| 1 | <html> |
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| 2 | <head> |
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| 3 | <!-- Generated by the Spirit (http://spirit.sf.net) QuickDoc --> |
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| 4 | <title>Distinct Parser</title> |
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| 12 | <td width="85%"> |
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| 13 | <font size="6" face="Verdana, Arial, Helvetica, sans-serif"><b>Distinct Parser </b></font></td> |
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| 24 | </tr> |
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| 25 | </table> |
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| 26 | <h3>Distinct Parsers</h3><p> |
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| 27 | The distinct parsers are utility parsers which ensure that matched input is |
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| 28 | not immediately followed by a forbidden pattern. Their typical usage is to |
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| 29 | distinguish keywords from identifiers.</p> |
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| 30 | <h3>distinct_parser</h3> |
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| 31 | <p> |
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| 32 | The basic usage of the <tt>distinct_parser</tt> is to replace the <tt>str_p</tt> parser. For |
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| 33 | example the <tt>declaration_rule</tt> in the following example:</p> |
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| 34 | <pre> |
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| 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>]; |
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| 36 | </span></code></pre> |
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| 37 | <p> |
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| 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 |
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| 39 | use <tt>distinct_parser</tt>:</p> |
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| 40 | <code> |
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| 41 | <pre> |
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| 42 | <span class=comment>// keyword_p may be defined in the global scope |
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| 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>); |
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| 44 | |
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| 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>]; |
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| 46 | </span></pre> |
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| 47 | </code> |
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| 48 | <p> |
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| 49 | The <tt>keyword_p</tt> works in the same way as the <tt>str_p</tt> parser but matches only |
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| 50 | when the matched input is not immediately followed by one of the characters |
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| 51 | from the set passed to the constructor of <tt>keyword_p</tt>. In the example the |
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| 52 | "declare" can't be immediately followed by any alphabetic character, any |
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| 53 | number or an underscore.</p> |
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| 54 | <p> |
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| 55 | See the full <a href="../example/fundamental/distinct/distinct_parser.cpp">example here </a>.</p> |
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| 56 | <h3>distinct_directive</h3><p> |
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| 57 | For more sophisticated cases, for example when keywords are stored in a |
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| 58 | symbol table, we can use <tt>distinct_directive</tt>.</p> |
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| 59 | <pre> |
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| 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>); |
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| 61 | |
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| 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>; |
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| 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>]; |
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| 64 | </span></code></pre> |
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| 65 | <h3>dynamic_distinct_parser and dynamic_distinct_directive</h3><p> |
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| 66 | In some cases a set of forbidden follow-up characters is not sufficient. |
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| 67 | For example ASN.1 naming conventions allows identifiers to contain dashes, |
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| 68 | but not double dashes (which marks the beginning of a comment). |
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| 69 | Furthermore, identifiers can't end with a dash. So, a matched keyword can't |
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| 70 | be followed by any alphanumeric character or exactly one dash, but can be |
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| 71 | followed by two dashes.</p> |
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| 72 | <p> |
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| 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 |
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| 74 | parser which matches any input that <strong>must NOT</strong> follow the keyword.</p> |
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| 75 | <pre> |
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| 76 | <code><span class=comment>// Alphanumeric characters and a dash followed by a non-dash |
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| 77 | // may not follow an ASN.1 identifier. |
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| 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>))); |
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| 79 | |
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| 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>]; |
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| 81 | </span></code></pre> |
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| 82 | <p> |
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| 83 | Since the <tt>dynamic_distinct_parser</tt> internally uses a rule, its type is |
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| 84 | dependent on the scanner type. So, the <tt>keyword_p</tt> shouldn't be defined |
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| 85 | globally, but rather within the grammar.</p> |
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| 86 | <p> |
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| 87 | See the full <a href="../example/fundamental/distinct/distinct_parser_dynamic.cpp">example here</a>.</p> |
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| 88 | <h3>How it works</h3><p> |
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| 89 | When the <tt>keyword_p_1</tt> and the <tt>keyword_p_2</tt> are defined as</p> |
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| 90 | <code><pre> |
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| 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>); |
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| 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>); |
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| 93 | </span></pre></code> |
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| 94 | <p> |
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| 95 | the parsers</p> |
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| 96 | <code><pre> |
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| 97 | <span class=identifier>keyword_p_1</span><span class=special>(</span><span class=identifier>str</span><span class=special>) |
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| 98 | </span><span class=identifier>keyword_p_2</span><span class=special>(</span><span class=identifier>str</span><span class=special>) |
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| 99 | </span></pre></code> |
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| 100 | <p> |
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| 101 | are equivalent to the rules</p> |
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| 102 | <code><pre> |
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| 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>))] |
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| 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>)] |
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| 105 | </span></pre></code> |
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| 106 | <table border="0"> |
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| 112 | </tr> |
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| 113 | </table> |
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| 114 | <br> |
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| 115 | <hr size="1"> |
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| 116 | <p class="copyright">Copyright © 2003-2004 |
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| 117 | |
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| 118 | |
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| 119 | Vaclav Vesely<br><br> |
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| 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> |
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