in reply to Macros in a RecDescent grammar

You can edit $text in a rule to achieve a "pass-and-a-half" parsing. When you recognize a macrodef, store the key into a hash, with the value being the replacement string. Then, whenever you are looking for an identifier, also look first to see if it's one of the keys, and if so, prepend $text with the corresponding value and accept it.

I've been thinking about writing a column on that, but haven't finished. I saw a good example of that in theDamian's class notes though.

-- Randal L. Schwartz, Perl hacker

update: The file demo_cpp.pl in the Parse::RecDescent distro has a good example of mangling $text.

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Re: •Re: Macros in a RecDescent grammar
by mush4brains (Acolyte) on Jun 21, 2002 at 21:51 UTC
    I'm currently doing something like that, though I'm not actually updating $text: 
    macro:  identifier ":" expr
{
   if (defined $_macro{$item[2]->getVal()}) {
      # ERROR -- duplicate macro definitions
   }
   else {
      $_macro{$item[2]->getVal()} = $item[4];
      $return = $item[2];
   }
}

    [download]
    Rather than update $text, I build my op-tree branch directly from expr. (And it's working so far...)
    Adding arg's to this seems harder, since the instantiation-points can be many subrules deeper.
    - Jim
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[d/l]
Re^2: Macros in a RecDescent grammar
by Aristotle (Chancellor) on Jun 22, 2002 at 16:12 UTC
    Macros will have to be defined prior to use then though, unless I'm a mile off on how this'd work. (Not that that's bad, but it would differ from a two pass compiler in this respect and might be a point to consider.)

    Makeshifts last the longest.

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Re: •Re: Macros in a RecDescent grammar
by mush4brains (Acolyte) on Jun 21, 2002 at 22:18 UTC
    (Merlyn, I look forward to seeing your column on this, by the way. - Jim)
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