Rafael Garcia-Suarez writes:
: Larry Wall wrote in perl.perl6.language :
: >
: > Such a grammar switching routine could operate either over a lexical
: > scope or over the rest of the file. The only restriction is that
: > one module not clobber the grammar of a different module.
: >
: > Basically, we're trying to make the opposite mistake of the one
: > we made with source filters. :-)
:
: I see that. But should it be possible to import grammar rules,
: to allow :
:
: use Some::Module::That::Modifies::A::Grammar::Rule;
: # continue to parse the perl program with the modified grammar
:
: or even :
: {
: use Some::Module::That::Modifies::A::Grammar::Rule;
: # continue to parse the block with the modified grammar
: }
Well, it's kind of klunky to do an import every time. More likely
you'd import a special subroutine once that you can use like this:
Java {
whatsit.blorf.unnecessary.bletch.extra.something.whatever(1);
}
Perl 5 is restricted to doing compile-time actions using BEGIN or use.
In Perl 6 there will be some way of marking a normal subroutine as as
grammatically active, so that it's called immediately, even before its
arguments are parsed. Our hypothetical Java subroutine above could
switch to a Java grammar, parse its block, and then restore the Perl
grammar at the end.
: And what about switching to a different or modified tokenizer ?
It's not clear that the lexer is a separate entity any more. Lexers
were originally invented as a way of abstracting out part of the
grammar so that it could be done in a separate pass, and to simplify
the grammar for the poor overworked parser. But you can write a
grammar for an identifier just about as easily as for an if-then-else.
More easily, if we're basing it on regexes. If we're viewing all
grammar through the lens of regexes, we're really starting out closer
to Lexerland anyway, and generalizing toward parsing, a traditionally
weaker area for Perl. And that's an odd weakness for a text
processing language.
Larry
