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    <td width="85%"> <font size="6" face="Verdana, Arial, Helvetica, sans-serif"><b>Rationale</b></font></td>
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<p><img src="theme/lens.gif" width="15" height="16"> <strong>Virtual functions: 
  From static to dynamic C++</strong></p>
<p>Rules straddle the border between static and dynamic C++. In effect, a rule 
  transforms compile-time polymorphism (using templates) into run-time polymorphism 
  (using virtual functions). This is necessary due to C++'s inability to automatically 
  declare a variable of a type deduced from an arbitrarily complex expression 
  in the right-hand side (rhs) of an assignment. Basically, we want to do something 
  like:</p>
<pre><code><font color="#000000">    <span class=identifier>T </span><span class=identifier>rule </span><span class=special>= </span><span class=identifier>an_arbitrarily_complex_expression</span><span class=special>;</span></font></code></pre>
<p>without having to know or care about the resulting type of the right-hand side 
  (rhs) of the assignment expression. Apart from this, we also need a facility 
  to forward declare an unknown type:</p>
<pre><code><font color="#000000"><span class=special>    </span><span class=identifier>T </span><span class=identifier>rule</span><span class=special>;
    </span><span class=special>...
    </span><span class=identifier>rule </span><span class=special>= </span><span class=identifier>a </span><span class=special>| </span><span class=identifier>b</span><span class=special>;</span></font></code></pre>
<p>These limitations lead us to this implementation of rules. This comes at the 
  expense of the overhead of a virtual function call, once through each invocation 
  of a rule.</p>
<p><img src="theme/lens.gif" width="15" height="16"> <strong>Multiple declaration 
  </strong> </p>
<p>Some BNF variants allow multiple declarations of a <tt>rule</tt>. The declarations 
  are taken as alternatives. Example:</p>
<pre>
   <span class=identifier><code>r </code></span><code><span class=special>= </span><span class=identifier>a</span><span class=special>;    </span><span class=identifier>
   r </span><span class=special>= </span><span class=identifier>b</span><span class=special>;</span></code></pre>
<p> is equivalent to: </p>
<pre>
   <span class=identifier><code>r </code></span><code><span class=special>= </span><span class=identifier>a </span><span class=special>| </span><span class=identifier>b</span><span class=special>;</span></code></pre>
<p>Spirit v1.3 allowed this behavior. However, the current version of Spirit <b>no 
  longer</b> allows this because experience shows that this behavior leads to 
  unwanted gotchas (for instance, it does not allow rules to be held in containers). 
  In the current release of Spirit, a second assignment to a rule will simply 
  redefine it. The old definition is destructed. This follows more closely C++ 
  semantics and is more in line with what the user expects the rule to behave.</p>
<p><img src="theme/lens.gif" width="15" height="16"> <b>Sequencing Syntax</b> 
  <br>
  <br>
  The comma operator as in a, b seems to be a better candidate, syntax-wise. But 
  then the problem is with its precedence. It has the lowest precedence in C/C++, 
  which makes it virtually useless. <br>
  <br>
  Bjarne Stroustrup, in his article <a href="references.html#generalized_overloading">&quot;Generalizing 
  Overloading for C++2000&quot;</a> talks about overloading whitespace. Such a 
  feature would allow juxtapositioning of parser objects exactly as we do in (E)BNF 
  (e.g. a b | c instead of a &gt;&gt; b | c). Unfortunately, the article was dated 
  April 1, 1998. Oh well.</p>
<p><img src="theme/lens.gif" width="15" height="16"> <b>Forward iterators</b><br>
  <br>
  In general, the scanner expects at least a standard conforming forward iterator. 
  Forward iterators are needed for backtracking where the iterator needs to be 
  saved and restored later. Generally speaking, Spirit is a backtracking parser. 
  The implication of this is that at some point, the iterator position will have 
  to be saved to allow the parser to backtrack to a previous point. Thus, for 
  backtracking to work, the framework requires at least a forward iterator.<br>
  <br>
  Some parsers might require more specialized iterators (bi-directional or even 
  random access). Perhaps in the future, deterministic parsers when added to the 
  framework, will perform no backtracking and will need just a single token lookahead, 
  hence will require input iterators only.</p>
<p><img src="theme/lens.gif" width="15" height="16"><b> Why are subrules important?</b><br>
  <br>
  Subrules open up the oportunity to do aggressive meta programming as well because 
  they do not rely on virtual functions. The virtual function is the meta-programmer's 
  hell. Not only does it slow down the program due to the virtual function indirect 
  call, it is also an opaque wall where no metaprogram can get past. It kills 
  all meta-information beyond the virtual function call. Worse, the virtual function 
  cannot be templated. Which means that its arguments have to be tied to a actual 
  types. Many problems stem from this limitation. <br>
  <br>
  While Spirit is a currently classified as a non-deterministic recursive-descent 
  parser, Doug Gregor first noted that other parsing techniques apart from top-down 
  recursive descent may be applied. For instance, apart from non-deterministic 
  recursive descent, deterministic LL(1) and LR(1) can theoretically be implemented 
  using the same expression template front end. Spirit rules use virtual functions 
  to encode the RHS parser expression in an opaque abstract parser type. While 
  it serves its purpose well, the rule's virtual functions are the stumbling blocks 
  to more advanced metaprogramming. Subrules are free from virtual functions.</p>
<p><img src="theme/lens.gif" width="15" height="16"><b> <a name="exhaustive_rd"></a>Exhaustive 
  backtracking and greedy RD</b></p>
<p>Spirit doesn't do exhaustive backtracking like regular expressions are expected 
  to. For example:</p>
<pre>    <span class="special">*</span>chlit_p<span class="special">(</span><span class="quotes">'a'</span><span class="special">) &gt;&gt;</span> chlit_p<span class="special">(</span><span class="quotes">'a'</span><span class="special">);</span></pre>
<p>will always fail to match because Spirit's Kleene star does not back off when 
  the rest of the rule fails to match. </p>
<p>Actually, there's a solution to this greedy RD problem. Such a scheme is discussed 
  in section 6.6.2 of <a 
href="http://www.cs.vu.nl/%7Edick/PTAPG.html">Parsing Techniques: A Practical 
  Guide</a>. The trick involves passing a <em>tail</em> parser (in addition to 
  the scanner) to each parser. The start parser will then simply be: <tt>start 
  &gt;&gt; end_p;</tt> (end_p is the start's tail). </p>
<p>Spirit is greedy --using straight forward, naive RD. It is certainly possible 
  to implement the fully backtracking scheme presented above, but there will be 
  also certainly be a performance hit. The scheme will always try to match all 
  possible parser paths (full parser hierarchy traversal) until it reaches a point 
  of certainty, that the whole thing matches or fails to match. </p>
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    <td class="note_box"><p><img src="theme/note.gif" width="16" height="16"><b>Backtracking 
        and Greedy RD </b><br>
        <br>
        Spirit is quite consistent and intuitive about when it backtracks and 
        to where, although it may not be obvious to those coming from different 
        backgrounds. In general, any (sub)parser will, given the same input, always 
        match the same portion of the input (or fail to match the input at all). 
        This means that Spirit is inherently greedy. Spirit will only backtrack 
        when a (sub)parser fails to match the input, and it will always backtrack 
        to the next choice point upward (not backward) in the parser structure. 
        In other words abb|ab will match &quot;ab&quot;, as will a(bb|b), but 
        (ab|a)b won't because the (ab|a) subparser will always match the 'b' after 
        the 'a' if it is available.</p>
      <p>--Rainer Deyke</p>
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<p>There's a strong preference on &quot;simplicity with all the knobs when you 
  need them&quot; approach, right now. On the other hand, the flexibility of Spirit 
  makes it possible to have different optional schemes available. It might be 
  possible to implement an exhaustive backtracking RD scheme as an optional feature 
  in the future. </p>
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<p class="copyright">Copyright &copy; 1998-2003 Joel de Guzman<br>
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  <font size="2">Use, modification and distribution is subject to the Boost Software
    License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at
    http://www.boost.org/LICENSE_1_0.txt)</font></p>
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