<html> <head> <!-- Generated by the Spirit (http://spirit.sf.net) QuickDoc --> <title>Distinct Parser</title> <link rel="stylesheet" href="theme/style.css" type="text/css"> </head> <body> <table width="100%" height="48" border="0" background="theme/bkd2.gif" cellspacing="2"> <tr> <td width="10"> </td> <td width="85%"> <font size="6" face="Verdana, Arial, Helvetica, sans-serif"><b>Distinct Parser </b></font></td> <td width="112"><a href="http://spirit.sf.net"><img src="theme/spirit.gif" align="right" border="0"></a></td> </tr> </table> <br> <table border="0"> <tr> <td width="10"></td> <td width="30"><a href="../index.html"><img src="theme/u_arr.gif" border="0"></a></td> <td width="30"><a href="scoped_lock.html"><img src="theme/l_arr.gif" border="0"></a></td> <td width="30"><a href="symbols.html"><img src="theme/r_arr.gif" border="0"></a></td> </tr> </table> <h3>Distinct Parsers</h3><p> The distinct parsers are utility parsers which ensure that matched input is not immediately followed by a forbidden pattern. Their typical usage is to distinguish keywords from identifiers.</p> <h3>distinct_parser</h3> <p> The basic usage of the <tt>distinct_parser</tt> is to replace the <tt>str_p</tt> parser. For example the <tt>declaration_rule</tt> in the following example:</p> <pre> <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>]; </span></code></pre> <p> 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 use <tt>distinct_parser</tt>:</p> <code> <pre> <span class=comment>// keyword_p may be defined in the global scope </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>); </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>]; </span></pre> </code> <p> The <tt>keyword_p</tt> works in the same way as the <tt>str_p</tt> parser but matches only when the matched input is not immediately followed by one of the characters from the set passed to the constructor of <tt>keyword_p</tt>. In the example the "declare" can't be immediately followed by any alphabetic character, any number or an underscore.</p> <p> See the full <a href="../example/fundamental/distinct/distinct_parser.cpp">example here </a>.</p> <h3>distinct_directive</h3><p> For more sophisticated cases, for example when keywords are stored in a symbol table, we can use <tt>distinct_directive</tt>.</p> <pre> <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>); </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>; </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>]; </span></code></pre> <h3>dynamic_distinct_parser and dynamic_distinct_directive</h3><p> In some cases a set of forbidden follow-up characters is not sufficient. For example ASN.1 naming conventions allows identifiers to contain dashes, but not double dashes (which marks the beginning of a comment). Furthermore, identifiers can't end with a dash. So, a matched keyword can't be followed by any alphanumeric character or exactly one dash, but can be followed by two dashes.</p> <p> 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 parser which matches any input that <strong>must NOT</strong> follow the keyword.</p> <pre> <code><span class=comment>// Alphanumeric characters and a dash followed by a non-dash // may not follow an ASN.1 identifier. </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>))); </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>]; </span></code></pre> <p> Since the <tt>dynamic_distinct_parser</tt> internally uses a rule, its type is dependent on the scanner type. So, the <tt>keyword_p</tt> shouldn't be defined globally, but rather within the grammar.</p> <p> See the full <a href="../example/fundamental/distinct/distinct_parser_dynamic.cpp">example here</a>.</p> <h3>How it works</h3><p> When the <tt>keyword_p_1</tt> and the <tt>keyword_p_2</tt> are defined as</p> <code><pre> <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>); </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>); </span></pre></code> <p> the parsers</p> <code><pre> <span class=identifier>keyword_p_1</span><span class=special>(</span><span class=identifier>str</span><span class=special>) </span><span class=identifier>keyword_p_2</span><span class=special>(</span><span class=identifier>str</span><span class=special>) </span></pre></code> <p> are equivalent to the rules</p> <code><pre> <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>))] </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>)] </span></pre></code> <table border="0"> <tr> <td width="10"></td> <td width="30"><a href="../index.html"><img src="theme/u_arr.gif" border="0"></a></td> <td width="30"><a href="scoped_lock.html"><img src="theme/l_arr.gif" border="0"></a></td> <td width="30"><a href="symbols.html"><img src="theme/r_arr.gif" border="0"></a></td> </tr> </table> <br> <hr size="1"> <p class="copyright">Copyright © 2003-2004 Vaclav Vesely<br><br> <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> </body> </html>