100 lines
8.0 KiB
HTML
100 lines
8.0 KiB
HTML
<html>
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<title>Storable Rules</title>
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<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
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<link rel="stylesheet" href="theme/style.css" type="text/css">
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<body>
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<table width="100%" border="0" background="theme/bkd2.gif" cellspacing="2">
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<td width="10">
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</td>
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<td width="85%"> <font size="6" face="Verdana, Arial, Helvetica, sans-serif"><b>Storable
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Rules</b></font></td>
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<td width="112"><a href="http://spirit.sf.net"><img src="theme/spirit.gif" width="112" height="48" align="right" border="0"></a></td>
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</tr>
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</table>
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<br>
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<table border="0">
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<tr>
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<td width="10"></td>
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<td width="30"><a href="../index.html"><img src="theme/u_arr.gif" border="0"></a></td>
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<td width="30"><a href="stored_rule.html"><img src="theme/l_arr.gif" border="0"></a></td>
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<td width="30"><a href="the_lazy_parser.html"><img src="theme/r_arr.gif" border="0"></a></td>
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</tr>
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</table>
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<p>The rule is a weird C++ citizen, unlike any other C++ object. It does not have
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the proper copy and assignment semantics and cannot be stored and passed around
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by value. You cannot store rules in STL containers (vector, stack, etc) for
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later use and you cannot pass and return rules to and from functions by value.</p>
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<p>EBNF is primarily declarative. Like in functional programming, an EBNF grammar
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is a static recipe and there's no notion of do this then that. However, in Spirit,
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we managed to coax imperative C++ to take in declarative EBNF. Hah! Fun!...
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We did that by masquerading the C++ assignment operator to mimic EBNF's <tt>::=</tt>.
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To do that, we gave the rule class' assignment operator and copy constructor
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a different meaning and semantics. The downside is that doing so made the rule
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unlike any other C++ object. You can't copy it. You can't assign it. </p>
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<p>We want to have the dynamic nature of C++ to our advantage. We've seen dynamic
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Spirit in action here and there. There are indeed some interesting applications
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of dynamic parsers using Spirit. Yet, we will not fully utilize the power of
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dynamic parsing, unless we have a rule that behaves like any other good C++
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object. With such a beast, we can write full parsers that's defined at run time,
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as opposed to compile time.</p>
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<p>We now have dynamic rules: <tt>stored_rules</tt>. Basically they are rules
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with perfect C++ assignment/copy-constructor semantics. This means that <tt>stored_rules</tt>
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can be stored in containers and/or dynamically created at run-time.</p>
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<pre><code><font color="#000000"><span class=identifier> </span><span class=keyword>template</span><span class=special><
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</span><span class=keyword>typename </span><span class=identifier>ScannerT </span><span class=special>= </span><span class=identifier>scanner</span><span class=special><>,
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</span><span class=keyword>typename </span><span class=identifier>ContextT </span><span class=special>= </span><span class=identifier>parser_context</span><span class=special><></span><span class=identifier>,
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</span><span class="keyword">typename</span><span class=identifier> TagT </span><span class="special">=</span><span class=identifier> parser_address_tag</span><span class=special>>
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</span><span class=keyword>class </span><span class=identifier>stored_rule</span><span class=special>;</span></font></code></pre>
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<p>The interface is exactly the same as with the rule class (see the <a href="rule.html">section
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on rules</a> for more information regarding the API). The only difference is
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with the copy and assignment semantics. Now, with <tt>stored_rule</tt>s, we
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can dynamically and algorithmically define our rules. Here are some samples...
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</p>
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<p>Say I want to dynamically create a rule for:</p>
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<pre>
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<span class=identifier> start </span><span class=special>= </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></pre>
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<p> I can write it dynamically step-by-step:</p>
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<pre> <span class=identifier> stored_rule</span><span class=special><> </span><span class=identifier>start</span><span class=special>;
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</span><span class=identifier>start </span><span class=special>= </span><span class=identifier>a</span><span class=special>;
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</span><span class=identifier>start </span><span class=special>= </span><span class=identifier>start</span><span class=special>.</span><span class=identifier>copy</span><span class=special>() </span><span class=special>| </span><span class=identifier>b</span><span class=special>;
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</span><span class=identifier>start </span><span class=special>= </span><span class=identifier>start</span><span class=special>.</span><span class=identifier>copy</span><span class=special>() </span><span class=special>| </span><span class=identifier>c</span><span class=special>;
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</span><span class=identifier>start </span><span class=special>= </span><span class=special>*(</span><span class=identifier>start</span><span class=special>.</span><span class=identifier>copy</span><span class=special>());</span></pre>
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<p>Later, I changed my mind and want to redefine it (again dynamically) as:</p>
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<pre><span class=identifier> start </span><span class=special>= </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=special>>> </span><span class=special>(</span><span class=identifier>start </span><span class=special>| </span><span class=identifier>b</span><span class=special>);</span>
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</pre>
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<p>I write:</p>
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<pre> <span class=special> </span><span class=identifier>start </span><span class=special>= </span><span class=identifier>b</span><span class=special>;
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</span><span class=identifier>start </span><span class=special>= </span><span class=identifier>a </span><span class=special>| </span><span class=identifier>start</span><span class=special>.</span><span class=identifier>copy</span><span class=special>();
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</span><span class=identifier>start </span><span class=special>= </span><span class=identifier>start</span><span class=special>.</span><span class=identifier>copy</span><span class=special>() </span><span class=special>>> </span><span class=special>(</span><span class=identifier>start </span><span class=special>| </span><span class=identifier>b</span><span class=special>);</span></pre>
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<p>Notice the statement:</p>
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<pre> <span class=special> </span><span class=identifier>start </span><span class=special>= </span><span class=identifier>start</span><span class=special>.</span><span class=identifier>copy</span><span class=special>() </span><span class=special>| </span><span class=identifier>b</span><span class=special>;</span></pre>
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<p>Why is start.copy() required? Well, because like rules, stored rules are still
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embedded by reference when found in the RHS (one reason is to avoid cyclic-shared-pointers).
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If we write:</p>
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<pre> <span class=special> </span><span class=identifier>start </span><span class=special>= </span><span class=identifier>start </span><span class=special>| </span><span class=identifier>b</span><span class=special>;</span></pre>
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<p>We have <strong>left-recursion</strong>! Copying copy of start avoids self
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referencing. What we are doing is making a copy of start, ORing it with b, then
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destructively assigning the result back to start.</p>
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<table border="0">
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<tr>
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<td width="10"></td>
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<td width="30"><a href="../index.html"><img src="theme/u_arr.gif" border="0"></a></td>
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<td width="30"><a href="stored_rule.html"><img src="theme/l_arr.gif" border="0"></a></td>
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<td width="30"><a href="the_lazy_parser.html"><img src="theme/r_arr.gif" border="0"></a></td>
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</tr>
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</table>
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<br>
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<hr size="1">
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<p class="copyright">Copyright © 1998-2003 Joel de Guzman<br>
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<br>
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<font size="2">Use, modification and distribution is subject to the Boost Software
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License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at
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http://www.boost.org/LICENSE_1_0.txt)</font></p>
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</body>
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</html>
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