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| 13 | <td width="85%"> <font size="6" face="Verdana, Arial, Helvetica, sans-serif"><b>Dynamic |
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| 14 | Parsers </b></font></td> |
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| 15 | <td width="112"><a href="http://spirit.sf.net"><img src="theme/spirit.gif" width="112" height="48" align="right" border="0"></a></td> |
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| 27 | <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> |
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| 28 | <p>Dynamic parsers modify the parsing behavior according to conditions. Constructing |
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| 29 | dynamic parsers requires a condition argument and a body parser argument. Additional |
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| 30 | arguments are required by some parsers.</p> |
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| 31 | <h2>Conditions</h2> |
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| 32 | <p>Functions or functors returning values convertable to bool can be used as conditions. |
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| 33 | When the evaluation of the function/functor yields true it will be considered |
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| 34 | as meeting the condition.</p> |
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| 35 | <p>Parsers can be used as conditions, as well. When the parser matches the condition |
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| 36 | is met. Parsers used as conditions work in an all-or-nothing manner: the scanner |
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| 37 | will not be advanced when they don't match.</p> |
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| 38 | <p>A failure to meet the condition will not result in a parse error.</p> |
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| 39 | <h2>if_p</h2> |
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| 40 | <p><tt>if_p</tt> can be used with or without an else-part. The syntax is:</p> |
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| 41 | <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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| 42 | <p><span class=special></span>or</p> |
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| 43 | <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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| 44 | <p>When the condition is met the then-parser is used next in the parsing process. |
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| 45 | When the condition is not met and an else-parser is available the else-parser |
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| 46 | is used next. When the condition isn't met and no else-parser is available then |
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| 47 | the whole parser matches the empty sequence. (<img src="theme/alert.gif" width="16" height="16"> |
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| 48 | Note: older versions of <tt>if_p</tt> report a failure when the condition isn't |
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| 49 | met and no else-parser is available.)</p> |
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| 50 | <p>Example:</p> |
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| 51 | <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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| 52 | <h2>while_p, do_p</h2> |
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| 53 | <p><tt>while_p</tt>/<tt>do_p</tt> syntax is:</p> |
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| 54 | <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>] |
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| 55 | </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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| 56 | <p>As long as the condition is met the dynamic parser constructed by <tt>while_p</tt> |
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| 57 | will try to match the body-parser. <tt>do_p</tt> returns a parser that tries |
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| 58 | to match the body-parser and then behaves just like the parser returned by <tt>while_p</tt>. |
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| 59 | A failure to match the body-parser will cause a failure to be reported by the |
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| 60 | while/do-parser.</p> |
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| 61 | <p>Example:</p> |
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| 62 | <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>)] |
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| 63 | </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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| 64 | </pre> |
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| 65 | <h2>for_p</h2> |
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| 66 | <p><tt>for_p</tt> requires four arguments. The syntax is:</p> |
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| 67 | <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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| 68 | <p>init and step have to be 0-ary functions/functors. for_p returns a parser that |
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| 69 | will:</p> |
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| 70 | <ol> |
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| 71 | <li> call init</li> |
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| 72 | <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> |
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| 73 | <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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| 74 | <li> calls step</li> |
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| 75 | <li> goes to 2.</li> |
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| 76 | </ol> |
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| 77 | <table border="0"> |
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| 78 | <tr> |
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| 79 | <td width="10"></td> |
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| 80 | <td width="30"><a href="../index.html"><img src="theme/u_arr.gif" border="0"></a></td> |
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| 84 | </table> |
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| 85 | <br> |
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| 86 | <hr size="1"> |
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| 87 | <p class="copyright">Copyright © 2002-2003 Joel de Guzman<br> |
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| 88 | Copyright © 2002-2003 Martin Wille<br> |
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| 89 | <br> |
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| 90 | <font size="2">Use, modification and distribution is subject to the Boost Software |
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| 91 | License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at |
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| 92 | http://www.boost.org/LICENSE_1_0.txt)</font></p> |
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| 93 | <p class="copyright"> </p> |
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