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| 3 | <title>Epsilon</title> |
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| 11 | <td width="10"> |
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| 12 | </td> |
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| 13 | <td width="85%"> <font size="6" face="Verdana, Arial, Helvetica, sans-serif"><b>Epsilon</b></font> |
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| 14 | </td> |
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| 25 | </tr> |
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| 26 | </table> |
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| 27 | <p>The <strong>Epsilon</strong> (<tt>epsilon_p</tt> and <tt>eps_p</tt>) is a multi-purpose |
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| 28 | parser that returns a zero length match. </p> |
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| 29 | <h3>Simple Form</h3> |
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| 30 | <p>In its simplest form, epsilon_p matches the null string and always returns |
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| 31 | a match of zero length:</p> |
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| 32 | <pre><code><span class=special> </span><span class="identifier">epsilon_p </span><span class="comment">// always returns a zero-length match</span></code></pre> |
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| 33 | <p>This form is usually used to trigger a <a href="semantic_actions.html">semantic |
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| 34 | action</a> unconditionally. For example, it is useful in triggering error messages |
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| 35 | when a set of alternatives fail:</p> |
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| 36 | <pre><code><span class=special> </span><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_p</span><span class="special">[</span><span class="identifier">error</span><span class="special">];</span><span class="identifier"></span><span class="comment"> // error if A, B, or C fails to match</span></code></pre> |
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| 37 | <h3>Semantic Predicate</h3> |
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| 38 | <p>Semantic predicates allow you to attach a function anywhere in the grammar. |
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| 39 | In this role, the epsilon takes a 0-ary (nullary) function/functor. The run-time |
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| 40 | function/functor is typically a test that is called upon to resolve ambiguity |
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| 41 | in the grammar. A parse failure will be reported when the function/functor result |
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| 42 | evaluates to false. Otherwise an empty match will be reported. The general form |
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| 43 | is:</p> |
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| 44 | <pre> eps_p<span class="special">(</span>f<span class="special">) >></span> rest<span class="special">;</span> |
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| 45 | </pre> |
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| 46 | <p>The nullary function <tt>f</tt> is called to do a semantic test (say, checking |
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| 47 | if a symbol is in the <a href="symbols.html">symbol table</a>). If test returns |
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| 48 | <tt>true</tt>, <tt>rest</tt> will be evaluated. Otherwise, the production will |
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| 49 | return early with a no-match without ever touching <tt>rest</tt>.</p> |
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| 50 | <h3>Syntactic Predicate</h3> |
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| 51 | <p>Similar to Semantic predicates, Syntactic predicates assert a certain conditional |
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| 52 | syntax to be satisfied before evaluating another production. This time, epsilon_p |
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| 53 | accepts a (conditional) parser. The general form is:</p> |
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| 54 | <pre> eps_p<span class="special">(</span>p<span class="special">) >></span> rest<span class="special">;</span> |
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| 55 | </pre> |
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| 56 | <p>If <tt>p</tt> is matched on the input stream then attempt to recognize <tt>rest</tt>. |
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| 57 | The parser <tt>p </tt>is called to do a syntax check. Regardless of <tt>p</tt>'s |
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| 58 | success, <tt>eps_p(p)</tt> will always return a zero length match (i.e. the |
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| 59 | input is not consumed). If test returns <tt>true</tt>, <tt>rest</tt> will be |
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| 60 | evaluated. Otherwise, the production will return early with a no-match without |
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| 61 | ever touching <tt>rest</tt>.</p> |
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| 62 | <p>Example:</p> |
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| 63 | <pre><code><span class=special> </span><span class="identifier">eps_p</span><span class="special">(</span><span class="literal">'0'</span><span class="special">) >> </span><span class="identifier">oct_p </span><span class="comment">// note that '0' is actually a ch_p('0')</span><span class="identifier"> </span></code></pre> |
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| 64 | <p>Epsilon here is used as a syntactic predicate. <tt>oct_p</tt> (see <a href="numerics.html">numerics</a>) |
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| 65 | is parsed only if we see a leading <tt>'0'</tt>. Wrapping the leading <tt>'0'</tt> |
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| 66 | inside an epsilon makes the parser not consume anything from the input. If a |
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| 67 | <tt>'0'</tt> is seen, <tt>epsilon_p</tt> reports a successful match with zero |
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| 68 | length. </p> |
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| 69 | <table width="80%" border="0" align="center"> |
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| 70 | <tr> |
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| 71 | <td class="note_box"><div align="justify"><img src="theme/note.gif" width="16" height="16"> |
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| 72 | <b>Primitive arguments</b> <br> |
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| 73 | <br> |
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| 74 | Epsilon allows primitive type arguments such as <tt>char</tt>, <tt>int</tt>, |
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| 75 | <tt>wchar_t</tt>, <tt>char const<span class="operators">*</span></tt>, |
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| 76 | <tt>wchar_t const<span class="operators">*</span></tt> and so on. Examples: |
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| 77 | <tt><br> |
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| 78 | <br> |
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| 79 | </tt><code><span class="identifier">eps_p</span><tt><span class=special>(</span><span class=string>"hello"</span><span class=special>)</span><span class=comment> |
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| 80 | // same as eps_p(str_p("hello"))</span></tt><span class=identifier><br> |
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| 81 | eps_p</span><span class=special>(</span><span class=literal>'x'</span><span class="special">) |
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| 82 | </span><span class=comment>// same as eps_p(ch_p('x'))</span></code></div></td> |
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| 83 | </tr> |
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| 84 | </table> |
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| 85 | <h3><img src="theme/alert.gif" width="16" height="16"> Inhibiting Semantic Actions</h3> |
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| 86 | <p>In a syntactic predicate <tt>eps_p(p)</tt>, any semantic action directly or |
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| 87 | indirectly attached to the conditional parser <tt>p</tt> will not be called. |
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| 88 | However, semantic actions attached to epsilon itself will always be called. |
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| 89 | The following code snippets illustrates the behavior:</p> |
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| 90 | <pre> eps_p<span class="special">(</span>c<span class="special">[</span>f<span class="special">])</span> <span class="comment">// f not called</span><br> eps_p<span class="special">(</span>c<span class="special">)[</span>f<span class="special">]</span> <span class="comment">// f is called</span><br> eps_p<span class="special">[</span>f<span class="special">]</span> <span class="comment">// f is called</span></pre> |
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| 91 | <p>Actually, the conditional parser <tt>p</tt> is implicitly wrapped in a <tt><a href="scanner.html#no_actions_scanner">no_actions_d</a></tt> |
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| 92 | directive:</p> |
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| 93 | <pre><code><span class=special> </span>no_actions_d<span class="special">[</span>p<span class="special">]</span></code></pre> |
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| 94 | <p>The conditional parser is required to be free from side-effects (semantic actions). |
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| 95 | <code></code>The conditional parser's purpose is to resolve ambiguity by looking |
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| 96 | ahead in the input stream for a certain pattern. Ambiguity and semantic actions |
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| 97 | do not mix well. On an ambiguous grammar, backtracking happens. And when it |
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| 98 | happens, we cannot undo the effects of triggered semantic actions. </p> |
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| 99 | <h3>Negation</h3> |
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| 100 | <p>Operator <tt>~</tt> is defined for parsers constructed by <tt>epsilon_p</tt>/<tt>eps_p</tt>. |
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| 101 | It performs negation by complementing the results reported. <tt>~~eps_p(x)</tt> |
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| 102 | is identical to <tt>eps_p(x)</tt>.</p> |
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| 103 | <table border="0"> |
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| 104 | <tr> |
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| 105 | <td width="10"></td> |
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| 106 | <td width="30"><a href="../index.html"><img src="theme/u_arr.gif" border="0"></a></td> |
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| 107 | <td width="30"><a href="rule.html"><img src="theme/l_arr.gif" border="0"></a></td> |
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| 108 | <td width="30"><a href="directives.html"><img src="theme/r_arr.gif" border="0"></a></td> |
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| 109 | </tr> |
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| 110 | </table> |
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| 111 | <br> |
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| 112 | <hr size="1"> |
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| 113 | <p class="copyright">Copyright © 1998-2003 Joel de Guzman<br> |
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| 114 | Copyright © 2003 Martin Wille<br> |
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| 115 | <br> |
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| 116 | <font size="2">Use, modification and distribution is subject to the Boost Software |
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| 117 | License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at |
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| 118 | http://www.boost.org/LICENSE_1_0.txt) </font> </p> |
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| 119 | <p> </p> |
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| 120 | </body> |
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| 121 | </html> |
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