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| 4 | <title>Dynamic Parsers</title><meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1"> | |
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| 9 | <td width="10"> | |
| 10 | </td> | |
| 11 | <td width="85%"> <font face="Verdana, Arial, Helvetica, sans-serif" size="6"><b>Dynamic | |
| 12 | Parsers </b></font></td> | |
| 13 | <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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| 15 | </tbody></table> | |
| 16 | <br> | |
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| 23 | </tr> | |
| 24 | </tbody></table> | |
| 25 | <p>We see dynamic parsing everywhere in Spirit. A special group of | |
| 26 | parsers, aptly named dynamic parsers, form the most basic building | |
| 27 | blocks to dynamic parsing. This chapter focuses on these critters. | |
| 28 | You'll notice the similarity of these parsers with C++'s control | |
| 29 | structures. The similarity is not a coincidence. These parsers give an | |
| 30 | imperative flavor to parsing, and, since imperative constructs are not | |
| 31 | native to declarative EBNF, mimicking the host language, C++, should | |
| 32 | make their use immediately familiar. </p> | |
| 33 | <p>Dynamic parsers modify the parsing behavior according to conditions. Constructing | |
| 34 | dynamic parsers requires a condition argument and a body parser argument. Additional | |
| 35 | arguments are required by some parsers.</p> | |
| 36 | <h2>Conditions</h2> | |
| 37 | <p>Functions or functors returning values convertable to bool can be used as conditions. | |
| 38 | When the evaluation of the function/functor yields true it will be considered | |
| 39 | as meeting the condition.</p> | |
| 40 | <p>Parsers can be used as conditions, as well. When the parser matches the condition | |
| 41 | is met. Parsers used as conditions work in an all-or-nothing manner: the scanner | |
| 42 | will not be advanced when they don't match.</p> | |
| 43 | <p>A failure to meet the condition will not result in a parse error.</p> | |
| 44 | <h2>if_p</h2> | |
| 45 | <p><tt>if_p</tt> can be used with or without an else-part. The syntax is:</p> | |
| 46 | <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> | |
| 47 | <p><span class="special"></span>or</p> | |
| 48 | <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> | |
| 49 | <p>When the condition is met the then-parser is used next in the parsing process. | |
| 50 | When the condition is not met and an else-parser is available the else-parser | |
| 51 | is used next. When the condition isn't met and no else-parser is available then | |
| 52 | the whole parser matches the empty sequence. (<img src="theme/alert.gif" height="16" width="16"> | |
| 53 | Note: older versions of <tt>if_p</tt> report a failure when the condition isn't | |
| 54 | met and no else-parser is available.)</p> | |
| 55 | <p>Example:</p> | |
| 56 | <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> | |
| 57 | <h2>while_p, do_p</h2> | |
| 58 | <p><tt>while_p</tt>/<tt>do_p</tt> syntax is:</p> | |
| 59 | <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> | |
| 60 | <p>As long as the condition is met the dynamic parser constructed by <tt>while_p</tt> | |
| 61 | will try to match the body-parser. <tt>do_p</tt> returns a parser that tries | |
| 62 | to match the body-parser and then behaves just like the parser returned by <tt>while_p</tt>. | |
| 63 | A failure to match the body-parser will cause a failure to be reported by the | |
| 64 | while/do-parser.</p> | |
| 65 | <p>Example:</p> | |
| 66 | <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> | |
| 67 | </pre> | |
| 68 | <p>Assuming <span style="font-family: monospace;">add</span> is a user defined function object.<br></p><h2>for_p</h2> | |
| 69 | <p><tt>for_p</tt> requires four arguments. The syntax is:</p> | |
| 70 | <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> | |
| 71 | <p>init and step have to be 0-ary functions/functors. for_p returns a parser that | |
| 72 | will:</p> | |
| 73 | <ol> | |
| 74 | <li> call init</li> | |
| 75 | <li>check the condition, if the | |
| 76 | condition isn't met then a match is returned. The match will cover | |
| 77 | everything that has been matched successfully up to this point.</li> | |
| 78 | <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> | |
| 79 | <li> calls step</li> | |
| 80 | <li> goes to 2.</li> | |
| 81 | </ol> | |
| 82 | <table border="0"> | |
| 83 | <tbody><tr> | |
| 84 | <td width="10"></td> | |
| 85 | <td width="30"><a href="../index.html"><img src="theme/u_arr.gif" border="0"></a></td> | |
| 86 | <td width="30"><a href="closures.html"><img src="theme/l_arr.gif" border="0"></a></td> | |
| 87 | <td width="30"><a href="stored_rule.html"><img src="theme/r_arr.gif" border="0"></a></td> | |
| 88 | </tr> | |
| 89 | </tbody></table> | |
| 90 | <br> | |
| 91 | <hr size="1"> | |
| 92 |