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<title>Epsilon Generator (eps)</title>
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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 Generator (eps)">Epsilon Generator
          (<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>
<h6>
<a name="spirit.karma.reference.auxiliary.eps.h0"></a>
            <span class="phrase"><a name="spirit.karma.reference.auxiliary.eps.description"></a></span><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>
<h6>
<a name="spirit.karma.reference.auxiliary.eps.h1"></a>
            <span class="phrase"><a name="spirit.karma.reference.auxiliary.eps.simple_form"></a></span><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>
<h6>
<a name="spirit.karma.reference.auxiliary.eps.h2"></a>
            <span class="phrase"><a name="spirit.karma.reference.auxiliary.eps.semantic_predicate"></a></span><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="../basics.html#spirit.karma.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.karma.reference.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="../basics.html#spirit.karma.reference.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">&lt;&lt;</span> <span class="identifier">rest</span><span class="special">;</span>
</pre>
<p>
            The <a class="link" href="../basics.html#spirit.karma.reference.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>
<h6>
<a name="spirit.karma.reference.auxiliary.eps.h3"></a>
            <span class="phrase"><a name="spirit.karma.reference.auxiliary.eps.header"></a></span><a class="link" href="eps.html#spirit.karma.reference.auxiliary.eps.header">Header</a>
          </h6>
<pre class="programlisting"><span class="comment">// forwards to &lt;boost/spirit/home/karma/auxiliary/eps.hpp&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</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">&gt;</span>
</pre>
<p>
            Also, see <a class="link" href="../../../structure/include.html" title="Include">Include Structure</a>. 
          </p>
<h6>
<a name="spirit.karma.reference.auxiliary.eps.h4"></a>
            <span class="phrase"><a name="spirit.karma.reference.auxiliary.eps.namespace"></a></span><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>
<h6>
<a name="spirit.karma.reference.auxiliary.eps.h5"></a>
            <span class="phrase"><a name="spirit.karma.reference.auxiliary.eps.model_of"></a></span><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 class="variablelist">
<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="../basics.html#spirit.karma.reference.basics.lazy_argument">Lazy
                  Argument</a> that evaluates to a boolean value. 
                </p></dd>
</dl>
</div>
<h6>
<a name="spirit.karma.reference.auxiliary.eps.h6"></a>
            <span class="phrase"><a name="spirit.karma.reference.auxiliary.eps.expression_semantics"></a></span><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>
<h6>
<a name="spirit.karma.reference.auxiliary.eps.h7"></a>
            <span class="phrase"><a name="spirit.karma.reference.auxiliary.eps.attributes"></a></span><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>
<h6>
<a name="spirit.karma.reference.auxiliary.eps.h8"></a>
            <span class="phrase"><a name="spirit.karma.reference.auxiliary.eps.complexity"></a></span><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>
<h6>
<a name="spirit.karma.reference.auxiliary.eps.h9"></a>
            <span class="phrase"><a name="spirit.karma.reference.auxiliary.eps.example"></a></span><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="../basics.html#spirit.karma.reference.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">&lt;</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">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</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">support_utree</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</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">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</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">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</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">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">iostream</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">string</span><span class="special">&gt;</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">&lt;&lt;</span> <span class="identifier">val</span><span class="special">(</span><span class="string">"starting eps example"</span><span class="special">)]</span> <span class="special">&lt;&lt;</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">&lt;&lt;</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">&lt;&lt;</span> <span class="string">"abc"</span><span class="special">);</span>      <span class="comment">// fails as eps expression is 'false'</span>
</pre>
<p>
          </p>
</div>
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<td align="right"><div class="copyright-footer">Copyright &#169; 2001-2011 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>
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