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| 3 | <!-- Generated by the Spirit (http://spirit.sf.net) QuickDoc --> | |
| 4 | <title>Distinct Parser</title> | |
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| 12 | <td width="85%"> | |
| 13 | <font size="6" face="Verdana, Arial, Helvetica, sans-serif"><b>Distinct Parser </b></font></td> | |
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| 21 | <td width="30"><a href="../index.html"><img src="theme/u_arr.gif" border="0"></a></td> | |
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| 24 | </tr> | |
| 25 | </table> | |
| 26 | <h3>Distinct Parsers</h3><p> | |
| 27 | The distinct parsers are utility parsers which ensure that matched input is | |
| 28 | not immediately followed by a forbidden pattern. Their typical usage is to | |
| 29 | distinguish keywords from identifiers.</p> | |
| 30 | <h3>distinct_parser</h3> | |
| 31 | <p> | |
| 32 | The basic usage of the <tt>distinct_parser</tt> is to replace the <tt>str_p</tt> parser. For | |
| 33 | example the <tt>declaration_rule</tt> in the following example:</p> | |
| 34 | <pre> | |
| 35 | <code><span class=identifier>rule</span><span class=special><</span><span class="identifier">ScannerT</span><span class=special>> </span><span class=identifier>declaration_rule </span><span class=special>= </span><span class=identifier>str_p</span><span class=special>(</span><span class=string>"declare"</span><span class=special>) >> </span><span class=identifier>lexeme_d</span><span class=special>[+</span><span class=identifier>alpha_p</span><span class=special>]; | |
| 36 | </span></code></pre> | |
| 37 | <p> | |
| 38 | would correctly match an input "declare abc", but as well an input"declareabc" what is usually not intended. In order to avoid this, we can | |
| 39 | use <tt>distinct_parser</tt>:</p> | |
| 40 | <code> | |
| 41 | <pre> | |
| 42 | <span class=comment>// keyword_p may be defined in the global scope | |
| 43 | </span><span class=identifier>distinct_parser</span><span class=special><> </span><span class=identifier>keyword_p</span><span class=special>(</span><span class=string>"a-zA-Z0-9_"</span><span class=special>); | |
| 44 | ||
| 45 | </span><span class=identifier>rule</span><span class=special><</span><span class="identifier">ScannerT</span><span class=special>> </span><span class=identifier>declaration_rule </span><span class=special>= </span><span class=identifier>keyword_p</span><span class=special>(</span><span class=string>"declare"</span><span class=special>) >> </span><span class=identifier>lexeme_d</span><span class=special>[+</span><span class=identifier>alpha_p</span><span class=special>]; | |
| 46 | </span></pre> | |
| 47 | </code> | |
| 48 | <p> | |
| 49 | The <tt>keyword_p</tt> works in the same way as the <tt>str_p</tt> parser but matches only | |
| 50 | when the matched input is not immediately followed by one of the characters | |
| 51 | from the set passed to the constructor of <tt>keyword_p</tt>. In the example the | |
| 52 | "declare" can't be immediately followed by any alphabetic character, any | |
| 53 | number or an underscore.</p> | |
| 54 | <p> | |
| 55 | See the full <a href="../example/fundamental/distinct/distinct_parser.cpp">example here </a>.</p> | |
| 56 | <h3>distinct_directive</h3><p> | |
| 57 | For more sophisticated cases, for example when keywords are stored in a | |
| 58 | symbol table, we can use <tt>distinct_directive</tt>.</p> | |
| 59 | <pre> | |
| 60 | <code><span class=identifier>distinct_directive</span><span class=special><> </span><span class=identifier>keyword_d</span><span class=special>(</span><span class=string>"a-zA-Z0-9_"</span><span class=special>); | |
| 61 | ||
| 62 | </span><span class=identifier>symbol</span><span class=special><> </span><span class=identifier>keywords </span><span class=special>= </span><span class=string>"declare"</span><span class=special>, </span><span class=string>"begin"</span><span class=special>, </span><span class=string>"end"</span><span class=special>; | |
| 63 | </span><span class=identifier>rule</span><span class=special><</span><span class="identifier">ScannerT</span><span class=special>> </span><span class=identifier>keyword </span><span class=special>= </span><span class=identifier>keyword_d</span><span class=special>[</span><span class=identifier>keywords</span><span class=special>]; | |
| 64 | </span></code></pre> | |
| 65 | <h3>dynamic_distinct_parser and dynamic_distinct_directive</h3><p> | |
| 66 | In some cases a set of forbidden follow-up characters is not sufficient. | |
| 67 | For example ASN.1 naming conventions allows identifiers to contain dashes, | |
| 68 | but not double dashes (which marks the beginning of a comment). | |
| 69 | Furthermore, identifiers can't end with a dash. So, a matched keyword can't | |
| 70 | be followed by any alphanumeric character or exactly one dash, but can be | |
| 71 | followed by two dashes.</p> | |
| 72 | <p> | |
| 73 | This is when <tt>dynamic_distinct_parser</tt> and the <tt>dynamic_distinct_directive </tt>come into play. The constructor of the <tt>dynamic_distinct_parser</tt> accepts a | |
| 74 | parser which matches any input that <strong>must NOT</strong> follow the keyword.</p> | |
| 75 | <pre> | |
| 76 | <code><span class=comment>// Alphanumeric characters and a dash followed by a non-dash | |
| 77 | // may not follow an ASN.1 identifier. | |
| 78 | </span><span class=identifier>dynamic_distinct_parser</span><span class=special><> </span><span class=identifier>keyword_p</span><span class=special>(</span><span class=identifier>alnum_p </span><span class=special>| (</span><span class=literal>'-' </span><span class=special>>> ~</span><span class=identifier>ch_p</span><span class=special>(</span><span class=literal>'-'</span><span class=special>))); | |
| 79 | ||
| 80 | </span><span class=identifier>rule</span><span class=special><</span><span class="identifier">ScannerT</span><span class=special>> </span><span class=identifier>declaration_rule </span><span class=special>= </span><span class=identifier>keyword_p</span><span class=special>(</span><span class=string>"declare"</span><span class=special>) >> </span><span class=identifier>lexeme_d</span><span class=special>[+</span><span class=identifier>alpha_p</span><span class=special>]; | |
| 81 | </span></code></pre> | |
| 82 | <p> | |
| 83 | Since the <tt>dynamic_distinct_parser</tt> internally uses a rule, its type is | |
| 84 | dependent on the scanner type. So, the <tt>keyword_p</tt> shouldn't be defined | |
| 85 | globally, but rather within the grammar.</p> | |
| 86 | <p> | |
| 87 | See the full <a href="../example/fundamental/distinct/distinct_parser_dynamic.cpp">example here</a>.</p> | |
| 88 | <h3>How it works</h3><p> | |
| 89 | When the <tt>keyword_p_1</tt> and the <tt>keyword_p_2</tt> are defined as</p> | |
| 90 | <code><pre> | |
| 91 | <span class=identifier>distinct_parser</span><span class=special><> </span><span class=identifier>keyword_p</span><span class=special>(</span><span class=identifier>forbidden_chars</span><span class=special>); | |
| 92 | </span><span class=identifier>distinct_parser_dynamic</span><span class=special><> </span><span class=identifier>keyword_p</span><span class=special>(</span><span class=identifier>forbidden_tail_parser</span><span class=special>); | |
| 93 | </span></pre></code> | |
| 94 | <p> | |
| 95 | the parsers</p> | |
| 96 | <code><pre> | |
| 97 | <span class=identifier>keyword_p_1</span><span class=special>(</span><span class=identifier>str</span><span class=special>) | |
| 98 | </span><span class=identifier>keyword_p_2</span><span class=special>(</span><span class=identifier>str</span><span class=special>) | |
| 99 | </span></pre></code> | |
| 100 | <p> | |
| 101 | are equivalent to the rules</p> | |
| 102 | <code><pre> | |
| 103 | <span class=identifier>lexeme_d</span><span class=special>[</span><span class=identifier>chseq_p</span><span class=special>(</span><span class=identifier>str</span><span class=special>) >> ~</span><span class=identifier>epsilon_p</span><span class=special>(</span><span class=identifier>chset_p</span><span class=special>(</span><span class=identifier>forbidden_chars</span><span class=special>))] | |
| 104 | </span><span class=identifier>lexeme_d</span><span class=special>[</span><span class=identifier>chseq_p</span><span class=special>(</span><span class=identifier>str</span><span class=special>) >> ~</span><span class=identifier>epsilon_p</span><span class=special>(</span><span class=identifier>forbidden_tail_parser</span><span class=special>)] | |
| 105 | </span></pre></code> | |
| 106 | <table border="0"> | |
| 107 | <tr> | |
| 108 | <td width="10"></td> | |
| 109 | <td width="30"><a href="../index.html"><img src="theme/u_arr.gif" border="0"></a></td> | |
| 110 | <td width="30"><a href="scoped_lock.html"><img src="theme/l_arr.gif" border="0"></a></td> | |
| 111 | <td width="30"><a href="symbols.html"><img src="theme/r_arr.gif" border="0"></a></td> | |
| 112 | </tr> | |
| 113 | </table> | |
| 114 | <br> | |
| 115 | <hr size="1"> | |
| 116 | <p class="copyright">Copyright © 2003-2004 | |
| 117 | ||
| 118 | ||
| 119 | Vaclav Vesely<br><br> | |
| 120 | <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> | |
| 121 | </body> | |
| 122 | </html> |