ceph/src/boost/libs/spirit/classic/doc/rationale.html

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  13     <td width="85%"> <font size="6" face="Verdana, Arial, Helvetica, sans-serif"><b>Rationale</b></font></td>
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  26 <p><img src="theme/lens.gif" width="15" height="16"> <strong>Virtual functions:
  27   From static to dynamic C++</strong></p>
  28 <p>Rules straddle the border between static and dynamic C++. In effect, a rule
  29   transforms compile-time polymorphism (using templates) into run-time polymorphism
  30   (using virtual functions). This is necessary due to C++'s inability to automatically
  31   declare a variable of a type deduced from an arbitrarily complex expression
  32   in the right-hand side (rhs) of an assignment. Basically, we want to do something
  33   like:</p>
  34 <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>
  35 <p>without having to know or care about the resulting type of the right-hand side
  36   (rhs) of the assignment expression. Apart from this, we also need a facility
  37   to forward declare an unknown type:</p>
  38 <pre><code><font color="#000000"><span class=special>    </span><span class=identifier>T </span><span class=identifier>rule</span><span class=special>;
  39     </span><span class=special>...
  40     </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>
  41 <p>These limitations lead us to this implementation of rules. This comes at the
  42   expense of the overhead of a virtual function call, once through each invocation
  43   of a rule.</p>
  44 <p><img src="theme/lens.gif" width="15" height="16"> <strong>Multiple declaration
  45   </strong> </p>
  46 <p>Some BNF variants allow multiple declarations of a <tt>rule</tt>. The declarations
  47   are taken as alternatives. Example:</p>
  48 <pre>
  49    <span class=identifier><code>r </code></span><code><span class=special>= </span><span class=identifier>a</span><span class=special>;    </span><span class=identifier>
  50    r </span><span class=special>= </span><span class=identifier>b</span><span class=special>;</span></code></pre>
  51 <p> is equivalent to: </p>
  52 <pre>
  53    <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>
  54 <p>Spirit v1.3 allowed this behavior. However, the current version of Spirit <b>no
  55   longer</b> allows this because experience shows that this behavior leads to
  56   unwanted gotchas (for instance, it does not allow rules to be held in containers).
  57   In the current release of Spirit, a second assignment to a rule will simply
  58   redefine it. The old definition is destructed. This follows more closely C++
  59   semantics and is more in line with what the user expects the rule to behave.</p>
  60 <p><img src="theme/lens.gif" width="15" height="16"> <b>Sequencing Syntax</b>
  61   <br>
  62   <br>
  63   The comma operator as in a, b seems to be a better candidate, syntax-wise. But
  64   then the problem is with its precedence. It has the lowest precedence in C/C++,
  65   which makes it virtually useless. <br>
  66   <br>
  67   Bjarne Stroustrup, in his article <a href="references.html#generalized_overloading">&quot;Generalizing
  68   Overloading for C++2000&quot;</a> talks about overloading whitespace. Such a
  69   feature would allow juxtapositioning of parser objects exactly as we do in (E)BNF
  70   (e.g. a b | c instead of a &gt;&gt; b | c). Unfortunately, the article was dated
  71   April 1, 1998. Oh well.</p>
  72 <p><img src="theme/lens.gif" width="15" height="16"> <b>Forward iterators</b><br>
  73   <br>
  74   In general, the scanner expects at least a standard conforming forward iterator.
  75   Forward iterators are needed for backtracking where the iterator needs to be
  76   saved and restored later. Generally speaking, Spirit is a backtracking parser.
  77   The implication of this is that at some point, the iterator position will have
  78   to be saved to allow the parser to backtrack to a previous point. Thus, for
  79   backtracking to work, the framework requires at least a forward iterator.<br>
  80   <br>
  81   Some parsers might require more specialized iterators (bi-directional or even
  82   random access). Perhaps in the future, deterministic parsers when added to the
  83   framework, will perform no backtracking and will need just a single token lookahead,
  84   hence will require input iterators only.</p>
  85 <p><img src="theme/lens.gif" width="15" height="16"><b> Why are subrules important?</b><br>
  86   <br>
  87   Subrules open up the oportunity to do aggressive meta programming as well because
  88   they do not rely on virtual functions. The virtual function is the meta-programmer's
  89   hell. Not only does it slow down the program due to the virtual function indirect
  90   call, it is also an opaque wall where no metaprogram can get past. It kills
  91   all meta-information beyond the virtual function call. Worse, the virtual function
  92   cannot be templated. Which means that its arguments have to be tied to a actual
  93   types. Many problems stem from this limitation. <br>
  94   <br>
  95   While Spirit is a currently classified as a non-deterministic recursive-descent
  96   parser, Doug Gregor first noted that other parsing techniques apart from top-down
  97   recursive descent may be applied. For instance, apart from non-deterministic
  98   recursive descent, deterministic LL(1) and LR(1) can theoretically be implemented
  99   using the same expression template front end. Spirit rules use virtual functions
 100   to encode the RHS parser expression in an opaque abstract parser type. While
 101   it serves its purpose well, the rule's virtual functions are the stumbling blocks
 102   to more advanced metaprogramming. Subrules are free from virtual functions.</p>
 103 <p><img src="theme/lens.gif" width="15" height="16"><b> <a name="exhaustive_rd"></a>Exhaustive
 104   backtracking and greedy RD</b></p>
 105 <p>Spirit doesn't do exhaustive backtracking like regular expressions are expected
 106   to. For example:</p>
 107 <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>
 108 <p>will always fail to match because Spirit's Kleene star does not back off when
 109   the rest of the rule fails to match. </p>
 110 <p>Actually, there's a solution to this greedy RD problem. Such a scheme is discussed
 111   in section 6.6.2 of <a
 112 href="http://www.cs.vu.nl/%7Edick/PTAPG.html">Parsing Techniques: A Practical
 113   Guide</a>. The trick involves passing a <em>tail</em> parser (in addition to
 114   the scanner) to each parser. The start parser will then simply be: <tt>start
 115   &gt;&gt; end_p;</tt> (end_p is the start's tail). </p>
 116 <p>Spirit is greedy --using straight forward, naive RD. It is certainly possible
 117   to implement the fully backtracking scheme presented above, but there will be
 118   also certainly be a performance hit. The scheme will always try to match all
 119   possible parser paths (full parser hierarchy traversal) until it reaches a point
 120   of certainty, that the whole thing matches or fails to match. </p>
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 122   <tr>
 123     <td class="note_box"><p><img src="theme/note.gif" width="16" height="16"><b>Backtracking
 124         and Greedy RD </b><br>
 125         <br>
 126         Spirit is quite consistent and intuitive about when it backtracks and
 127         to where, although it may not be obvious to those coming from different
 128         backgrounds. In general, any (sub)parser will, given the same input, always
 129         match the same portion of the input (or fail to match the input at all).
 130         This means that Spirit is inherently greedy. Spirit will only backtrack
 131         when a (sub)parser fails to match the input, and it will always backtrack
 132         to the next choice point upward (not backward) in the parser structure.
 133         In other words abb|ab will match &quot;ab&quot;, as will a(bb|b), but
 134         (ab|a)b won't because the (ab|a) subparser will always match the 'b' after
 135         the 'a' if it is available.</p>
 136       <p>--Rainer Deyke</p>
 137       </td>
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 139 </table>
 140 <p>There's a strong preference on &quot;simplicity with all the knobs when you
 141   need them&quot; approach, right now. On the other hand, the flexibility of Spirit
 142   makes it possible to have different optional schemes available. It might be
 143   possible to implement an exhaustive backtracking RD scheme as an optional feature
 144   in the future. </p>
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 155 <p class="copyright">Copyright &copy; 1998-2003 Joel de Guzman<br>
 156   <br>
 157   <font size="2">Use, modification and distribution is subject to the Boost Software
 158     License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at
 159     http://www.boost.org/LICENSE_1_0.txt)</font></p>
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