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| <font size="6" face="Verdana, Arial, Helvetica, sans-serif"><b>Distinct Parser </b></font></td> |
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| <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> |
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| <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> |
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