ceph/src/boost/libs/spirit/classic/doc/distinct.html

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  13     <font size="6" face="Verdana, Arial, Helvetica, sans-serif"><b>Distinct Parser </b></font></td>
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  26 <h3>Distinct Parsers</h3><p>
  27 The distinct parsers are utility parsers which ensure that matched input is
  28 not immediately followed by a forbidden pattern. Their typical usage is to
  29 distinguish keywords from identifiers.</p>
  30 <h3>distinct_parser</h3>
  31 <p>
  32 The basic usage of the <tt>distinct_parser</tt> is to replace the <tt>str_p</tt> parser. For
  33 example the <tt>declaration_rule</tt> in the following example:</p>
  34 <pre>
  35     <code><span class=identifier>rule</span><span class=special>&lt;</span><span class="identifier">ScannerT</span><span class=special>&gt; </span><span class=identifier>declaration_rule </span><span class=special>= </span><span class=identifier>str_p</span><span class=special>(</span><span class=string>&quot;declare&quot;</span><span class=special>) &gt;&gt; </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>
  38 would correctly match an input &quot;declare abc&quot;, but as well an input&quot;declareabc&quot; what is usually not intended. In order to avoid this, we can
  39 use <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>&lt;&gt; </span><span class=identifier>keyword_p</span><span class=special>(</span><span class=string>&quot;a-zA-Z0-9_&quot;</span><span class=special>);
  44
  45     </span><span class=identifier>rule</span><span class=special>&lt;</span><span class="identifier">ScannerT</span><span class=special>&gt; </span><span class=identifier>declaration_rule </span><span class=special>= </span><span class=identifier>keyword_p</span><span class=special>(</span><span class=string>&quot;declare&quot;</span><span class=special>) &gt;&gt; </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>
  49 The <tt>keyword_p</tt> works in the same way as the <tt>str_p</tt> parser but matches only
  50 when the matched input is not immediately followed by one of the characters
  51 from the set passed to the constructor of <tt>keyword_p</tt>. In the example the
  52 &quot;declare&quot; can't be immediately followed by any alphabetic character, any
  53 number or an underscore.</p>
  54 <p>
  55 See the full <a href="../example/fundamental/distinct/distinct_parser.cpp">example here </a>.</p>
  56 <h3>distinct_directive</h3><p>
  57 For more sophisticated cases, for example when keywords are stored in a
  58 symbol table, we can use <tt>distinct_directive</tt>.</p>
  59 <pre>
  60     <code><span class=identifier>distinct_directive</span><span class=special>&lt;&gt; </span><span class=identifier>keyword_d</span><span class=special>(</span><span class=string>&quot;a-zA-Z0-9_&quot;</span><span class=special>);
  61
  62     </span><span class=identifier>symbol</span><span class=special>&lt;&gt; </span><span class=identifier>keywords </span><span class=special>= </span><span class=string>&quot;declare&quot;</span><span class=special>, </span><span class=string>&quot;begin&quot;</span><span class=special>, </span><span class=string>&quot;end&quot;</span><span class=special>;
  63     </span><span class=identifier>rule</span><span class=special>&lt;</span><span class="identifier">ScannerT</span><span class=special>&gt; </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>
  66 In some cases a set of forbidden follow-up characters is not sufficient.
  67 For example ASN.1 naming conventions allows identifiers to contain dashes,
  68 but not double dashes (which marks the beginning of a comment).
  69 Furthermore, identifiers can't end with a dash. So, a matched keyword can't
  70 be followed by any alphanumeric character or exactly one dash, but can be
  71 followed by two dashes.</p>
  72 <p>
  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
  74 parser 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>&lt;&gt; </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>&gt;&gt; ~</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>&lt;</span><span class="identifier">ScannerT</span><span class=special>&gt; </span><span class=identifier>declaration_rule </span><span class=special>= </span><span class=identifier>keyword_p</span><span class=special>(</span><span class=string>&quot;declare&quot;</span><span class=special>) &gt;&gt; </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>
  83 Since the <tt>dynamic_distinct_parser</tt> internally uses a rule, its type is
  84 dependent on the scanner type. So, the <tt>keyword_p</tt> shouldn't be defined
  85 globally, but rather within the grammar.</p>
  86 <p>
  87 See the full <a href="../example/fundamental/distinct/distinct_parser_dynamic.cpp">example here</a>.</p>
  88 <h3>How it works</h3><p>
  89 When 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>&lt;&gt; </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>&lt;&gt; </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>
  95 the 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>
 101 are 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>) &gt;&gt; ~</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>) &gt;&gt; ~</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>
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 116 <p class="copyright">Copyright &copy; 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>
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