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| <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="id867060"></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="id867084"></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="id867184"></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="id867292"></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="id867367"></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="id867445"></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> |
| <a class="link" href="../parser_concepts/primitiveparser.html" title="PrimitiveParser"><code class="computeroutput"><span class="identifier">PrimitiveParser</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">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="id867513"></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="id867650"></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="id867730"></a> |
| <a class="link" href="eps.html#spirit.qi.reference.auxiliary.eps.complexity">Complexity</a> |
| </h6> |
| <div class="blockquote"><blockquote class="blockquote"><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></blockquote></div> |
| <a name="spirit.qi.reference.auxiliary.eps.example"></a><h6> |
| <a name="id867784"></a> |
| <a class="link" href="eps.html#spirit.qi.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="../basics.html#spirit.qi.reference.basics.examples">Basics Examples</a> |
| section. |
| </p></td></tr> |
| </table></div> |
| <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">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> |
| Basic <code class="computeroutput"><span class="identifier">eps</span></code>: |
| </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> |
| 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> |
| <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> |
| 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, <code class="computeroutput"><span class="identifier">c</span></code>, becomes true. Notice that the condition, |
| <code class="computeroutput"><span class="identifier">c</span></code>, is turned to <code class="computeroutput"><span class="keyword">false</span></code> when we get to parse <code class="computeroutput"><span class="number">4</span></code>. |
| </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> |
| </div> |
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| <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>) |
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