<html> <head> <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> <title>Epsilon (eps)</title> <link rel="stylesheet" href="../../../../../../../../doc/src/boostbook.css" type="text/css"> <meta name="generator" content="DocBook XSL Stylesheets V1.75.0"> <link rel="home" href="../../../../index.html" title="Spirit 2.4"> <link rel="up" href="../auxiliary.html" title="Auxiliary"> <link rel="prev" href="eol.html" title="End of Line (eol)"> <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="../../../../../../../../libs/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="eol.html"><img src="../../../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../auxiliary.html"><img src="../../../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../../../index.html"><img src="../../../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="lazy.html"><img src="../../../../../../../../doc/src/images/next.png" alt="Next"></a> </div> <div class="section"> <div class="titlepage"><div><div><h5 class="title"> <a name="spirit.karma.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> <p> The family of <code class="computeroutput"><span class="identifier">eps</span></code> components allows to create pseudo generators generating an empty string. This feature is sometimes useful either to force a generator to fail or to succeed or to insert semantic actions into the generation process. </p> <a name="spirit.karma.reference.auxiliary.eps.description"></a><h6> <a name="id1054080"></a> <a class="link" href="eps.html#spirit.karma.reference.auxiliary.eps.description">Description</a> </h6> <p> The Epsilon (<code class="computeroutput"><span class="identifier">eps</span></code>) is a multi-purpose generator that emits a yero length string. </p> <a name="spirit.karma.reference.auxiliary.eps.simple_form"></a><h6> <a name="id1054104"></a> <a class="link" href="eps.html#spirit.karma.reference.auxiliary.eps.simple_form">Simple Form</a> </h6> <p> In its simplest form, <code class="computeroutput"><span class="identifier">eps</span></code> creates a component generating an empty string while always succeeding: </p> <pre class="programlisting"><span class="identifier">eps</span> <span class="comment">// always emits a zero-length string </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 generate </span></pre> <a name="spirit.karma.reference.auxiliary.eps.semantic_predicate"></a><h6> <a name="id1054205"></a> <a class="link" href="eps.html#spirit.karma.reference.auxiliary.eps.semantic_predicate">Semantic Predicate</a> </h6> <p> The <code class="computeroutput"><span class="identifier">eps</span><span class="special">(</span><span class="identifier">b</span><span class="special">)</span></code> component generates an empty string as well, but succeeds only if <code class="computeroutput"><span class="identifier">b</span></code> is <code class="computeroutput"><span class="keyword">true</span></code> and fails otherwise. It's lazy variant <code class="computeroutput"><span class="identifier">eps</span><span class="special">(</span><span class="identifier">fb</span><span class="special">)</span></code> is equivalent to <code class="computeroutput"><span class="identifier">eps</span><span class="special">(</span><span class="identifier">b</span><span class="special">)</span></code> except it evaluates the supplied function <code class="computeroutput"><span class="identifier">fb</span></code> at generate time, while using the return value as the criteria to succeed. </p> <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="../karma_basics.html#spirit.karma.reference.karma_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="../karma_basics.html#spirit.karma.reference.karma_basics.lazy_argument">Lazy Argument</a> is typically a test that is called to resolve ambiguity in the grammar. A generator failure will be reported when the <a class="link" href="../karma_basics.html#spirit.karma.reference.karma_basics.lazy_argument">Lazy Argument</a> result evaluates to <code class="computeroutput"><span class="keyword">false</span></code>. Otherwise an empty string will be emitted. The general form is: </p> <pre class="programlisting"><span class="identifier">eps_p</span><span class="special">(</span><span class="identifier">fb</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="../karma_basics.html#spirit.karma.reference.karma_basics.lazy_argument">Lazy Argument</a> <code class="computeroutput"><span class="identifier">fb</span></code> is called to do a semantic test. If the test returns true, <code class="computeroutput"><span class="identifier">rest</span></code> will be evaluated. Otherwise, the production will return early without ever touching rest. </p> <a name="spirit.karma.reference.auxiliary.eps.header"></a><h6> <a name="id1054392"></a> <a class="link" href="eps.html#spirit.karma.reference.auxiliary.eps.header">Header</a> </h6> <pre class="programlisting"><span class="comment">// forwards to <boost/spirit/home/karma/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">karma_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.karma.reference.auxiliary.eps.namespace"></a><h6> <a name="id1054467"></a> <a class="link" href="eps.html#spirit.karma.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::karma::eps</span></code> </p> </td></tr></tbody> </table></div> <a name="spirit.karma.reference.auxiliary.eps.model_of"></a><h6> <a name="id1054546"></a> <a class="link" href="eps.html#spirit.karma.reference.auxiliary.eps.model_of">Model of</a> </h6> <div class="blockquote"><blockquote class="blockquote"><p> <a class="link" href="../generator_concepts/primitivegenerator.html" title="PrimitiveGenerator"><code class="computeroutput"><span class="identifier">PrimitiveGenerator</span></code></a> </p></blockquote></div> <div class="variablelist"> <p class="title"><b>Notation</b></p> <dl> <dt><span class="term"><code class="computeroutput"><span class="identifier">b</span></code></span></dt> <dd><p> A boolean value. </p></dd> <dt><span class="term"><code class="computeroutput"><span class="identifier">fb</span></code></span></dt> <dd><p> A <a class="link" href="../karma_basics.html#spirit.karma.reference.karma_basics.lazy_argument">Lazy Argument</a> that evaluates to a boolean value. </p></dd> </dl> </div> <a name="spirit.karma.reference.auxiliary.eps.expression_semantics"></a><h6> <a name="id1054631"></a> <a class="link" href="eps.html#spirit.karma.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="../generator_concepts/primitivegenerator.html" title="PrimitiveGenerator"><code class="computeroutput"><span class="identifier">PrimitiveGenerator</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> Creates a component generating an empty string. Succeeds always. </p> </td> </tr> <tr> <td> <p> <code class="computeroutput"><span class="identifier">eps</span><span class="special">(</span><span class="identifier">b</span><span class="special">)</span></code> </p> </td> <td> <p> Creates a component generating an empty string. Succeeds if <code class="computeroutput"><span class="identifier">b</span></code> is <code class="computeroutput"><span class="keyword">true</span></code> (unless the underlying output stream reports an error). </p> </td> </tr> <tr> <td> <p> <code class="computeroutput"><span class="identifier">eps</span><span class="special">(</span><span class="identifier">fb</span><span class="special">)</span></code> </p> </td> <td> <p> Creates a component generating an empty string. Succeeds if <code class="computeroutput"><span class="identifier">fb</span></code> returns <code class="computeroutput"><span class="keyword">true</span></code> at generate time (unless the underlying output stream reports an error). </p> </td> </tr> </tbody> </table></div> <a name="spirit.karma.reference.auxiliary.eps.attributes"></a><h6> <a name="id1054823"></a> <a class="link" href="eps.html#spirit.karma.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> <tr> <td> <p> <code class="computeroutput"><span class="identifier">eps</span><span class="special">(</span><span class="identifier">b</span><span class="special">)</span></code> </p> </td> <td> <p> <code class="computeroutput"><span class="identifier">unused</span></code> </p> </td> </tr> <tr> <td> <p> <code class="computeroutput"><span class="identifier">eps</span><span class="special">(</span><span class="identifier">fb</span><span class="special">)</span></code> </p> </td> <td> <p> <code class="computeroutput"><span class="identifier">unused</span></code> </p> </td> </tr> </tbody> </table></div> <a name="spirit.karma.reference.auxiliary.eps.complexity"></a><h6> <a name="id1054990"></a> <a class="link" href="eps.html#spirit.karma.reference.auxiliary.eps.complexity">Complexity</a> </h6> <div class="blockquote"><blockquote class="blockquote"><p> O(1) </p></blockquote></div> <p> The complexity is constant as no output is generated. </p> <a name="spirit.karma.reference.auxiliary.eps.example"></a><h6> <a name="id1055014"></a> <a class="link" href="eps.html#spirit.karma.reference.auxiliary.eps.example">Example</a> </h6> <div class="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="../karma_basics.html#spirit.karma.reference.karma_basics.examples">Basics Examples</a> section. </p></td></tr> </table></div> <p> Some includes: </p> <p> </p> <pre class="programlisting"><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">karma</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></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">phoenix_core</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></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">phoenix_operator</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> <span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">fusion</span><span class="special">/</span><span class="identifier">include</span><span class="special">/</span><span class="identifier">std_pair</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> <span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">iostream</span><span class="special">></span> <span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">string</span><span class="special">></span> </pre> <p> </p> <p> Some using declarations: </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">karma</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">phoenix</span><span class="special">::</span><span class="identifier">val</span><span class="special">;</span> </pre> <p> </p> <p> Basic usage of the <code class="computeroutput"><span class="identifier">eps</span></code> generator: </p> <p> </p> <pre class="programlisting"><span class="identifier">test_generator</span><span class="special">(</span><span class="string">"abc"</span><span class="special">,</span> <span class="identifier">eps</span><span class="special">[</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">val</span><span class="special">(</span><span class="string">"starting eps example"</span><span class="special">)]</span> <span class="special"><<</span> <span class="string">"abc"</span><span class="special">);</span> <span class="identifier">test_generator</span><span class="special">(</span><span class="string">"abc"</span><span class="special">,</span> <span class="identifier">eps</span><span class="special">(</span><span class="keyword">true</span><span class="special">)</span> <span class="special"><<</span> <span class="string">"abc"</span><span class="special">);</span> <span class="identifier">test_generator</span><span class="special">(</span><span class="string">""</span><span class="special">,</span> <span class="identifier">eps</span><span class="special">(</span><span class="keyword">false</span><span class="special">)</span> <span class="special"><<</span> <span class="string">"abc"</span><span class="special">);</span> <span class="comment">// fails as eps expression is 'false' </span></pre> <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="eol.html"><img src="../../../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../auxiliary.html"><img src="../../../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../../../index.html"><img src="../../../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="lazy.html"><img src="../../../../../../../../doc/src/images/next.png" alt="Next"></a> </div> </body> </html>