1<html> 2<head> 3<title>Directives</title> 4<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1"> 5<link rel="stylesheet" href="theme/style.css" type="text/css"> 6</head> 7 8<body> 9<table width="100%" border="0" background="theme/bkd2.gif" cellspacing="2"> 10 <tr> 11 <td width="10"> 12 </td> 13 <td width="85%"> 14 <font size="6" face="Verdana, Arial, Helvetica, sans-serif"><b>Directives</b></font> 15 </td> 16 <td width="112"><a href="http://spirit.sf.net"><img src="theme/spirit.gif" width="112" height="48" align="right" border="0"></a></td> 17 </tr> 18</table> 19<br> 20<table border="0"> 21 <tr> 22 <td width="10"></td> 23 <td width="30"><a href="../index.html"><img src="theme/u_arr.gif" border="0"></a></td> 24 <td width="30"><a href="epsilon.html"><img src="theme/l_arr.gif" border="0"></a></td> 25 <td width="30"><a href="scanner.html"><img src="theme/r_arr.gif" border="0"></a></td> 26 </tr> 27</table> 28<p>Parser directives have the form: <b>directive[expression]</b></p> 29<p>A directive modifies the behavior of its enclosed expression, essentially <em>decorating</em> 30 it. The framework pre-defines a few directives. Clients of the framework are 31 free to define their own directives as needed. Information on how this is done 32 will be provided later. For now, we shall deal only with predefined directives.</p> 33<h2>lexeme_d</h2> 34<p>Turns off white space skipping. At the phrase level, the parser ignores white 35 spaces, possibly including comments. Use <tt>lexeme_d</tt> in situations where 36 we want to work at the character level instead of the phrase level. Parsers 37 can be made to work at the character level by enclosing the pertinent parts 38 inside the lexeme_d directive. For example, let us complete the example presented 39 in the <a href="introduction.html">Introduction</a>. There, we skipped the definition 40 of the <tt>integer</tt> rule. Here's how it is actually defined:</p> 41<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> 42<p>The <tt>lexeme_d</tt> directive instructs the parser to work on the character 43 level. Without it, the <tt>integer</tt> rule would have allowed erroneous embedded 44 white spaces in inputs such as <span class="quotes">"1 2 345"</span> 45 which will be parsed as <span class="quotes">"12345"</span>.</p> 46<h2>as_lower_d</h2> 47<p>There are times when we want to inhibit case sensitivity. The <tt>as_lower_d</tt> 48 directive converts all characters from the input to lower-case.</p> 49<table width="80%" border="0" align="center"> 50 <tr> 51 <td class="note_box"><img src="theme/alert.gif" width="16" height="16"><b> 52 as_lower_d behavior</b> <br> 53 <br> 54 It is important to note that only the input is converted to lower case. 55 Parsers enclosed inside the <tt>as_lower_d</tt> expecting upper case characters 56 will fail to parse. Example: <tt>as_lower_d[<span class="quotes">'X'</span>]</tt> 57 will never succeed because it expects an upper case <tt class="quotes">'X'</tt> 58 that the <tt>as_lower_d</tt> directive will never supply.</td> 59 </tr> 60</table> 61<p>For example, in Pascal, keywords and identifiers are case insensitive. Pascal 62 ignores the case of letters in identifiers and keywords. Identifiers Id, ID 63 and id are indistinguishable in Pascal. Without the as_lower_d directive, it 64 would be awkward to define a rule that recognizes this. Here's a possibility:</p> 65<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> 66<p>Now, try doing that with the case insensitive Pascal keyword <span class="quotes">"BEGIN"</span>. 67 The <tt>as_lower_d</tt> directive makes this simple:</p> 68<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> 69<table width="80%" border="0" align="center"> 70 <tr> 71 <td class="note_box"><div align="justify"><img src="theme/note.gif" width="16" height="16"> 72 <b>Primitive arguments</b> <br> 73 <br> 74 The astute reader will notice that we did not explicitly wrap <span class="quotes">"begin"</span> 75 inside an <tt>str_p</tt>. Whenever appropriate, directives should be able 76 to allow primitive types such as <tt>char</tt>, <tt>int</tt>, <tt>wchar_t</tt>, 77 <tt>char const<span class="operators">*</span></tt>, <tt>wchar_t const<span class="operators">*</span></tt> 78 and so on. Examples: <tt><br> 79 <br> 80 </tt><code><span class=identifier>as_lower_d</span><tt><span class=special>[</span><span class=string>"hello"</span><span class=special>] 81 </span><span class=comment>// same as as_lower_d[str_p("hello")]</span></tt><code></code><span class=identifier><br> 82 as_lower_d</span><span class=special>[</span><span class=literal>'x'</span><span class=special>] 83 </span><span class=comment>// same as as_lower_d[ch_p('x')]</span></code></div></td> 84 </tr> 85</table> 86<h3>no_actions_d</h3> 87<p>There are cases where you want <a href="semantic_actions.html">semantic actions</a> 88 not to be triggered. By enclosing a parser in the <tt>no_actions_d</tt> directive, 89 all semantic actions directly or indirectly attached to the parser will not 90 fire. </p> 91<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> 92<h3>Tweaking the Scanner Type</h3> 93<p><img src="theme/note.gif" width="16" height="16"> How does <tt>lexeme_d, as_lower_d</tt> 94 and <font color="#000000"><tt>no_actions_d</tt></font> work? These directives 95 do their magic by tweaking the scanner policies. Well, you don't need to know 96 what that means for now. Scanner policies are discussed <a href="indepth_the_scanner.html">later</a>. 97 However, it is important to note that when the scanner policy is tweaked, the 98 result is a different scanner. Why is this important to note? The <a href="rule.html">rule</a> 99 is tied to a particular scanner (one or more scanners, to be precise). If you 100 wrap a rule inside a <tt>lexeme_d, as_lower_d</tt> or <font color="#000000"><tt>no_actions_d,</tt>the 101 compiler will complain about <a href="faq.html#scanner_business">scanner mismatch</a> 102 unless you associate the required scanner with the rule. </font></p> 103<p><tt>lexeme_scanner</tt>, <tt>as_lower_scanner</tt> and <tt>no_actions_scanner</tt> 104 are your friends if the need to wrap a rule inside these directives arise. Learn 105 bout these beasts in the next chapter on <a href="scanner.html#lexeme_scanner">The 106 Scanner and Parsing</a>.</p> 107<h2>longest_d</h2> 108<p>Alternatives in the Spirit parser compiler are short-circuited (see <a href="operators.html">Operators</a>). 109 Sometimes, this is not what is desired. The <tt>longest_d</tt> directive instructs 110 the parser not to short-circuit alternatives enclosed inside this directive, 111 but instead makes the parser try all possible alternatives and choose the one 112 matching the longest portion of the input stream.</p> 113<p>Consider the parsing of integers and real numbers:</p> 114<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> 115<p>A number can be a real or an integer. This grammar is ambiguous. An input <span class="quotes">"1234"</span> 116 should potentially match both real and integer. Recall though that alternatives 117 are short-circuited . Thus, for inputs such as above, the real alternative always 118 wins. However, if we swap the alternatives:</p> 119<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> 120<p>we still have a problem. Now, an input <span class="quotes">"123.456"</span> 121 will be partially matched by integer until the decimal point. This is not what 122 we want. The solution here is either to fix the ambiguity by factoring out the 123 common prefixes of real and integer or, if that is not possible nor desired, 124 use the <tt>longest_d</tt> directive:</p> 125<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> 126<h2>shortest_d</h2> 127<p>Opposite of the <tt>longest_d</tt> directive.</p> 128<table width="80%" border="0" align="center"> 129 <tr> 130 <td class="note_box"><img src="theme/note.gif" width="16" height="16"> <b>Multiple 131 alternatives</b> <br> 132 <br> 133 The <tt>longest_d</tt> and <tt>shortest_d</tt> directives can accept two 134 or more alternatives. Examples:<br> 135 <br> 136 <font color="#000000"><span class=identifier><code>longest</code></span><code><span class=special>[ 137 </span><span class=identifier>a </span><span class=special>| </span><span class=identifier>b 138 </span><span class=special>| </span><span class=identifier>c </span><span class=special>]; 139 </span><span class=identifier><br> 140 shortest</span><span class=special>[ </span><span class=identifier>a </span><span class=special>| 141 </span><span class=identifier>b </span><span class=special>| </span><span class=identifier>c 142 </span><span class=special>| </span><span class=identifier>d </span><span class=special>];</span></code></font></td> 143 </tr> 144</table> 145<h2>limit_d</h2> 146<p>Ensures that the result of a parser is constrained to a given min..max range 147 (inclusive). If not, then the parser fails and returns a no-match.</p> 148<p><b>Usage:</b></p> 149<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> 150<p>This directive is particularly useful in conjunction with parsers that parse 151 specific scalar ranges (for example, <a href="numerics.html">numeric parsers</a>). 152 Here's a practical example. Although the numeric parsers can be configured to 153 accept only a limited number of digits (say, 0..2), there is no way to limit 154 the result to a range (say -1.0..1.0). This design is deliberate. Doing so would 155 have undermined Spirit's design rule that <i><span class="quotes">"the 156 client should not pay for features that she does not use"</span></i>. We 157 would have stored the min, max values in the numeric parser itself, used or 158 unused. Well, we could get by by using static constants configured by a non-type 159 template parameter, but that is not acceptable because that way, we can only 160 accommodate integers. What about real numbers or user defined numbers such as 161 big-ints?</p> 162<p><b>Example</b>, parse time of the form <b>HH:MM:SS</b>:</p> 163<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>; 164 165 </span><span class=identifier>r </span><span class=special>= </span><span class=identifier>lexeme_d 166 </span><span class=special>[ 167 </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..23 168 </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..59 169 </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..59 170 </span><span class=special>];</span></font></code> 171</pre> 172<h2>min_limit_d</h2> 173<p>Sometimes, it is useful to unconstrain just the maximum limit. This will allow 174 for an interval that's unbounded in one direction. The directive min_limit_d 175 ensures that the result of a parser is not less than minimum. If not, then the 176 parser fails and returns a no-match.</p> 177<p><b>Usage:</b></p> 178<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> 179<p><b>Example</b>, ensure that a date is not less than 1900</p> 180<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> 181<h2>max_limit_d</h2> 182<p>Opposite of <tt>min_limit_d</tt>. Take note that <tt>limit_d[p]</tt> is equivalent 183 to:</p> 184<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> 185<table border="0"> 186 <tr> 187 <td width="10"></td> 188 <td width="30"><a href="../index.html"><img src="theme/u_arr.gif" border="0"></a></td> 189 <td width="30"><a href="epsilon.html"><img src="theme/l_arr.gif" border="0"></a></td> 190 <td width="30"><a href="scanner.html"><img src="theme/r_arr.gif" border="0"></a></td> 191 </tr> 192</table> 193<br> 194<hr size="1"> 195<p class="copyright">Copyright © 1998-2003 Joel de Guzman<br> 196 <br> 197<font size="2">Use, modification and distribution is subject to the Boost Software 198 License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at 199 http://www.boost.org/LICENSE_1_0.txt) </font> </p> 200<p> </p> 201</body> 202</html> 203
