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<font size="6" face="Verdana, Arial, Helvetica, sans-serif"><b>Directives</b></font>
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<p>Parser directives have the form: <b>directive[expression]</b></p>
<p>A directive modifies the behavior of its enclosed expression, essentially <em>decorating</em>
it. The framework pre-defines a few directives. Clients of the framework are
free to define their own directives as needed. Information on how this is done
will be provided later. For now, we shall deal only with predefined directives.</p>
<h2>lexeme_d</h2>
<p>Turns off white space skipping. At the phrase level, the parser ignores white
spaces, possibly including comments. Use <tt>lexeme_d</tt> in situations where
we want to work at the character level instead of the phrase level. Parsers
can be made to work at the character level by enclosing the pertinent parts
inside the lexeme_d directive. For example, let us complete the example presented
in the <a href="introduction.html">Introduction</a>. There, we skipped the definition
of the <tt>integer</tt> rule. Here's how it is actually defined:</p>
<pre><code><font color="#000000"><span class=identifier> </span><span class=identifier>integer </span><span class=special>= </span><span class=identifier>lexeme_d</span><span class=special>[ </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=special>| </span><span class=literal>'-'</span><span class=special>) </span><span class=special>>> </span><span class=special>+</span><span class=identifier>digit </span><span class=special>];</span></font></code></pre>
<p>The <tt>lexeme_d</tt> directive instructs the parser to work on the character
level. Without it, the <tt>integer</tt> rule would have allowed erroneous embedded
white spaces in inputs such as <span class="quotes">"1 2 345"</span>
which will be parsed as <span class="quotes">"12345"</span>.</p>
<h2>as_lower_d</h2>
<p>There are times when we want to inhibit case sensitivity. The <tt>as_lower_d</tt>
directive converts all characters from the input to lower-case.</p>
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as_lower_d behavior</b> <br>
<br>
It is important to note that only the input is converted to lower case.
Parsers enclosed inside the <tt>as_lower_d</tt> expecting upper case characters
will fail to parse. Example: <tt>as_lower_d[<span class="quotes">'X'</span>]</tt>
will never succeed because it expects an upper case <tt class="quotes">'X'</tt>
that the <tt>as_lower_d</tt> directive will never supply.</td>
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<p>For example, in Pascal, keywords and identifiers are case insensitive. Pascal
ignores the case of letters in identifiers and keywords. Identifiers Id, ID
and id are indistinguishable in Pascal. Without the as_lower_d directive, it
would be awkward to define a rule that recognizes this. Here's a possibility:</p>
<pre><code><font color="#000000"><span class=special> </span><span class=identifier>r </span><span class=special>= </span><span class=identifier>str_p</span><span class=special>(</span><span class=string>"id"</span><span class=special>) </span><span class=special>| </span><span class=string>"Id" </span><span class=special>| </span><span class=string>"iD" </span><span class=special>| </span><span class=string>"ID"</span><span class=special>;</span></font></code></pre>
<p>Now, try doing that with the case insensitive Pascal keyword <span class="quotes">"BEGIN"</span>.
The <tt>as_lower_d</tt> directive makes this simple:</p>
<pre><code><font color="#000000"><span class=special> </span><span class=identifier>r </span><span class=special>= </span><span class=identifier>as_lower_d</span><span class=special>[</span><span class=string>"begin"</span><span class=special>];</span></font></code></pre>
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<b>Primitive arguments</b> <br>
<br>
The astute reader will notice that we did not explicitly wrap <span class="quotes">"begin"</span>
inside an <tt>str_p</tt>. Whenever appropriate, directives should be able
to allow primitive types such as <tt>char</tt>, <tt>int</tt>, <tt>wchar_t</tt>,
<tt>char const<span class="operators">*</span></tt>, <tt>wchar_t const<span class="operators">*</span></tt>
and so on. Examples: <tt><br>
<br>
</tt><code><span class=identifier>as_lower_d</span><tt><span class=special>[</span><span class=string>"hello"</span><span class=special>]
</span><span class=comment>// same as as_lower_d[str_p("hello")]</span></tt><code></code><span class=identifier><br>
as_lower_d</span><span class=special>[</span><span class=literal>'x'</span><span class=special>]
</span><span class=comment>// same as as_lower_d[ch_p('x')]</span></code></div></td>
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<h3>no_actions_d</h3>
<p>There are cases where you want <a href="semantic_actions.html">semantic actions</a>
not to be triggered. By enclosing a parser in the <tt>no_actions_d</tt> directive,
all semantic actions directly or indirectly attached to the parser will not
fire. </p>
<pre><code><font color="#000000"><span class=special> </span>no_actions_d<span class=special>[</span><span class=identifier>expression</span><span class=special>]</span></font></code><code><font color="#000000"><span class=special></span></font></code></pre>
<h3>Tweaking the Scanner Type</h3>
<p><img src="theme/note.gif" width="16" height="16"> How does <tt>lexeme_d, as_lower_d</tt>
and <font color="#000000"><tt>no_actions_d</tt></font> work? These directives
do their magic by tweaking the scanner policies. Well, you don't need to know
what that means for now. Scanner policies are discussed <a href="indepth_the_scanner.html">later</a>.
However, it is important to note that when the scanner policy is tweaked, the
result is a different scanner. Why is this important to note? The <a href="rule.html">rule</a>
is tied to a particular scanner (one or more scanners, to be precise). If you
wrap a rule inside a <tt>lexeme_d, as_lower_d</tt> or <font color="#000000"><tt>no_actions_d,</tt>the
compiler will complain about <a href="faq.html#scanner_business">scanner mismatch</a>
unless you associate the required scanner with the rule. </font></p>
<p><tt>lexeme_scanner</tt>, <tt>as_lower_scanner</tt> and <tt>no_actions_scanner</tt>
are your friends if the need to wrap a rule inside these directives arise. Learn
bout these beasts in the next chapter on <a href="scanner.html#lexeme_scanner">The
Scanner and Parsing</a>.</p>
<h2>longest_d</h2>
<p>Alternatives in the Spirit parser compiler are short-circuited (see <a href="operators.html">Operators</a>).
Sometimes, this is not what is desired. The <tt>longest_d</tt> directive instructs
the parser not to short-circuit alternatives enclosed inside this directive,
but instead makes the parser try all possible alternatives and choose the one
matching the longest portion of the input stream.</p>
<p>Consider the parsing of integers and real numbers:</p>
<pre><code><font color="#000000"><span class=comment> </span><span class=identifier>number </span><span class=special>= </span><span class=identifier>real </span><span class=special>| </span><span class=identifier>integer</span><span class=special>;</span></font></code></pre>
<p>A number can be a real or an integer. This grammar is ambiguous. An input <span class="quotes">"1234"</span>
should potentially match both real and integer. Recall though that alternatives
are short-circuited . Thus, for inputs such as above, the real alternative always
wins. However, if we swap the alternatives:</p>
<pre><code><font color="#000000"><span class=special> </span><span class=identifier>number </span><span class=special>= </span><span class=identifier>integer </span><span class=special>| </span><span class=identifier>real</span><span class=special>;</span></font></code></pre>
<p>we still have a problem. Now, an input <span class="quotes">"123.456"</span>
will be partially matched by integer until the decimal point. This is not what
we want. The solution here is either to fix the ambiguity by factoring out the
common prefixes of real and integer or, if that is not possible nor desired,
use the <tt>longest_d</tt> directive:</p>
<pre><code><font color="#000000"><span class=special> </span><span class=identifier>number </span><span class=special>= </span><span class=identifier>longest_d</span><span class=special>[ </span><span class=identifier>integer </span><span class=special>| </span><span class=identifier>real </span><span class=special>];</span></font></code></pre>
<h2>shortest_d</h2>
<p>Opposite of the <tt>longest_d</tt> directive.</p>
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<td class="note_box"><img src="theme/note.gif" width="16" height="16"> <b>Multiple
alternatives</b> <br>
<br>
The <tt>longest_d</tt> and <tt>shortest_d</tt> directives can accept two
or more alternatives. Examples:<br>
<br>
<font color="#000000"><span class=identifier><code>longest</code></span><code><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>c </span><span class=special>];
</span><span class=identifier><br>
shortest</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>c
</span><span class=special>| </span><span class=identifier>d </span><span class=special>];</span></code></font></td>
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<h2>limit_d</h2>
<p>Ensures that the result of a parser is constrained to a given min..max range
(inclusive). If not, then the parser fails and returns a no-match.</p>
<p><b>Usage:</b></p>
<pre><code><font color="#000000"><span class=special> </span><span class=identifier>limit_d</span><span class=special>(</span><span class=identifier>min</span><span class=special>, </span><span class=identifier>max</span><span class=special>)[</span><span class=identifier>expression</span><span class=special>]</span></font></code></pre>
<p>This directive is particularly useful in conjunction with parsers that parse
specific scalar ranges (for example, <a href="numerics.html">numeric parsers</a>).
Here's a practical example. Although the numeric parsers can be configured to
accept only a limited number of digits (say, 0..2), there is no way to limit
the result to a range (say -1.0..1.0). This design is deliberate. Doing so would
have undermined Spirit's design rule that <i><span class="quotes">"the
client should not pay for features that she does not use"</span></i>. We
would have stored the min, max values in the numeric parser itself, used or
unused. Well, we could get by by using static constants configured by a non-type
template parameter, but that is not acceptable because that way, we can only
accommodate integers. What about real numbers or user defined numbers such as
big-ints?</p>
<p><b>Example</b>, parse time of the form <b>HH:MM:SS</b>:</p>
<pre><code><font color="#000000"><span class=special> </span><span class=identifier>uint_parser</span><span class=special><</span><span class=keyword>int</span><span class=special>, </span><span class=number>10</span><span class=special>, </span><span class=number>2</span><span class=special>, </span><span class=number>2</span><span class=special>> </span><span class=identifier>uint2_p</span><span class=special>;
</span><span class=identifier>r </span><span class=special>= </span><span class=identifier>lexeme_d
</span><span class=special>[
</span><span class=identifier>limit_d</span><span class=special>(</span><span class=number>0u</span><span class=special>, </span><span class=number>23u</span><span class=special>)[</span><span class=identifier>uint2_p</span><span class=special>] </span><span class=special>>> </span><span class=literal>':' </span><span class=comment>// Hours 00..23
</span><span class=special>>> </span><span class=identifier>limit_d</span><span class=special>(</span><span class=number>0u</span><span class=special>, </span><span class=number>59u</span><span class=special>)[</span><span class=identifier>uint2_p</span><span class=special>] </span><span class=special>>> </span><span class=literal>':' </span><span class=comment>// Minutes 00..59
</span><span class=special>>> </span><span class=identifier>limit_d</span><span class=special>(</span><span class=number>0u</span><span class=special>, </span><span class=number>59u</span><span class=special>)[</span><span class=identifier>uint2_p</span><span class=special>] </span><span class=comment>// Seconds 00..59
</span><span class=special>];</span></font></code>
</pre>
<h2>min_limit_d</h2>
<p>Sometimes, it is useful to unconstrain just the maximum limit. This will allow
for an interval that's unbounded in one direction. The directive min_limit_d
ensures that the result of a parser is not less than minimun. If not, then the
parser fails and returns a no-match.</p>
<p><b>Usage:</b></p>
<pre><code><font color="#000000"><span class=special> </span><span class=identifier>min_limit_d</span><span class=special>(</span><span class=identifier>min</span><span class=special>)[</span><span class=identifier>expression</span><span class=special>]</span></font></code></pre>
<p><b>Example</b>, ensure that a date is not less than 1900</p>
<pre><code><font color="#000000"><span class=special> </span><span class=identifier>min_limit_d</span><span class=special>(</span><span class=number>1900u</span><span class=special>)[</span><span class=identifier>uint_p</span><span class=special>]</span></font></code></pre>
<h2>max_limit_d</h2>
<p>Opposite of <tt>min_limit_d</tt>. Take note that <tt>limit_d[p]</tt> is equivalent
to:</p>
<pre><code><font color="#000000"><span class=special> </span><span class=identifier>min_limit_d</span><span class=special>(</span><span class=identifier>min</span><span class=special>)[</span><span class=identifier>max_limit_d</span><span class=special>(</span><span class=identifier>max</span><span class=special>)[</span><span class=identifier>p</span><span class=special>]]</span></font></code><code><font color="#000000"><span class=special></span></font></code></pre>
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