<html> <head> <title>Epsilon</title> <meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1"> <link rel="stylesheet" href="theme/style.css" type="text/css"> </head> <body> <table width="100%" border="0" background="theme/bkd2.gif" cellspacing="2"> <tr> <td width="10"> </td> <td width="85%"> <font size="6" face="Verdana, Arial, Helvetica, sans-serif"><b>Epsilon</b></font> </td> <td width="112"><a href="http://spirit.sf.net"><img src="theme/spirit.gif" width="112" height="48" align="right" border="0"></a></td> </tr> </table> <br> <table border="0"> <tr> <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="rule.html"><img src="theme/l_arr.gif" border="0"></a></td> <td width="30"><a href="directives.html"><img src="theme/r_arr.gif" border="0"></a></td> </tr> </table> <p>The <strong>Epsilon</strong> (<tt>epsilon_p</tt> and <tt>eps_p</tt>) is a multi-purpose parser that returns a zero length match. </p> <h3>Simple Form</h3> <p>In its simplest form, epsilon_p matches the null string and always returns a match of zero length:</p> <pre><code><span class=special> </span><span class="identifier">epsilon_p </span><span class="comment">// always returns a zero-length match</span></code></pre> <p>This form is usually used to trigger a <a href="semantic_actions.html">semantic action</a> unconditionally. For example, it is useful in triggering error messages when a set of alternatives fail:</p> <pre><code><span class=special> </span><span class="identifier">r</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">eps_p</span><span class="special">[</span><span class="identifier">error</span><span class="special">];</span><span class="identifier"></span><span class="comment"> // error if A, B, or C fails to match</span></code></pre> <h3>Semantic Predicate</h3> <p>Semantic predicates allow you to attach a function anywhere in the grammar. In this role, the epsilon takes a 0-ary (nullary) function/functor. The run-time function/functor is typically a test that is called upon to resolve ambiguity in the grammar. A parse failure will be reported when the function/functor result evaluates to false. Otherwise an empty match will be reported. The general form is:</p> <pre> eps_p<span class="special">(</span>f<span class="special">) >></span> rest<span class="special">;</span> </pre> <p>The nullary function <tt>f</tt> is called to do a semantic test (say, checking if a symbol is in the <a href="symbols.html">symbol table</a>). If test returns <tt>true</tt>, <tt>rest</tt> will be evaluated. Otherwise, the production will return early with a no-match without ever touching <tt>rest</tt>.</p> <h3>Syntactic Predicate</h3> <p>Similar to Semantic predicates, Syntactic predicates assert a certain conditional syntax to be satisfied before evaluating another production. This time, epsilon_p accepts a (conditional) parser. The general form is:</p> <pre> eps_p<span class="special">(</span>p<span class="special">) >></span> rest<span class="special">;</span> </pre> <p>If <tt>p</tt> is matched on the input stream then attempt to recognize <tt>rest</tt>. The parser <tt>p </tt>is called to do a syntax check. Regardless of <tt>p</tt>'s success, <tt>eps_p(p)</tt> will always return a zero length match (i.e. the input is not consumed). If test returns <tt>true</tt>, <tt>rest</tt> will be evaluated. Otherwise, the production will return early with a no-match without ever touching <tt>rest</tt>.</p> <p>Example:</p> <pre><code><span class=special> </span><span class="identifier">eps_p</span><span class="special">(</span><span class="literal">'0'</span><span class="special">) >> </span><span class="identifier">oct_p </span><span class="comment">// note that '0' is actually a ch_p('0')</span><span class="identifier"> </span></code></pre> <p>Epsilon here is used as a syntactic predicate. <tt>oct_p</tt> (see <a href="numerics.html">numerics</a>) is parsed only if we see a leading <tt>'0'</tt>. Wrapping the leading <tt>'0'</tt> inside an epsilon makes the parser not consume anything from the input. If a <tt>'0'</tt> is seen, <tt>epsilon_p</tt> reports a successful match with zero length. </p> <table width="80%" border="0" align="center"> <tr> <td class="note_box"><div align="justify"><img src="theme/note.gif" width="16" height="16"> <b>Primitive arguments</b> <br> <br> Epsilon allows primitive type arguments 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">eps_p</span><tt><span class=special>(</span><span class=string>"hello"</span><span class=special>)</span><span class=comment> // same as eps_p(str_p("hello"))</span></tt><span class=identifier><br> eps_p</span><span class=special>(</span><span class=literal>'x'</span><span class="special">) </span><span class=comment>// same as eps_p(ch_p('x'))</span></code></div></td> </tr> </table> <h3><img src="theme/alert.gif" width="16" height="16"> Inhibiting Semantic Actions</h3> <p>In a syntactic predicate <tt>eps_p(p)</tt>, any semantic action directly or indirectly attached to the conditional parser <tt>p</tt> will not be called. However, semantic actions attached to epsilon itself will always be called. The following code snippets illustrates the behavior:</p> <pre> eps_p<span class="special">(</span>c<span class="special">[</span>f<span class="special">])</span> <span class="comment">// f not called</span><br> eps_p<span class="special">(</span>c<span class="special">)[</span>f<span class="special">]</span> <span class="comment">// f is called</span><br> eps_p<span class="special">[</span>f<span class="special">]</span> <span class="comment">// f is called</span></pre> <p>Actually, the conditional parser <tt>p</tt> is implicitly wrapped in a <tt><a href="scanner.html#no_actions_scanner">no_actions_d</a></tt> directive:</p> <pre><code><span class=special> </span>no_actions_d<span class="special">[</span>p<span class="special">]</span></code></pre> <p>The conditional parser is required to be free from side-effects (semantic actions). <code></code>The conditional parser's purpose is to resolve ambiguity by looking ahead in the input stream for a certain pattern. Ambiguity and semantic actions do not mix well. On an ambiguous grammar, backtracking happens. And when it happens, we cannot undo the effects of triggered semantic actions. </p> <h3>Negation</h3> <p>Operator <tt>~</tt> is defined for parsers constructed by <tt>epsilon_p</tt>/<tt>eps_p</tt>. It performs negation by complementing the results reported. <tt>~~eps_p(x)</tt> is identical to <tt>eps_p(x)</tt>.</p> <table border="0"> <tr> <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="rule.html"><img src="theme/l_arr.gif" border="0"></a></td> <td width="30"><a href="directives.html"><img src="theme/r_arr.gif" border="0"></a></td> </tr> </table> <br> <hr size="1"> <p class="copyright">Copyright © 1998-2003 Joel de Guzman<br> Copyright © 2003 Martin Wille<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> </p> </body> </html>