<html> <head> <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> <title>Epsilon (eps)</title> <link rel="stylesheet" href="../../../../../../../../doc/html/boostbook.css" type="text/css"> <meta name="generator" content="DocBook XSL Stylesheets V1.75.0"> <link rel="home" href="../../../../index.html" title="Spirit 2.2"> <link rel="up" href="../auxiliary.html" title="Auxiliary"> <link rel="prev" href="eoi.html" title="End of Input (eoi)"> <link rel="next" href="lazy.html" title="Lazy (lazy)"> </head> <body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> <table cellpadding="2" width="100%"><tr> <td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../../../../boost.png"></td> <td align="center"><a href="../../../../../../../../index.html">Home</a></td> <td align="center"><a href="../../../../../../../libraries.htm">Libraries</a></td> <td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> <td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> <td align="center"><a href="../../../../../../../../more/index.htm">More</a></td> </tr></table> <hr> <div class="spirit-nav"> <a accesskey="p" href="eoi.html"><img src="../../../../../../../../doc/html/images/prev.png" alt="Prev"></a><a accesskey="u" href="../auxiliary.html"><img src="../../../../../../../../doc/html/images/up.png" alt="Up"></a><a accesskey="h" href="../../../../index.html"><img src="../../../../../../../../doc/html/images/home.png" alt="Home"></a><a accesskey="n" href="lazy.html"><img src="../../../../../../../../doc/html/images/next.png" alt="Next"></a> </div> <div class="section" title="Epsilon (eps)"> <div class="titlepage"><div><div><h5 class="title"> <a name="spirit.qi.reference.auxiliary.eps"></a><a class="link" href="eps.html" title="Epsilon (eps)"> Epsilon (<code class="computeroutput"><span class="identifier">eps</span></code>)</a> </h5></div></div></div> <a name="spirit.qi.reference.auxiliary.eps.description"></a><h6> <a name="id689932"></a> <a class="link" href="eps.html#spirit.qi.reference.auxiliary.eps.description">Description</a> </h6> <p> The Epsilon (<code class="computeroutput"><span class="identifier">eps</span></code>) is a multi-purpose parser that returns a zero length match. </p> <a name="spirit.qi.reference.auxiliary.eps.simple_form"></a><h6> <a name="id689956"></a> <a class="link" href="eps.html#spirit.qi.reference.auxiliary.eps.simple_form">Simple Form</a> </h6> <p> In its simplest form, <code class="computeroutput"><span class="identifier">eps</span></code> matches the null string and always returns a match of zero length: </p> <pre class="programlisting"><span class="identifier">eps</span> <span class="comment">// always returns a zero-length match </span></pre> <p> This form is usually used to trigger a semantic action unconditionally. For example, it is useful in triggering error messages when a set of alternatives fail: </p> <pre class="programlisting"><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</span><span class="special">[</span><span class="identifier">error</span><span class="special">()];</span> <span class="comment">// Call error if a, b, and c fail to match </span></pre> <a name="spirit.qi.reference.auxiliary.eps.semantic_predicate"></a><h6> <a name="id690057"></a> <a class="link" href="eps.html#spirit.qi.reference.auxiliary.eps.semantic_predicate">Semantic Predicate</a> </h6> <p> Semantic predicates allow you to attach a conditional function anywhere in the grammar. In this role, the epsilon takes a <a class="link" href="../basics.html#spirit.qi.reference.basics.lazy_argument">Lazy Argument</a> that returns <code class="computeroutput"><span class="keyword">true</span></code> or <code class="computeroutput"><span class="keyword">false</span></code>. The <a class="link" href="../basics.html#spirit.qi.reference.basics.lazy_argument">Lazy Argument</a> is typically a test that is called to resolve ambiguity in the grammar. A parse failure will be reported when the <a class="link" href="../basics.html#spirit.qi.reference.basics.lazy_argument">Lazy Argument</a> result evaluates to <code class="computeroutput"><span class="keyword">false</span></code>. Otherwise an empty match will be reported. The general form is: </p> <pre class="programlisting"><span class="identifier">eps</span><span class="special">(</span><span class="identifier">f</span><span class="special">)</span> <span class="special">>></span> <span class="identifier">rest</span><span class="special">;</span> </pre> <p> The <a class="link" href="../basics.html#spirit.qi.reference.basics.lazy_argument">Lazy Argument</a> <code class="computeroutput"><span class="identifier">f</span></code> is called to do a semantic test (say, checking if a symbol is in the symbol table). If test returns true, <code class="computeroutput"><span class="identifier">rest</span></code> will be evaluated. Otherwise, the production will return early with a no-match without ever touching rest. </p> <a name="spirit.qi.reference.auxiliary.eps.header"></a><h6> <a name="id690165"></a> <a class="link" href="eps.html#spirit.qi.reference.auxiliary.eps.header">Header</a> </h6> <pre class="programlisting"><span class="comment">// forwards to <boost/spirit/home/qi/auxiliary/eps.hpp> </span><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">spirit</span><span class="special">/</span><span class="identifier">include</span><span class="special">/</span><span class="identifier">qi_eps</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> </pre> <p> Also, see <a class="link" href="../../../structure/include.html" title="Include">Include Structure</a>. </p> <a name="spirit.qi.reference.auxiliary.eps.namespace"></a><h6> <a name="id690239"></a> <a class="link" href="eps.html#spirit.qi.reference.auxiliary.eps.namespace">Namespace</a> </h6> <div class="informaltable"><table class="table"> <colgroup><col></colgroup> <thead><tr><th> <p> Name </p> </th></tr></thead> <tbody><tr><td> <p> <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">eps</span> <span class="comment">// alias: boost::spirit::qi::eps</span></code> </p> </td></tr></tbody> </table></div> <a name="spirit.qi.reference.auxiliary.eps.model_of"></a><h6> <a name="id690317"></a> <a class="link" href="eps.html#spirit.qi.reference.auxiliary.eps.model_of">Model of</a> </h6> <div class="blockquote"><blockquote class="blockquote"> <p> </p> <p> <a class="link" href="../parser_concepts/primitiveparser.html" title="PrimitiveParser"><code class="computeroutput"><span class="identifier">PrimitiveParser</span></code></a> </p> <p> </p> </blockquote></div> <div class="variablelist" title="Notation"> <p class="title"><b>Notation</b></p> <dl> <dt><span class="term"><code class="computeroutput"><span class="identifier">f</span></code></span></dt> <dd><p> A <a class="link" href="../basics.html#spirit.qi.reference.basics.lazy_argument">Lazy Argument</a> that evaluates <code class="computeroutput"><span class="keyword">bool</span></code>. </p></dd> </dl> </div> <a name="spirit.qi.reference.auxiliary.eps.expression_semantics"></a><h6> <a name="id690388"></a> <a class="link" href="eps.html#spirit.qi.reference.auxiliary.eps.expression_semantics">Expression Semantics</a> </h6> <p> Semantics of an expression is defined only where it differs from, or is not defined in <a class="link" href="../parser_concepts/primitiveparser.html" title="PrimitiveParser"><code class="computeroutput"><span class="identifier">PrimitiveParser</span></code></a>. </p> <div class="informaltable"><table class="table"> <colgroup> <col> <col> </colgroup> <thead><tr> <th> <p> Expression </p> </th> <th> <p> Semantics </p> </th> </tr></thead> <tbody> <tr> <td> <p> <code class="computeroutput"><span class="identifier">eps</span></code> </p> </td> <td> <p> Match an empty string (always matches). </p> </td> </tr> <tr> <td> <p> <code class="computeroutput"><span class="identifier">eps</span><span class="special">(</span><span class="identifier">f</span><span class="special">)</span></code> </p> </td> <td> <p> If <code class="computeroutput"><span class="identifier">f</span></code> evaluates to <code class="computeroutput"><span class="keyword">true</span></code>, return a zero length match. </p> </td> </tr> </tbody> </table></div> <a name="spirit.qi.reference.auxiliary.eps.attributes"></a><h6> <a name="id690521"></a> <a class="link" href="eps.html#spirit.qi.reference.auxiliary.eps.attributes">Attributes</a> </h6> <div class="informaltable"><table class="table"> <colgroup> <col> <col> </colgroup> <thead><tr> <th> <p> Expression </p> </th> <th> <p> Attribute </p> </th> </tr></thead> <tbody><tr> <td> <p> <code class="computeroutput"><span class="identifier">eps</span></code> </p> </td> <td> <p> <code class="computeroutput"><span class="identifier">unused</span></code> </p> </td> </tr></tbody> </table></div> <a name="spirit.qi.reference.auxiliary.eps.complexity"></a><h6> <a name="id690599"></a> <a class="link" href="eps.html#spirit.qi.reference.auxiliary.eps.complexity">Complexity</a> </h6> <div class="blockquote"><blockquote class="blockquote"> <p> </p> <p> For plain (<code class="computeroutput"><span class="identifier">eps</span></code>) the complexity is O(1). For Semantic predicates (<code class="computeroutput"><span class="identifier">eps</span><span class="special">(</span><span class="identifier">f</span><span class="special">)</span></code>) the complexity is defined by the function <code class="computeroutput"><span class="identifier">f</span></code>. </p> <p> </p> </blockquote></div> <a name="spirit.qi.reference.auxiliary.eps.example"></a><h6> <a name="id690657"></a> <a class="link" href="eps.html#spirit.qi.reference.auxiliary.eps.example">Example</a> </h6> <div class="note" title="Note"><table border="0" summary="Note"> <tr> <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../images/note.png"></td> <th align="left">Note</th> </tr> <tr><td align="left" valign="top"><p> The test harness for the example(s) below is presented in the <a class="link" href="../basics.html#spirit.qi.reference.basics.examples">Basics Examples</a> section. </p></td></tr> </table></div> <p> Some using declarations: </p> <p> </p> <p> </p> <pre class="programlisting"><span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">qi</span><span class="special">::</span><span class="identifier">eps</span><span class="special">;</span> <span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">qi</span><span class="special">::</span><span class="identifier">int_</span><span class="special">;</span> <span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">qi</span><span class="special">::</span><span class="identifier">_1</span><span class="special">;</span> <span class="keyword">namespace</span> <span class="identifier">phx</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">phoenix</span><span class="special">;</span> </pre> <p> </p> <p> </p> <p> </p> <p> Basic <code class="computeroutput"><span class="identifier">eps</span></code>: </p> <p> </p> <p> </p> <pre class="programlisting"><span class="identifier">test_parser</span><span class="special">(</span><span class="string">""</span><span class="special">,</span> <span class="identifier">eps</span><span class="special">);</span> <span class="comment">// always matches </span></pre> <p> </p> <p> </p> <p> </p> <p> This example simulates the "classic" <code class="computeroutput"><span class="identifier">if_p</span></code> parser. Here, <code class="computeroutput"><span class="identifier">int_</span></code> will be tried only if the condition, <code class="computeroutput"><span class="identifier">c</span></code>, is true. </p> <p> </p> <p> </p> <pre class="programlisting"><span class="keyword">bool</span> <span class="identifier">c</span> <span class="special">=</span> <span class="keyword">true</span><span class="special">;</span> <span class="comment">// a flag </span><span class="identifier">test_parser</span><span class="special">(</span><span class="string">"1234"</span><span class="special">,</span> <span class="identifier">eps</span><span class="special">(</span><span class="identifier">phx</span><span class="special">::</span><span class="identifier">ref</span><span class="special">(</span><span class="identifier">c</span><span class="special">)</span> <span class="special">==</span> <span class="keyword">true</span><span class="special">)</span> <span class="special">>></span> <span class="identifier">int_</span><span class="special">);</span> </pre> <p> </p> <p> </p> <p> </p> <p> This example simulates the "classic" <code class="computeroutput"><span class="identifier">while_p</span></code> parser. Here, the kleene loop will exit once the condition, c, becomes true. Notice that the condition, <code class="computeroutput"><span class="identifier">c</span><span class="special">,</span> <span class="identifier">is</span> <span class="identifier">turned</span> <span class="identifier">to</span> </code>false<code class="computeroutput"> <span class="identifier">when</span> <span class="identifier">we</span> <span class="identifier">get</span> <span class="identifier">to</span> <span class="identifier">parse</span> </code>4`. </p> <p> </p> <p> </p> <pre class="programlisting"><span class="identifier">test_phrase_parser</span><span class="special">(</span><span class="string">"1 2 3 4"</span><span class="special">,</span> <span class="special">*(</span><span class="identifier">eps</span><span class="special">(</span><span class="identifier">phx</span><span class="special">::</span><span class="identifier">ref</span><span class="special">(</span><span class="identifier">c</span><span class="special">)</span> <span class="special">==</span> <span class="keyword">true</span><span class="special">)</span> <span class="special">>></span> <span class="identifier">int_</span><span class="special">[</span><span class="identifier">phx</span><span class="special">::</span><span class="identifier">ref</span><span class="special">(</span><span class="identifier">c</span><span class="special">)</span> <span class="special">=</span> <span class="special">(</span><span class="identifier">_1</span> <span class="special">==</span> <span class="number">4</span><span class="special">)]));</span> </pre> <p> </p> <p> </p> </div> <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> <td align="left"></td> <td align="right"><div class="copyright-footer">Copyright © 2001-2010 Joel de Guzman, Hartmut Kaiser<p> Distributed under the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>) </p> </div></td> </tr></table> <hr> <div class="spirit-nav"> <a accesskey="p" href="eoi.html"><img src="../../../../../../../../doc/html/images/prev.png" alt="Prev"></a><a accesskey="u" href="../auxiliary.html"><img src="../../../../../../../../doc/html/images/up.png" alt="Up"></a><a accesskey="h" href="../../../../index.html"><img src="../../../../../../../../doc/html/images/home.png" alt="Home"></a><a accesskey="n" href="lazy.html"><img src="../../../../../../../../doc/html/images/next.png" alt="Next"></a> </div> </body> </html>