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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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- Vaclav Vesely<br><br>
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