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