1<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> 2<html> 3<head> 4<title>Confix Parsers</title> 5<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1"> 6<link rel="stylesheet" href="theme/style.css" type="text/css"> 7</head> 8 9<body> 10<table width="100%" border="0" background="theme/bkd2.gif" cellspacing="2"> 11 <tr> 12 <td width="10"> <font size="6" face="Verdana, Arial, Helvetica, sans-serif"><b> </b></font></td> 13 <td width="85%"> <font size="6" face="Verdana, Arial, Helvetica, sans-serif"><b>Confix Parsers</b></font></td> 14 <td width="112"><a href="http://spirit.sf.net"><img src="theme/spirit.gif" width="112" height="48" align="right" border="0"></a></td> 15 </tr> 16</table> 17<br> 18<table border="0"> 19 <tr> 20 <td width="10"></td> 21 <td width="30"><a href="../index.html"><img src="theme/u_arr.gif" border="0"></a></td> 22 <td width="30"><a href="character_sets.html"><img src="theme/l_arr.gif" border="0"></a></td> 23 <td width="30"><a href="list_parsers.html"><img src="theme/r_arr.gif" border="0"></a></td> 24 </tr> 25</table> 26<p><a name="confix_parser"></a><b>Confix Parsers</b></p> 27<p>Confix Parsers recognize a sequence out of three independent elements: an 28 opening, an expression and a closing. A simple example is a C comment: 29</p> 30<pre><code class="comment"> /* This is a C comment */</code></pre> 31<p>which could be parsed through the following rule definition:<code><font color="#000000"> 32 </font></code> </p> 33<pre><span class=identifier> </span><span class=identifier>rule</span><span class=special><> </span><span class=identifier>c_comment_rule 34 </span><span class=special>= </span><span class=identifier>confix_p</span><span class=special>(</span><span class=literal>"/*"</span><span class=special>, </span><span class=special>*</span><span class=identifier>anychar_p</span><span class=special>, </span><span class=literal>"*/"</span><span class=special>) 35 </span><span class=special>;</span></pre> 36<p>The <tt>confix_p</tt> parser generator 37 should be used for generating the required Confix Parser. The 38 three parameters to <tt>confix_p</tt> can be single 39 characters (as above), strings or, if more complex parsing logic is required, 40 auxiliary parsers, each of which is automatically converted to the corresponding 41 parser type needed for successful parsing.</p> 42<p>The generated parser is equivalent to the following rule: </p> 43<pre><code> <span class=identifier>open </span><span class=special>>> (</span><span class=identifier>expr </span><span class=special>- </span><span class=identifier>close</span><span class=special>) >> </span><span class=identifier>close</span></code></pre> 44<p>If the expr parser is an <tt>action_parser_category</tt> type parser (a parser 45 with an attached semantic action) we have to do something special. This happens, 46 if the user wrote something like:</p> 47<pre><code><span class=identifier> confix_p</span><span class=special>(</span><span class=identifier>open</span><span class=special>, </span><span class=identifier>expr</span><span class=special>[</span><span class=identifier>func</span><span class=special>], </span><span class=identifier>close</span><span class=special>)</span></code></pre> 48<p>where <code>expr</code> is the parser matching the expr of the confix sequence 49 and <code>func</code> is a functor to be called after matching the <code>expr</code>. 50 If we would do nothing, the resulting code would parse the sequence as follows:</p> 51<pre><code> <span class=identifier>open </span><span class=special>>> (</span><span class=identifier>expr</span><span class=special>[</span><span class=identifier>func</span><span class=special>] - </span><span class=identifier>close</span><span class=special>) >> </span><span class=identifier>close</span></code></pre> 52<p>which in most cases is not what the user expects. (If this <u>is</u> what you've 53 expected, then please use the <tt>confix_p</tt> generator 54 function <tt>direct()</tt>, which will inhibit the parser refactoring). To make 55 the confix parser behave as expected:</p> 56<pre><code><span class=identifier> open </span><span class=special>>> (</span><span class=identifier>expr </span><span class=special>- </span><span class=identifier>close</span><span class=special>)[</span><span class=identifier>func</span><span class=special>] >> </span><span class=identifier>close</span></code></pre> 57<p>the actor attached to the <code>expr</code> parser has to be re-attached to 58 the <code>(expr - close)</code> parser construct, which will make the resulting 59 confix parser 'do the right thing'. This refactoring is done by the help of 60 the <a href="refactoring.html">Refactoring Parsers</a>. Additionally special 61 care must be taken, if the expr parser is a <tt>unary_parser_category</tt> type 62 parser as </p> 63<pre><code><span class=identifier> confix_p</span><span class=special>(</span><span class=identifier>open</span><span class=special>, *</span><span class=identifier>anychar_p</span><span class=special>, </span><span class=identifier>close</span><span class=special>)</span></code></pre> 64<p>which without any refactoring would result in </p> 65<pre><code> <span class=identifier>open</span> <span class=special>>> (*</span><span class=identifier>anychar_p </span><span class=special>- </span><span class=identifier>close</span><span class=special>) >> </span><span class=identifier>close</span></code></pre> 66<p>and will not give the expected result (*anychar_p will eat up all the input up 67to the end of the input stream). So we have to refactor this into: 68<pre><code><span class=identifier> open </span><span class=special>>> *(</span><span class=identifier>anychar_p </span><span class=special>- </span><span class=identifier>close</span><span class=special>) >> </span><span class=identifier>close</span></code></pre> 69<p>what will give the correct result. </p> 70<p>The case, where the expr parser is a combination of the two mentioned problems 71 (i.e. the expr parser is a unary parser with an attached action), is handled 72 accordingly too, so: </p> 73<pre><code><span class=identifier> confix_p</span><span class=special>(</span><span class=identifier>open</span><span class=special>, (*</span><span class=identifier>anychar_p</span><span class=special>)[</span><span class=identifier>func</span><span class=special>], </span>close<span class=special>)</span></code></pre> 74<p>will be parsed as expected: </p> 75<pre><code> <span class=identifier>open</span> <span class=special>>> (*(</span><span class=identifier>anychar_p </span><span class=special>- </span><span class=identifier>end</span><span class=special>))[</span><span class=identifier>func</span><span class=special>] >> </span>close</code></pre> 76<p>The required refactoring is implemented here with the help of the <a href="refactoring.html">Refactoring 77 Parsers</a> too.</p> 78<table width="90%" border="0" align="center"> 79 <tr> 80 <td colspan="2" class="table_title"><b>Summary of Confix Parser refactorings</b></td> 81 </tr> 82 <tr class="table_title"> 83 <td width="40%"><b>You write it as:</b></td> 84 <td width="60%"><code><font face="Verdana, Arial, Helvetica, sans-serif">It 85 is refactored to:</font></code></td> 86 </tr> 87 <tr> 88 <td width="40%" class="table_cells"><code>confix_p<span class="special">(</span><span class=identifier>open</span><span class="special">,</span> 89 expr<span class="special">,</span> close<span class="special">)</span></code></td> 90 <td width="60%" class="table_cells"> <p><code>open <span class=special>>> 91 (</span>expr <span class=special>-</span> close<span class=special>)</span><font color="#0000FF"> 92 </font><span class=special>>></span> close</code></p> 93 </td> 94 </tr> 95 <tr> 96 <td width="40%" class="table_cells"><code>confix_p<span class="special">(</span><span class=identifier>open</span><span class="special">,</span> 97 expr<span class="special">[</span>func<span class="special">],</span> close<span class="special">)</span></code></td> 98 <td width="60%" class="table_cells"> <p><code>open <span class=special>>> 99 (</span>expr <span class=special>-</span> close<span class="special">)[</span>func<span class="special">] 100 <font color="#0000FF" class="special">>></font></span> close</code></p> 101 </td> 102 </tr> 103 <tr> 104 <td width="40%" class="table_cells" height="9"><code>confix_p<span class="special">(</span><span class=identifier>open</span><span class="special">, 105 *</span>expr<span class="special">,</span> close<span class="special">)</span></code></td> 106 <td width="60%" class="table_cells" height="9"> <p><code>open <font color="#0000FF"><span class="special">>></span></font> 107 <span class="special"><font color="#0000FF" class="special">*</font>(</span>expr 108 <font color="#0000FF" class="special">-</font> close<span class="special">) 109 <font color="#0000FF" class="special">>></font></span> close</code></p> 110 </td> 111 </tr> 112 <tr> 113 <td width="40%" class="table_cells"><code>confix_p<span class="special">(</span><span class=identifier>open</span><span class="special">, 114 (*</span>expr<span class="special">)[</span>func<span class="special">], 115 close</span><span class="special">)</span></code></td> 116 <td width="60%" class="table_cells"> <p><code>open <font color="#0000FF"><span class="special">>></span></font><span class="special"> 117 (<font color="#0000FF" class="special">*</font>(</span>expr <font color="#0000FF" class="special">-</font> 118 close<span class="special">))[</span>func<span class="special">] <font color="#0000FF" class="special">>></font></span> 119 close</code></p> 120 </td> 121 </tr> 122</table> 123<p><a name="comment_parsers"></a><b>Comment Parsers</b></p> 124<p>The Comment Parser generator template <tt>comment_p</tt> 125 is helper for generating a correct <a href="#confix_parser">Confix Parser</a> 126 from auxiliary parameters, which is able to parse comment constructs as follows: 127</p> 128<pre><code> StartCommentToken <span class="special">>></span> Comment text <span class="special">>></span> EndCommentToken</code></pre> 129<p>There are the following types supported as parameters: parsers, single 130 characters and strings (see as_parser). If it 131 is used with one parameter, a comment starting with the given first parser 132 parameter up to the end of the line is matched. So for instance the following 133 parser matches C++ style comments:</p> 134 135<pre><code><span class=identifier> comment_p</span><span class=special>(</span><span class=string>"//"</span><span class=special>)</span></code></pre> 136<p>If it is used with two parameters, a comment starting with the first parser 137 parameter up to the second parser parameter is matched. For instance a C style 138 comment parser could be constrcuted as:</p> 139<pre><code> <span class=identifier>comment_p</span><span class=special>(</span><span class=string>"/*"</span><span class=special>, </span><span class=string>"*/"</span><span class=special>)</span></code></pre> 140<p>The <tt>comment_p</tt> parser generator allows to generate parsers for matching 141 non-nested comments (as for C/C++ comments). Sometimes it is necessary to parse 142 nested comments as for instance allowed in Pascal.</p> 143<pre><code class="comment"> { This is a { nested } PASCAL-comment }</code></pre> 144<p>Such nested comments are 145 parseable through parsers generated by the <tt>comment_nest_p</tt> generator 146 template functor. The following example shows a parser, which can be used for 147 parsing the two different (nestable) Pascal comment styles:</p> 148<pre><code> <span class=identifier>rule</span><span class=special><> </span><span class=identifier>pascal_comment 149 </span><span class=special>= </span><span class=identifier>comment_nest_p</span><span class=special>(</span><span class=string>"(*"</span><span class=special>, </span><span class=string>"*)"</span><span class=special>) 150 | </span><span class=identifier>comment_nest_p</span><span class=special>(</span><span class=literal>'{'</span><span class=special>, </span><span class=literal>'}'</span><span class=special>) 151 ;</span></code></pre> 152<p>Please note, that a comment is parsed implicitly as if the whole <tt>comment_p(...)</tt> 153 statement were embedded into a <tt>lexeme_d[]</tt> directive, i.e. during parsing 154 of a comment no token skipping will occur, even if you've defined a skip parser 155for your whole parsing process.</p> 156<p> <img height="16" width="15" src="theme/lens.gif"> <a href="../example/fundamental/comments.cpp">comments.cpp</a> demonstrates various comment parsing schemes: </p> 157<ol> 158 <li>Parsing of different comment styles </li> 159 <ul> 160 <li>parsing C/C++-style comment</li> 161 <li>parsing C++-style comment</li> 162 <li>parsing PASCAL-style comment</li> 163 </ul> 164 <li>Parsing tagged data with the help of the confix_parser</li> 165 <li>Parsing tagged data with the help of the confix_parser but the semantic<br> 166 action is directly attached to the body sequence parser</li> 167</ol> 168<p>This is part of the Spirit distribution.</p> 169<table border="0"> 170 <tr> 171 <td width="10"></td> 172 <td width="30"><a href="../index.html"><img src="theme/u_arr.gif" border="0"></a></td> 173 <td width="30"><a href="character_sets.html"><img src="theme/l_arr.gif" border="0"></a></td> 174 <td width="30"><a href="list_parsers.html"><img src="theme/r_arr.gif" border="0"></a></td> 175 </tr> 176</table> 177<br> 178<hr size="1"> 179<p class="copyright">Copyright © 2001-2002 Hartmut Kaiser<br> 180 <br> 181 <font size="2">Use, modification and distribution is subject to the Boost Software 182 License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at 183 http://www.boost.org/LICENSE_1_0.txt) </font> </p> 184</body> 185</html> 186
