Greetings Jeffrey and all,
I'm just getting started with Marpa::R2::Scanless. I have a lot of high
hopes related to using Marpa:
* (currently in progress) migrate Language::Expr from using
Regexp::Grammar. With RG there are lots of problems: limitations when
writing grammar (e.g. have to avoid left recursion), exponential parsing
time as the length of input string increases, the whole debacle of failure
to run under Perl 5.18, limited error message/diagnostics, reentrace
problem (can't use regex matching in action code), etc.
* migrate Org::Parser from parsing with regex, with the hope of speeding up
the parsing (and improve the readability of the parser code :-) ). Parsing
my 400KB (8000 lines) todo.org file takes about 0.8-1s on my Core i7-4770
PC (and probably a couple of seconds on my Core i5 laptop), I wish the time
could go down to at least 0.1-0.2s.
* write a markdown parser and markdown-to-POD converter. The current
Markdown::POD module uses Markdent which is Moose-based and has a heavy
startup cost, about 0.4s on a fast computer and 1+s on a rather slow one,
which is annoying for command-line scripts. It also has trouble parsing _
(emphasis), causing text like 'some_identifier and another_identifier' to
be converted to POD 'someI<identifier and another>identifier'.
* rewrite my Ledger::Parser to using Marpa and increase its compliance and
feature support.
* write more parsers and converters for other formats which I so far
haven't done because the tools I had at my hand are just Perl regex and
Regexp::Grammars.
For now I'm playing and exercising with some simple grammars. I was
wondering whether Marpa BNF can (or will) support the "zero-or-one"
quantifier ? like commonly found in regexp. This is convenient when stating
a list of things that are optional but need to be in order. For example,
consider the case of parsing ISO 8601 date duration (I apologize in advance
for using MarpaX::Simple, it's just a thin wrapper to keep things as simple
and as short as possible):
----------------------------
#!/usr/bin/env perl
# parses ISO 8601 duration literal
use 5.010;
use MarpaX::Simple qw(gen_parser);
my $parser = gen_parser(
grammar => <<'_',
:start ::= duration_literal
duration_literal ~ 'P' year_opt month_opt week_opt day_opt
| 'P' year_opt month_opt week_opt day_opt 'T' hour_opt minute_opt
second_opt
year_opt ~ posnum 'Y'
year_opt ~
month_opt ~ posnum 'M'
month_opt ~
week_opt ~ posnum 'W'
week_opt ~
day_opt ~ posnum 'D'
day_opt ~
hour_opt ~ posnum 'H'
hour_opt ~
minute_opt ~ posnum 'M'
minute_opt ~
second_opt ~ posnum 'S'
second_opt ~
posnum ~ digits
| digits '.' digits
digits ~ [0-9]+
_
);
$parser->('P');
$parser->('P1Y');
$parser->('P2M');
$parser->('P2MT2M');
---------------------
It would be nice if I could write (or can I?) something like this like in a
regexp:
---------------------
duration_literal ~ 'P' year? month? week? day?
| 'P' year? month? week? 'T' hour? minute? second?
year ~ posnum 'Y'
month ~ posnum 'M'
# and so on
---------------------
Expect more (stupid) questions from me :-)
Regards,
Steven
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