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Joel de Guzman 994d4e48cc moving stuff to classic spirit 
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<title>The Scanner and Parsing</title>
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<font size="6" face="Verdana, Arial, Helvetica, sans-serif"><b>The Scanner and Parsing</b></font>
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<p>The <b>scanner</b>'s task is to feed the sequential input data stream to the
parser. The scanner extracts data from the input, parceling, potentially modifying
or filtering, and then finally relegating the result to individual parser elements
on demand until the input is exhausted. The scanner is composed of two STL conforming
forward iterators, first and last, where first is held by reference and last,
by value. The first iterator is held by reference to allow it to be re-positioned.
The following diagram illustrates what's happening:</p>
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<td><img src="theme/scanner1.png"></td>
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<p>The scanner manages various aspects of the parsing process through a set of
policies. There are three sets of policies that govern:</p>
<blockquote>
<p><img src="theme/bullet.gif" width="12" height="12"> Iteration and filtering<br>
<img src="theme/bullet.gif" width="12" height="12"> Recognition and matching<br>
<img src="theme/bullet.gif" width="12" height="12"> Handling semantic actions</p>
</blockquote>
<p>These policies are mostly hidden from view and users generally need not know
about them. Advanced users might however provide their own policies that override
the ones that are already in place to fine tune the parsing process
to fit their own needs. We shall see how this can be done. This will be covered
in further detail later.</p>
<p>The <tt>scanner</tt> is a template class expecting two parameters: <tt>IteratorT</tt>,
the iterator type and <tt>PoliciesT</tt>, its set of policies. <tt>IteratorT</tt>
defaults to <tt>char const*</tt> while <tt>PoliciesT</tt> defaults to <tt>scanner_policies&lt;&gt;</tt>,
a predefined set of scanner policies that we can use straight out of the box.</p>
<pre><code><font color="#000000"><span class=keyword> template</span><span class=special>&lt;
</span><span class=keyword>typename </span><span class=identifier>IteratorT </span><span class=special>= </span><span class=keyword>char </span><span class=keyword>const</span><span class=special>*,
</span><span class=keyword>typename </span><span class=identifier>PoliciesT </span><span class=special>= </span><span class=identifier>scanner_policies</span><span class=special>&lt;&gt; </span><span class=special>&gt;
</span><span class=keyword>class </span><span class=identifier>scanner</span><span class=special>;</span></font></code></pre>
<p>Spirit uses the same iterator concepts and interface formally defined by the
C++ Standard Template Library (STL). We can use iterators supplied by STL's
containers (e.g. <tt>list</tt>, <tt>vector</tt>, <tt>string</tt>, etc.) as is,
or perhaps write our own. Iterators can be as simple as a pointer (e.g. <tt>char
const<span class="operators">*</span></tt>). At the other end of the spectrum,
iterators can be quite complex; for instance, an iterator adapter that wraps
a lexer such as LEX.</p>
<h2>The Free Parse Functions</h2>
<p>The framework provides a couple of free functions to make parsing a snap. These
parser functions have two forms. The first form works on the <b>character level</b>.
The second works on the <b>phrase level</b> and asks for a <b>skip parser</b>.</p>
<p>The <b>skip parser</b> is just about any parser primitive or composite. Its
purpose is to move the scanner's <tt>first</tt> iterator to valid tokens by
skipping white spaces. In C for instance, the tab <tt class="quotes">'\t'</tt>,
the newline <tt class="quotes">'\n'</tt>, return <tt><span class="quotes">'\r'</span></tt>,
space <tt class="quotes">' '</tt> and characters inside comments <tt class="quotes">/*...*/</tt>
are considered as white spaces.</p>
<p><b>Character level parsing</b></p>
<pre><code><font color="#000000"><span class=special> </span><span class=keyword>template </span><span class=special>&lt;</span><span class=keyword>typename </span><span class=identifier>IteratorT</span><span class=special>, </span><span class=keyword>typename </span><span class=identifier>DerivedT</span><span class=special>&gt;
</span><span class=identifier>parse_info</span><span class=special>&lt;</span><span class=identifier>IteratorT</span><span class=special>&gt;
</span><span class=identifier>parse
</span><span class=special>(
</span><span class=identifier>IteratorT </span><span class=keyword>const</span><span class=special>& </span><span class=identifier>first</span><span class=special>,
</span><span class=identifier>IteratorT </span><span class=keyword>const</span><span class=special>& </span><span class=identifier>last</span><span class=special>,
</span><span class=identifier>parser</span><span class=special>&lt;</span><span class=identifier>DerivedT</span><span class=special>&gt; </span><span class=keyword>const</span><span class=special>& </span><span class=identifier>p
</span><span class=special>);</span></font></code></pre>
<pre><code><font color="#000000"><span class=special> </span><span class=keyword>template </span><span class=special>&lt;</span><span class=keyword>typename </span><span class=identifier>CharT</span><span class=special>, </span><span class=keyword>typename </span><span class=identifier>DerivedT</span><span class=special>&gt;
</span><span class=identifier>parse_info</span><span class=special>&lt;</span><span class=identifier>CharT </span><span class=keyword>const</span><span class=special>*&gt;
</span><span class=identifier>parse
</span><span class=special>(
</span><span class=identifier>CharT </span><span class=keyword>const</span><span class=special>* </span><span class=identifier>str</span><span class=special>,
</span><span class=identifier>parser</span><span class=special>&lt;</span><span class=identifier>DerivedT</span><span class=special>&gt; </span><span class=keyword>const</span><span class=special>& </span><span class=identifier>p
</span><span class=special>);</span></font></code></pre>
<p>There are two variants. The first variant accepts a <tt>first</tt>, <tt>last</tt>
iterator pair like you do STL algorithms. The second variant accepts a null
terminated string. The last argument is a parser <tt>p</tt> which will be used
to parse the input.</p>
<p><b>Phrase level parsing</b></p>
<pre><code><font color="#000000"><span class=special> </span><span class=keyword>template </span><span class=special>&lt;</span><span class=keyword>typename </span><span class=identifier>IteratorT</span><span class=special>, </span><span class=keyword>typename </span><span class=identifier>ParserT</span><span class=special>, </span><span class=keyword>typename </span><span class=identifier>SkipT</span><span class=special>&gt;
</span><span class=identifier>parse_info</span><span class=special>&lt;</span><span class=identifier>IteratorT</span><span class=special>&gt;
</span><span class=identifier>parse
</span><span class=special>(
</span><span class=identifier>IteratorT </span><span class=keyword>const</span><span class=special>& </span><span class=identifier>first</span><span class=special>,
</span><span class=identifier>IteratorT </span><span class=keyword>const</span><span class=special>& </span><span class=identifier>last</span><span class=special>,
</span><span class=identifier>parser</span><span class=special>&lt;</span><span class=identifier>ParserT</span><span class=special>&gt; </span><span class=keyword>const</span><span class=special>& </span><span class=identifier>p</span><span class=special>,
</span><span class=identifier>parser</span><span class=special>&lt;</span><span class=identifier>SkipT</span><span class=special>&gt; </span><span class=keyword>const</span><span class=special>& </span><span class=identifier>skip
</span><span class=special>);</span></font></code></pre>
<pre><code><font color="#000000"><span class=special> </span><span class=keyword>template </span><span class=special>&lt;</span><span class=keyword>typename </span><span class=identifier>CharT</span><span class=special>, </span><span class=keyword>typename </span><span class=identifier>ParserT</span><span class=special>, </span><span class=keyword>typename </span><span class=identifier>SkipT</span><span class=special>&gt;
</span><span class=identifier>parse_info</span><span class=special>&lt;</span><span class=identifier>CharT </span><span class=keyword>const</span><span class=special>*&gt;
</span><span class=identifier>parse
</span><span class=special>(
</span><span class=identifier>CharT </span><span class=keyword>const</span><span class=special>* </span><span class=identifier>str</span><span class=special>,
</span><span class=identifier>parser</span><span class=special>&lt;</span><span class=identifier>ParserT</span><span class=special>&gt; </span><span class=keyword>const</span><span class=special>& </span><span class=identifier>p</span><span class=special>,
</span><span class=identifier>parser</span><span class=special>&lt;</span><span class=identifier>SkipT</span><span class=special>&gt; </span><span class=keyword>const</span><span class=special>& </span><span class=identifier>skip
</span><span class=special>);</span></font></code></pre>
<p>Like above, there are two variants. The first variant accepts a <tt>first</tt>,
<tt>last</tt> iterator pair like you do STL algorithms. The second variant accepts
a null terminated string. The argument <tt>p</tt> is the parser which will be
used to parse the input. The last argument <tt>skip</tt> is the skip parser.</p>
<p><b>The parse_info structure</b></p>
<p>The functions above return a <tt>parse_info</tt> structure parameterized by
the iterator type passed in. The parse_info struct has these members:</p>
<table width="90%" border="0" align="center">
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<td colspan="2" class="table_title"><b>parse_info</b></td>
</tr>
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<td width="14%" class="table_cells"><b>stop</b></td>
<td width="86%" class="table_cells">Points to the final parse position (i.e
The parser recognized and processed the input up to this point)</td>
</tr>
<tr>
<td width="14%" class="table_cells"><b>hit</b></td>
<td width="86%" class="table_cells">True if parsing is successful. This may
be full: the parser consumed all the input, or partial: the parser consumed
only a portion of the input.</td>
</tr>
<tr>
<td width="14%" class="table_cells"><b>full</b></td>
<td width="86%" class="table_cells">True when we have a full match (i.e The
parser consumed all the input).</td>
</tr>
<tr>
<td width="14%" class="table_cells"><b>length</b></td>
<td width="86%" class="table_cells">The number of characters consumed by the
parser. This is valid only if we have a successful match (either partial
or full). </td>
</tr>
</table>
<h2><a name="phrase_scanner_t" id="phrase_scanner_t"></a><img src="theme/lens.gif" width="15" height="16">
The phrase_scanner_t and wide_phrase_scanner_t</h2>
<p>For convenience, Spirit declares these typedefs:</p>
<pre>
<span class="keyword">typedef</span> scanner<span class="special">&lt;</span><span class="keyword">char const</span><span class="special">*,</span> unspecified<span class="special">&gt;</span> phrase_scanner_t<span class="special">;</span>
<span class="keyword">typedef</span> scanner<span class="special">&lt;</span><span class="keyword">wchar_t const</span><span class="special">*,</span> <span class="identifier">unspecified</span><span class="special">&gt;</span> wide_phrase_scanner_t<span class="special">;</span>
</pre>
<p>These are the exact scanner types used by Spirit on calls to the parse function
passing in a <tt>char const*</tt> (C string) or a <tt>wchar_t const*</tt> (wide
string) as the first parameter and a <tt>space_p</tt> as skip-parser (the third
parameter). For instance, we can use these typedefs to declare some rules. Example:</p>
<pre> rule<span class="special">&lt;</span>phrase_scanner_t<span class="special">&gt; </span><span class="identifier">my_rule</span><span class="special">;
</span><span class="identifier">parse</span><span class="special">(</span><span class="string">&quot;abrakadabra&quot;</span><span class="special">, </span><span class="identifier">my_rule</span><span class="special">,</span> <span class="identifier">space_p</span><span class="special">);</span></pre>
<h2><img src="theme/lens.gif" width="15" height="16"> Direct parsing with Iterators</h2>
<p>The free parse functions make it easy for us. By using them, we need not bother
with the scanner intricacies. The free parse functions hide the dirty details.
However, sometime in the future, we will need to get under the hood. It's nice
that we know what we are dealing with when that need comes. We will need to
go low-level and call the parser's parse member function directly. </p>
<p>If we wish to work on the <b>character level</b>, the procedure is quite simple:</p>
<pre><span class=identifier> </span><span class=identifier>scanner</span><span class=special>&lt;</span><span class=identifier>IteratorT</span><span class=special>&gt; </span><span class=identifier>scan</span><span class=special>(</span><span class=identifier>first</span><span class=special>, </span><span class=identifier>last</span><span class=special>);
</span><span class=keyword>if </span><span class=special>(</span><span class=identifier>p</span><span class=special>.</span><span class=identifier>parse</span><span class=special>(</span><span class=identifier>scan</span><span class=special>))
</span><span class=special>{
</span><span class=comment>// Parsed successfully. If first == last, then we have
// a full parse, the parser recognized the input in whole.
</span><span class=special>}
</span><span class=keyword>else
</span><span class=special>{
</span><span class=comment>// Parsing failure. The parser failed to recognize the input
</span><span class=special>}</span></pre>
<table width="80%" border="0" align="center">
<tr>
<td class="note_box"><img src="theme/alert.gif" width="16" height="16"> <strong>The
scanner position on an unsucessful match</strong><br> <br>
On a successful match, the input is advanced accordingly. But what happens
on an unsuccessful match? Be warned. It might be intuitive to think that
the scanner position is reset to its initial position prior to parsing.
No, the position is not reset. On an unsuccessful match, the position of
the scanner is <strong>undefined</strong>! Usually, it is positioned at
the farthest point where the error was found somewhere down the recursive
descent. If this behavior is not desired, you may need to position the scanner
yourself. The <a href="numerics.html#scanner_save">example in the numerics
chapter</a> illustrates how the scanner position can be saved and later
restored.</td>
</tr>
</table>
<p>Where <tt>p</tt> is the parser we want to use, and <tt>first</tt>/<tt>last</tt>
are the iterator pairs referring to the input. We just create a scanner given
the iterators. The scanner type we will use here uses the default <tt>scanner_policies&lt;&gt;</tt>.</p>
<p>The situation is a bit more complex when we wish to work on the <b>phrase level</b>:</p>
<pre><span class=special> </span><span class=keyword>typedef </span><span class=identifier>skip_parser_iteration_policy</span><span class=special>&lt;</span><span class=identifier>SkipT</span><span class=special>&gt; </span><span class=identifier>iter_policy_t</span><span class=special>;
</span><span class=keyword>typedef </span><span class=identifier>scanner_policies</span><span class=special>&lt;</span><span class=identifier>iter_policy_t</span><span class=special>&gt; </span><span class=identifier>scanner_policies_t</span><span class=special>;
</span><span class=keyword>typedef </span><span class=identifier>scanner</span><span class=special>&lt;</span><span class=identifier>IteratorT</span><span class=special>, </span><span class=identifier>scanner_policies_t</span><span class=special>&gt; </span><span class=identifier>scanner_t</span><span class=special>;
</span><span class=special> </span><span class=identifier>iter_policy_t </span><span class=identifier>iter_policy</span><span class=special>(</span><span class=identifier>skip</span><span class=special>);
</span><span class=identifier>scanner_policies_t </span><span class=identifier>policies</span><span class=special>(</span><span class=identifier>iter_policy</span><span class=special>);
</span><span class=identifier>scanner_t </span><span class=identifier>scan</span><span class=special>(</span><span class=identifier>first</span><span class=special>, </span><span class=identifier>last</span><span class=special>, </span><span class=identifier>policies</span><span class=special>);
</span>
<span class=keyword>if </span><span class=special>(</span><span class=identifier>p</span><span class=special>.</span><span class=identifier>parse</span><span class=special>(</span><span class=identifier>scan</span><span class=special>))
</span><span class=special>{
</span><span class=comment>// Parsed successfully. If first == last, then we have
// a full parse, the parser recognized the input in whole.
</span><span class=special>}
</span><span class=keyword>else
</span><span class=special>{
</span><span class=comment>// Parsing failure. The parser failed to recognize the input
</span><span class=special>}</span></pre>
<p>Where <tt>SkipT</tt> is the type of the skip-parser, <tt>skip</tt>. Again,
<tt>p</tt> is the parser we want to use, and <tt>first</tt>/<tt>last</tt> are
the iterator pairs referring to the input. Given a skip-parser type <tt>SkipT</tt>,
<span class=identifier><tt>skip_parser_iteration_policy</tt></span> creates
a scanner iteration policy that skips over portions that are recognized by the
skip-parser. This may then be used to create a scanner. The <tt>scanner_policies</tt>
class wraps all scanner related policies including the iteration policies.</p>
<h2><a name="lexeme_scanner"></a>lexeme_scanner</h2>
<p>When switching from phrase level to character level parsing, the <tt>lexeme_d</tt>
(see <a href="directives.html">directives.html</a>) does its magic by disabling
the skipping of white spaces. This is done by tweaking the <a href="scanner.html">scanner</a>.
However, when we do this, all parsers inside the lexeme gets a transformed scanner
type. This should not be a problem in most cases. However, when rules are called
inside the <tt>lexeme_d</tt>, the compiler will choke if the rule does not have
the proper scanner type. If a rule must be used inside a <tt>lexeme_d</tt>,
the rule's type must be:</p>
<pre> <span class=identifier>rule</span><span class=special>&lt;</span><span class=identifier>lexeme_scanner</span><span class="special">&lt;</span><span class=identifier>ScannerT</span><span class=special>&gt;::</span><span class="identifier">type</span><span class=special>&gt; </span>r<span class=special>;</span></pre>
<p>where <span class=identifier><tt>ScannerT</tt></span> is the actual type of
the scanner used. Take note that <tt>lexeme_scanner</tt> will only work for phrase level scanners. </p>
<h2><a name="as_lower_scanner"></a>as_lower_scanner</h2>
<p>Similarly, the <tt>as_lower_d</tt> does its work by filtering and converting
all characters received from the scanner to lower case. This is also done by
tweaking the <a href="scanner.html">scanner</a>. Then again, all parsers inside
the <tt>as_lower_d</tt> gets a transformed scanner type. If a rule must be used
inside a <tt>as_lower_d</tt>, the rule's type must be:</p>
<pre> <span class=identifier>rule</span><span class=special>&lt;</span><span class=identifier>as_lower_scanner</span><span class="special">&lt;</span><span class=identifier>ScannerT</span><span class=special>&gt;::</span><span class="identifier">type</span><span class=special>&gt; </span>r<span class=special>;</span></pre>
<p>where <span class=identifier><tt>ScannerT</tt></span> is the actual type of
the scanner used. </p>
<table width="80%" border="0" align="center">
<tr>
<td class="note_box"><img src="theme/bulb.gif" width="13" height="18"> See
the techniques section for an <a href="techniques.html#multiple_scanner_support">example</a>
of a <a href="grammar.html">grammar</a> using a <a href="rule.html#multiple_scanner_support">multiple
scanner enabled rule</a>, <a href="scanner.html#lexeme_scanner">lexeme_scanner</a>
and <a href="scanner.html#as_lower_scanner">as_lower_scanner.</a></td>
</tr>
</table>
<h3><a name="no_actions_scanner"></a>no_actions_scanner</h3>
<p>Again, <tt>no_actions_d</tt> directive tweaks the scanner to disable firing
semantic actions. Like before, all parsers inside the <tt>no_actions_d</tt>
gets a transformed scanner type. If a rule must be used inside a <tt>no_actions_d</tt>,
the rule's type must be:</p>
<pre> <span class=identifier>rule</span><span class=special>&lt;</span>no_actions_scanner<span class="special">&lt;</span><span class=identifier>ScannerT</span><span class=special>&gt;::</span><span class="identifier">type</span><span class=special>&gt; </span>r<span class=special>;</span></pre>
<p>where <tt>ScannerT</tt> is the actual type of the scanner used. <span class=special></span></p>
<table width="80%" border="0" align="center">
<tr>
<td class="note_box"><img src="theme/note.gif" width="16" height="16"> Be
sure to add &quot;<tt>typename</tt>&quot; before <tt><span class=identifier><tt>lexeme_scanner</tt>,
<tt>as_lower_scanner</tt></span></tt> and <tt>no_actions_scanner</tt> when
these are used inside a template class or function.</td>
</tr>
</table>
<p><img src="theme/lens.gif" width="15" height="16"> See <a href="../example/fundamental/no_actions.cpp">no_actions.cpp</a>. This is part of the Spirit distribution.</p>
<table border="0">
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<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="directives.html"><img src="theme/l_arr.gif" border="0"></a></td>
<td width="30"><a href="grammar.html"><img src="theme/r_arr.gif" border="0"></a></td>
</tr>
</table>
<br>
<hr size="1">
<p class="copyright">Copyright &copy; 1998-2003 Joel de Guzman<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>
<p>&nbsp;</p>
<p>&nbsp;</p>
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