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<title>Qi Distinct Parser Directive</title>
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<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="spirit_repository.qi_components.directives.distinct"></a><a class="link" href="distinct.html" title="Qi Distinct Parser Directive">Qi
Distinct Parser Directive</a>
</h4></div></div></div>
<h6>
<a name="spirit_repository.qi_components.directives.distinct.h0"></a>
<span><a name="spirit_repository.qi_components.directives.distinct.description"></a></span><a class="link" href="distinct.html#spirit_repository.qi_components.directives.distinct.description">Description</a>
</h6>
<p>
The <span class="emphasis"><em>Spirit.Qi</em></span> <code class="computeroutput"><span class="identifier">distinct</span></code>
parser is a directive component allowing to avoid partial matches while
parsing using a skipper. A simple example is the common task of matching
a C keyword. Consider:
</p>
<pre class="programlisting"><span class="string">"description"</span> <span class="special">>></span> <span class="special">-</span><span class="identifier">lit</span><span class="special">(</span><span class="string">":"</span><span class="special">)</span> <span class="special">>></span> <span class="special">*(</span><span class="identifier">char_</span> <span class="special">-</span> <span class="identifier">eol</span><span class="special">)</span>
</pre>
<p>
intended to match a line in a configuration file. Let's assume further,
that this rule is used with a <code class="computeroutput"><span class="identifier">space</span></code>
skipper and that we have the following strings in the input:
</p>
<pre class="programlisting"><span class="string">"description: ident\n"</span>
<span class="string">"description ident\n"</span>
<span class="string">"descriptionident\n"</span>
</pre>
<p>
It might seem unexpected, but the parser above matches all three inputs
just fine, even if the third input should not match at all! In order to
avoid the unwanted match we are forced to make our rule more complicated:
</p>
<pre class="programlisting"><span class="identifier">lexeme</span><span class="special">[</span><span class="string">"description"</span> <span class="special">>></span> <span class="special">!</span><span class="identifier">char_</span><span class="special">(</span><span class="string">"a-zA-Z_0-9"</span><span class="special">)]</span> <span class="special">>></span> <span class="special">-</span><span class="identifier">lit</span><span class="special">(</span><span class="string">":"</span><span class="special">)</span> <span class="special">>></span> <span class="special">*(</span><span class="identifier">char_</span> <span class="special">-</span> <span class="identifier">eol</span><span class="special">)</span>
</pre>
<p>
(the rule reads as: match <code class="computeroutput"><span class="string">"description"</span></code>
as long as it's not <span class="emphasis"><em>directly</em></span> followed by a valid identifier).
</p>
<p>
The <code class="computeroutput"><span class="identifier">distinct</span><span class="special">[]</span></code>
directive is meant to simplify the rule above:
</p>
<pre class="programlisting"><span class="identifier">distinct</span><span class="special">(</span><span class="identifier">char_</span><span class="special">(</span><span class="string">"a-zA-Z_0-9"</span><span class="special">))[</span><span class="string">"description"</span><span class="special">]</span> <span class="special">>></span> <span class="special">-</span><span class="identifier">lit</span><span class="special">(</span><span class="string">":"</span><span class="special">)</span> <span class="special">>></span> <span class="special">*(</span><span class="identifier">char_</span> <span class="special">-</span> <span class="identifier">eol</span><span class="special">)</span>
</pre>
<p>
Using the <code class="computeroutput"><span class="identifier">distinct</span><span class="special">[]</span></code>
component instead of the explicit sequence has the advantage of being able
to encapsulate the tail (i.e the <code class="computeroutput"><span class="identifier">char_</span><span class="special">(</span><span class="string">"a-zA-Z_0-9"</span><span class="special">)</span></code>) as a separate parser construct. The following
code snippet illustrates the idea (for the full code of this example please
see <a href="../../../../../test/qi/distinct.cpp" target="_top">distinct.cpp</a>):
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">spirit</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">;</span>
<span class="keyword">namespace</span> <span class="identifier">ascii</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">ascii</span><span class="special">;</span>
<span class="keyword">namespace</span> <span class="identifier">repo</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">repository</span><span class="special">;</span>
<span class="comment">// Define metafunctions allowing to compute the type of the distinct()</span>
<span class="comment">// and ascii::char_() constructs</span>
<span class="keyword">namespace</span> <span class="identifier">traits</span>
<span class="special">{</span>
<span class="comment">// Metafunction allowing to get the type of any repository::distinct(...) </span>
<span class="comment">// construct</span>
<span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Tail</span><span class="special">></span>
<span class="keyword">struct</span> <span class="identifier">distinct_spec</span>
<span class="special">:</span> <span class="identifier">spirit</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="identifier">repo</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">distinct</span><span class="special">(</span><span class="identifier">Tail</span><span class="special">)></span>
<span class="special">{};</span>
<span class="comment">// Metafunction allowing to get the type of any ascii::char_(...) construct</span>
<span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">String</span><span class="special">></span>
<span class="keyword">struct</span> <span class="identifier">char_spec</span>
<span class="special">:</span> <span class="identifier">spirit</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">ascii</span><span class="special">::</span><span class="identifier">char_</span><span class="special">(</span><span class="identifier">String</span><span class="special">)></span>
<span class="special">{};</span>
<span class="special">};</span>
<span class="comment">// Define a helper function allowing to create a distinct() construct from </span>
<span class="comment">// an arbitrary tail parser</span>
<span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Tail</span><span class="special">></span>
<span class="keyword">inline</span> <span class="keyword">typename</span> <span class="identifier">traits</span><span class="special">::</span><span class="identifier">distinct_spec</span><span class="special"><</span><span class="identifier">Tail</span><span class="special">>::</span><span class="identifier">type</span>
<span class="identifier">distinct_spec</span><span class="special">(</span><span class="identifier">Tail</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">tail</span><span class="special">)</span>
<span class="special">{</span>
<span class="keyword">return</span> <span class="identifier">repo</span><span class="special">::</span><span class="identifier">distinct</span><span class="special">(</span><span class="identifier">tail</span><span class="special">);</span>
<span class="special">}</span>
<span class="comment">// Define a helper function allowing to create a ascii::char_() construct </span>
<span class="comment">// from an arbitrary string representation</span>
<span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">String</span><span class="special">></span>
<span class="keyword">inline</span> <span class="keyword">typename</span> <span class="identifier">traits</span><span class="special">::</span><span class="identifier">char_spec</span><span class="special"><</span><span class="identifier">String</span><span class="special">>::</span><span class="identifier">type</span>
<span class="identifier">char_spec</span><span class="special">(</span><span class="identifier">String</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">str</span><span class="special">)</span>
<span class="special">{</span>
<span class="keyword">return</span> <span class="identifier">ascii</span><span class="special">::</span><span class="identifier">char_</span><span class="special">(</span><span class="identifier">str</span><span class="special">);</span>
<span class="special">}</span>
<span class="comment">// the following constructs the type of a distinct_spec holding a</span>
<span class="comment">// charset("0-9a-zA-Z_") as its tail parser</span>
<span class="keyword">typedef</span> <span class="identifier">traits</span><span class="special">::</span><span class="identifier">char_spec</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">>::</span><span class="identifier">type</span> <span class="identifier">charset_tag_type</span><span class="special">;</span>
<span class="keyword">typedef</span> <span class="identifier">traits</span><span class="special">::</span><span class="identifier">distinct_spec</span><span class="special"><</span><span class="identifier">charset_tag_type</span><span class="special">>::</span><span class="identifier">type</span> <span class="identifier">keyword_tag_type</span><span class="special">;</span>
<span class="comment">// Define a new Qi 'keyword' directive usable as a shortcut for a</span>
<span class="comment">// repository::distinct(char_(std::string("0-9a-zA-Z_")))</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="keyword">const</span> <span class="identifier">keyword_spec</span><span class="special">(</span><span class="string">"0-9a-zA-Z_"</span><span class="special">);</span>
<span class="identifier">keyword_tag_type</span> <span class="keyword">const</span> <span class="identifier">keyword</span> <span class="special">=</span> <span class="identifier">distinct_spec</span><span class="special">(</span><span class="identifier">char_spec</span><span class="special">(</span><span class="identifier">keyword_spec</span><span class="special">));</span>
</pre>
<p>
</p>
<p>
These definitions define a new Qi parser recognizing keywords! This allows
to rewrite our declaration parser expression as:
</p>
<pre class="programlisting"><span class="identifier">keyword</span><span class="special">[</span><span class="string">"description"</span><span class="special">]</span> <span class="special">>></span> <span class="special">-</span><span class="identifier">lit</span><span class="special">(</span><span class="string">":"</span><span class="special">)</span> <span class="special">>></span> <span class="special">*(</span><span class="identifier">char_</span> <span class="special">-</span> <span class="identifier">eol</span><span class="special">)</span>
</pre>
<p>
which is much more readable and concise if compared to the original parser
expression. In addition the new <code class="computeroutput"><span class="identifier">keyword</span><span class="special">[]</span></code> directive has the advantage to be usable
for wrapping any parser expression, not only strings as in the example
above.
</p>
<h6>
<a name="spirit_repository.qi_components.directives.distinct.h1"></a>
<span><a name="spirit_repository.qi_components.directives.distinct.header"></a></span><a class="link" href="distinct.html#spirit_repository.qi_components.directives.distinct.header">Header</a>
</h6>
<pre class="programlisting"><span class="comment">// forwards to <boost/spirit/repository/home/qi/directive/distinct.hpp></span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">spirit</span><span class="special">/</span><span class="identifier">repository</span><span class="special">/</span><span class="identifier">include</span><span class="special">/</span><span class="identifier">qi_distinct</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
</pre>
<h6>
<a name="spirit_repository.qi_components.directives.distinct.h2"></a>
<span><a name="spirit_repository.qi_components.directives.distinct.synopsis"></a></span><a class="link" href="distinct.html#spirit_repository.qi_components.directives.distinct.synopsis">Synopsis</a>
</h6>
<pre class="programlisting"><span class="identifier">distinct</span><span class="special">(</span><span class="identifier">tail</span><span class="special">)[</span><span class="identifier">subject</span><span class="special">]</span>
</pre>
<h6>
<a name="spirit_repository.qi_components.directives.distinct.h3"></a>
<span><a name="spirit_repository.qi_components.directives.distinct.parameters"></a></span><a class="link" href="distinct.html#spirit_repository.qi_components.directives.distinct.parameters">Parameters</a>
</h6>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
<p>
Parameter
</p>
</th>
<th>
<p>
Description
</p>
</th>
</tr></thead>
<tbody>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">tail</span></code>
</p>
</td>
<td>
<p>
The parser construct specifying what whould not follow the subject
in order to match the overall expression.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">subject</span></code>
</p>
</td>
<td>
<p>
The parser construct to use to match the current input. The distinct
directive makes sure that no unexpected partial matches occur.
</p>
</td>
</tr>
</tbody>
</table></div>
<p>
All two parameters can be arbitrary complex parsers themselves.
</p>
<h6>
<a name="spirit_repository.qi_components.directives.distinct.h4"></a>
<span><a name="spirit_repository.qi_components.directives.distinct.attribute"></a></span><a class="link" href="distinct.html#spirit_repository.qi_components.directives.distinct.attribute">Attribute</a>
</h6>
<p>
The <code class="computeroutput"><span class="identifier">distinct</span></code> component
exposes the attribute type of its subject as its own attribute type. If
the <code class="computeroutput"><span class="identifier">subject</span></code> does not expose
any attribute (the type is <code class="computeroutput"><span class="identifier">unused_type</span></code>),
then the <code class="computeroutput"><span class="identifier">distinct</span></code> does
not expose any attribute either.
</p>
<pre class="programlisting"><span class="identifier">a</span><span class="special">:</span> <span class="identifier">A</span><span class="special">,</span> <span class="identifier">b</span><span class="special">:</span> <span class="identifier">B</span> <span class="special">--></span> <span class="identifier">distinct</span><span class="special">(</span><span class="identifier">b</span><span class="special">)[</span><span class="identifier">a</span><span class="special">]:</span> <span class="identifier">A</span>
</pre>
<h6>
<a name="spirit_repository.qi_components.directives.distinct.h5"></a>
<span><a name="spirit_repository.qi_components.directives.distinct.example"></a></span><a class="link" href="distinct.html#spirit_repository.qi_components.directives.distinct.example">Example</a>
</h6>
<p>
The following example shows simple use cases of the <code class="computeroutput"><span class="identifier">distinct</span></code>
parser. <a href="../../../../../example/qi/distinct.cpp" target="_top">distinct.cpp</a>)
</p>
<h6>
<a name="spirit_repository.qi_components.directives.distinct.h6"></a>
<span><a name="spirit_repository.qi_components.directives.distinct.prerequisites"></a></span><a class="link" href="distinct.html#spirit_repository.qi_components.directives.distinct.prerequisites">Prerequisites</a>
</h6>
<p>
In addition to the main header file needed to include the core components
implemented in <span class="emphasis"><em>Spirit.Qi</em></span> we add the header file needed
for the new <code class="computeroutput"><span class="identifier">distinct</span></code> generator.
</p>
<p>
</p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">spirit</span><span class="special">/</span><span class="identifier">include</span><span class="special">/</span><span class="identifier">qi</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">spirit</span><span class="special">/</span><span class="identifier">repository</span><span class="special">/</span><span class="identifier">include</span><span class="special">/</span><span class="identifier">qi_distinct</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
</pre>
<p>
</p>
<p>
To make all the code below more readable we introduce the following namespaces.
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">using</span> <span class="keyword">namespace</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">;</span>
<span class="keyword">using</span> <span class="keyword">namespace</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">ascii</span><span class="special">;</span>
<span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">repository</span><span class="special">::</span><span class="identifier">distinct</span><span class="special">;</span>
</pre>
<p>
</p>
<h6>
<a name="spirit_repository.qi_components.directives.distinct.h7"></a>
<span><a name="spirit_repository.qi_components.directives.distinct.using_the_distinct_directive_to_match_keywords"></a></span><a class="link" href="distinct.html#spirit_repository.qi_components.directives.distinct.using_the_distinct_directive_to_match_keywords">Using
The Distinct Directive to Match keywords</a>
</h6>
<p>
We show several examples of how the <code class="computeroutput"><span class="identifier">distinct</span><span class="special">[]</span></code> directive can be used to force correct
behavior while matching keywords. The first two code snippets show the
correct matching of the <code class="computeroutput"><span class="identifier">description</span></code>
keyword (in this hypothetical example we allow keywords to be directly
followed by an optional <code class="computeroutput"><span class="string">"--"</span></code>):
</p>
<p>
</p>
<pre class="programlisting"><span class="special">{</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">str</span><span class="special">(</span><span class="string">"description ident"</span><span class="special">);</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">::</span><span class="identifier">iterator</span> <span class="identifier">first</span><span class="special">(</span><span class="identifier">str</span><span class="special">.</span><span class="identifier">begin</span><span class="special">());</span>
<span class="keyword">bool</span> <span class="identifier">r</span> <span class="special">=</span> <span class="identifier">qi</span><span class="special">::</span><span class="identifier">phrase_parse</span><span class="special">(</span><span class="identifier">first</span><span class="special">,</span> <span class="identifier">str</span><span class="special">.</span><span class="identifier">end</span><span class="special">()</span>
<span class="special">,</span> <span class="identifier">distinct</span><span class="special">(</span><span class="identifier">alnum</span> <span class="special">|</span> <span class="char">'_'</span><span class="special">)[</span><span class="string">"description"</span><span class="special">]</span> <span class="special">>></span> <span class="special">-</span><span class="identifier">lit</span><span class="special">(</span><span class="string">"--"</span><span class="special">)</span> <span class="special">>></span> <span class="special">+(</span><span class="identifier">alnum</span> <span class="special">|</span> <span class="char">'_'</span><span class="special">)</span>
<span class="special">,</span> <span class="identifier">space</span><span class="special">);</span>
<span class="identifier">BOOST_ASSERT</span><span class="special">(</span><span class="identifier">r</span> <span class="special">&&</span> <span class="identifier">first</span> <span class="special">==</span> <span class="identifier">str</span><span class="special">.</span><span class="identifier">end</span><span class="special">());</span>
<span class="special">}</span>
</pre>
<p>
</p>
<p>
</p>
<pre class="programlisting"><span class="special">{</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">str</span><span class="special">(</span><span class="string">"description--ident"</span><span class="special">);</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">::</span><span class="identifier">iterator</span> <span class="identifier">first</span><span class="special">(</span><span class="identifier">str</span><span class="special">.</span><span class="identifier">begin</span><span class="special">());</span>
<span class="keyword">bool</span> <span class="identifier">r</span> <span class="special">=</span> <span class="identifier">qi</span><span class="special">::</span><span class="identifier">phrase_parse</span><span class="special">(</span><span class="identifier">first</span><span class="special">,</span> <span class="identifier">str</span><span class="special">.</span><span class="identifier">end</span><span class="special">()</span>
<span class="special">,</span> <span class="identifier">distinct</span><span class="special">(</span><span class="identifier">alnum</span> <span class="special">|</span> <span class="char">'_'</span><span class="special">)[</span><span class="string">"description"</span><span class="special">]</span> <span class="special">>></span> <span class="special">-</span><span class="identifier">lit</span><span class="special">(</span><span class="string">"--"</span><span class="special">)</span> <span class="special">>></span> <span class="special">+(</span><span class="identifier">alnum</span> <span class="special">|</span> <span class="char">'_'</span><span class="special">)</span>
<span class="special">,</span> <span class="identifier">space</span><span class="special">);</span>
<span class="identifier">BOOST_ASSERT</span><span class="special">(</span><span class="identifier">r</span> <span class="special">&&</span> <span class="identifier">first</span> <span class="special">==</span> <span class="identifier">str</span><span class="special">.</span><span class="identifier">end</span><span class="special">());</span>
<span class="special">}</span>
</pre>
<p>
</p>
<p>
The last example shows that the <code class="computeroutput"><span class="identifier">distinct</span><span class="special">[]</span></code> parser component correctly refuses to
match "description-ident":
</p>
<p>
</p>
<pre class="programlisting"><span class="special">{</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">str</span><span class="special">(</span><span class="string">"description-ident"</span><span class="special">);</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">::</span><span class="identifier">iterator</span> <span class="identifier">first</span><span class="special">(</span><span class="identifier">str</span><span class="special">.</span><span class="identifier">begin</span><span class="special">());</span>
<span class="keyword">bool</span> <span class="identifier">r</span> <span class="special">=</span> <span class="identifier">qi</span><span class="special">::</span><span class="identifier">phrase_parse</span><span class="special">(</span><span class="identifier">first</span><span class="special">,</span> <span class="identifier">str</span><span class="special">.</span><span class="identifier">end</span><span class="special">()</span>
<span class="special">,</span> <span class="identifier">distinct</span><span class="special">(</span><span class="identifier">alnum</span> <span class="special">|</span> <span class="char">'_'</span><span class="special">)[</span><span class="string">"description"</span><span class="special">]</span> <span class="special">>></span> <span class="special">-</span><span class="identifier">lit</span><span class="special">(</span><span class="string">"--"</span><span class="special">)</span> <span class="special">>></span> <span class="special">+(</span><span class="identifier">alnum</span> <span class="special">|</span> <span class="char">'_'</span><span class="special">)</span>
<span class="special">,</span> <span class="identifier">space</span><span class="special">);</span>
<span class="identifier">BOOST_ASSERT</span><span class="special">(!</span><span class="identifier">r</span> <span class="special">&&</span> <span class="identifier">first</span> <span class="special">==</span> <span class="identifier">str</span><span class="special">.</span><span class="identifier">begin</span><span class="special">());</span>
<span class="special">}</span>
</pre>
<p>
</p>
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