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| <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="id1012376"></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 zero length string. |
| </p> |
| <a name="spirit.karma.reference.auxiliary.eps.simple_form"></a><h6> |
| <a name="id1012400"></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="id1012500"></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="id1012688"></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="id1012762"></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="id1012841"></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="id1012926"></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="id1013118"></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="id1013285"></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="id1013309"></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> |
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