, should
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make their use immediately familiar. </p>
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<p>Dynamic parsers modify the parsing behavior according to conditions. Constructing
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dynamic parsers requires a condition argument and a body parser argument. Additional
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arguments are required by some parsers.</p>
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<h2>Conditions</h2>
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<p>Functions or functors returning values convertable to bool can be used as conditions.
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When the evaluation of the function/functor yields true it will be considered
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as meeting the condition.</p>
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<p>Parsers can be used as conditions, as well. When the parser matches the condition
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is met. Parsers used as conditions work in an all-or-nothing manner: the scanner
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will not be advanced when they don't match.</p>
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<p>A failure to meet the condition will not result in a parse error.</p>
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<h2>if_p</h2>
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<p><tt>if_p</tt> can be used with or without an else-part. The syntax is:</p>
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<pre> <span class="identifier">if_p</span><span class="special">(</span><span class="identifier">condition</span><span class="special">)[</span><span class="identifier">then</span><span class="special">-</span><span class="identifier">parser</span><span class="special">]</span></pre>
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<p><span class="special"></span>or</p>
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<pre><span class="identifier"> if_p</span><span class="special">(</span><span class="identifier">condition</span><span class="special">)[</span><span class="identifier">then</span><span class="special">-</span><span class="identifier">parser</span><span class="special">].</span><span class="identifier">else_p</span><span class="special">[</span><span class="identifier">else</span><span class="special">-</span><span class="identifier">parser</span><span class="special">]</span></pre>
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<p>When the condition is met the then-parser is used next in the parsing process.
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When the condition is not met and an else-parser is available the else-parser
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is used next. When the condition isn't met and no else-parser is available then
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the whole parser matches the empty sequence. (<img src="theme/alert.gif" height="16" width="16">
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Note: older versions of <tt>if_p</tt> report a failure when the condition isn't
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met and no else-parser is available.)</p>
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<p>Example:</p>
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<pre> <span class="special"></span><span class="identifier">if_p</span><span class="special">(</span><span class="string">"0x"</span><span class="special">)[</span><span class="identifier">hex_p</span><span class="special">].</span><span class="identifier">else_p</span><span class="special">[</span><span class="identifier">uint_p</span><span class="special">]</span></pre>
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<h2>while_p, do_p</h2>
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<p><tt>while_p</tt>/<tt>do_p</tt> syntax is:</p>
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<pre> <span class="identifier">while_p</span><span class="special">(</span><span class="identifier">condition</span><span class="special">)[</span><span class="identifier">body</span><span class="special">-</span><span class="identifier">parser</span><span class="special">]<br> </span><span class="identifier">do_p</span><span class="special">[</span><span class="identifier">body</span><span class="special">-</span><span class="identifier">parser</span><span class="special">].</span><span class="identifier">while_p</span><span class="special">(</span><span class="identifier">condition</span><span class="special">)</span></pre>
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<p>As long as the condition is met the dynamic parser constructed by <tt>while_p</tt>
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will try to match the body-parser. <tt>do_p</tt> returns a parser that tries
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to match the body-parser and then behaves just like the parser returned by <tt>while_p</tt>.
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A failure to match the body-parser will cause a failure to be reported by the
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while/do-parser.</p>
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<p>Example:</p>
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<pre><span class="special"> </span><span class="identifier">uint_p</span><span class="special">[</span><span class="identifier">assign_a</span><span class="special">(</span><span class="identifier">sum</span><span class="special">)] >> </span><span class="identifier">while_p</span><span class="special">(</span><span class="literal">'+'</span><span class="special">)[</span><span class="identifier">uint_p</span><span class="special">[</span><span class="identifier">add</span><span class="special">(</span><span class="identifier">sum</span><span class="special">)]]<br> </span><span class="literal">'"' </span><span class="special">>> </span><span class="identifier">while_p</span><span class="special">(~</span><span class="identifier">eps_p</span><span class="special">(</span><span class="literal">'"'</span><span class="special">))[</span><span class="identifier">c_escape_ch_p</span><span class="special">[</span><span class="identifier">push_back_a</span><span class="special">(</span><span class="identifier">result</span><span class="special">)]] >> </span><span class="literal">'"'</span>
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</pre>
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<p>Assuming <span style="font-family: monospace;">add</span> is a user defined function object.<br></p><h2>for_p</h2>
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<p><tt>for_p</tt> requires four arguments. The syntax is:</p>
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<pre> <span class="literal"></span><span class="identifier">for_p</span><span class="special">(</span><span class="identifier">init</span><span class="special">, </span><span class="identifier">condition</span><span class="special">, </span><span class="identifier">step</span><span class="special">)[</span><span class="identifier">body</span><span class="special">-</span><span class="identifier">parser</span><span class="special">]</span></pre>
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<p>init and step have to be 0-ary functions/functors. for_p returns a parser that
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will:</p>
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<ol>
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<li> call init</li>
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<li>check the condition, if the
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condition isn't met then a match is returned. The match will cover
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everything that has been matched successfully up to this point.</li>
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<li> tries to match the body-parser. A failure to match the body-parser will cause a failure to be reported by the for-parser</li>
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<li> calls step</li>
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<li> goes to 2.</li>
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</ol>
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<table border="0">
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<tbody><tr>
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<td width="10"></td>
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<td width="30"><a href="../index.html"><img src="theme/u_arr.gif" border="0"></a></td>
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<td width="30"><a href="closures.html"><img src="theme/l_arr.gif" border="0"></a></td>
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<td width="30"><a href="stored_rule.html"><img src="theme/r_arr.gif" border="0"></a></td>
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</tr>
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</tbody></table>
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<br>
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<hr size="1">
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<p class="copyright">Copyright <20> 2002-2003 Joel de Guzman<br>
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Copyright <20> 2002-2003 Martin Wille<br>
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<br>
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<font size="2">Use, modification and distribution is subject to the Boost Software
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License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at
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http://www.boost.org/LICENSE_1_0.txt)</font></p>
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<p class="copyright"> </p>
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</body></html> |