1<html> 2<head> 3<!-- Generated by the Spirit (http://spirit.sf.net) QuickDoc --> 4<title>Distinct Parser</title> 5<link rel="stylesheet" href="theme/style.css" type="text/css"> 6</head> 7<body> 8<table width="100%" height="48" border="0" background="theme/bkd2.gif" cellspacing="2"> 9 <tr> 10 <td width="10"> 11 </td> 12 <td width="85%"> 13 <font size="6" face="Verdana, Arial, Helvetica, sans-serif"><b>Distinct Parser </b></font></td> 14 <td width="112"><a href="http://spirit.sf.net"><img src="theme/spirit.gif" align="right" border="0"></a></td> 15 </tr> 16</table> 17<br> 18<table border="0"> 19 <tr> 20 <td width="10"></td> 21 <td width="30"><a href="../index.html"><img src="theme/u_arr.gif" border="0"></a></td> 22 <td width="30"><a href="scoped_lock.html"><img src="theme/l_arr.gif" border="0"></a></td> 23 <td width="30"><a href="symbols.html"><img src="theme/r_arr.gif" border="0"></a></td> 24 </tr> 25</table> 26<h3>Distinct Parsers</h3><p> 27The distinct parsers are utility parsers which ensure that matched input is 28not immediately followed by a forbidden pattern. Their typical usage is to 29distinguish keywords from identifiers.</p> 30<h3>distinct_parser</h3> 31<p> 32The basic usage of the <tt>distinct_parser</tt> is to replace the <tt>str_p</tt> parser. For 33example 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> 38would correctly match an input "declare abc", but as well an input"declareabc" what is usually not intended. In order to avoid this, we can 39use <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> 49The <tt>keyword_p</tt> works in the same way as the <tt>str_p</tt> parser but matches only 50when the matched input is not immediately followed by one of the characters 51from 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 53number or an underscore.</p> 54<p> 55See the full <a href="../example/fundamental/distinct/distinct_parser.cpp">example here </a>.</p> 56<h3>distinct_directive</h3><p> 57For more sophisticated cases, for example when keywords are stored in a 58symbol 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> 66In some cases a set of forbidden follow-up characters is not sufficient. 67For example ASN.1 naming conventions allows identifiers to contain dashes, 68but not double dashes (which marks the beginning of a comment). 69Furthermore, identifiers can't end with a dash. So, a matched keyword can't 70be followed by any alphanumeric character or exactly one dash, but can be 71followed by two dashes.</p> 72<p> 73This 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 74parser 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> 83Since the <tt>dynamic_distinct_parser</tt> internally uses a rule, its type is 84dependent on the scanner type. So, the <tt>keyword_p</tt> shouldn't be defined 85globally, but rather within the grammar.</p> 86<p> 87See the full <a href="../example/fundamental/distinct/distinct_parser_dynamic.cpp">example here</a>.</p> 88<h3>How it works</h3><p> 89When 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> 95the 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> 101are 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> 123
