Ocean of Awareness
Thu, 05 Jan 2012
What! No Lexer?
To those who have noted that
does not come with a lexer,
I'd respond that,
in a very real sense it does --
Perl5 is a powerful lexical analyzer.
If you're trying to figure out how to write your first Marpa
I'd recommend a close look at
Wolfgang Kinkeldei's recent posting about his
Marpa-powered CSS parser.
Wolfgang lays his parser out in a very elegant fashion,
and I find his code makes an excellent template.
Especially nice-looking is Wolfgang's lexer.
Wolfgang follows one of the two
main strategies for lexical analysis in
he consumes the input using substitution
s/ ... / ... /) commands.
The other strategy is to use the Perl regex search position
to track the progress of the lexical analysis.
In the search-position strategy,
your cases consist of a lot of
match commands using the
m/\G ... /gc.
An excellent tutorial on this
kind of lexing,
albeit in a non-Marpa context,
can be found in
Mark Jason Dominus's book,
Higher Order Perl.
Mark's coverage of lexing is in
Chapter 8, "Parsing", on pages 359-375.
Mark's book can be read
I highly recommend Mark's book
and own a paper copy.
Actually, regular expressions are well within Marpa's capabilites,
and lexical analysis could be done in Marpa.
But a look at Mark and Wolfgang's code should convince you that
lexical analysis is easy to do in Perl.
posted at: 18:31 |
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