Wonderful! I've added the GH issue:
https://github.com/jeffreykegler/Marpa--R2/issues/144
Regards,
Steven
Pada Kamis, 15 Mei 2014 21:49:37 UTC+7, Jeffrey Kegler menulis:
>
> On reflection, I think I will add ? quantification in the Libmarpa C
> code. If it's in the C code, it will be available universally. Also, at
> this point, the Perl layer has to support layers of legacy code and changes
> of the sort to the Perl level tend to paint me more into a corner.
>
> The bad news about this is that it may take me a while -- it has to come
> after several other changes. A good way to track this (and make sure I
> don't forget) is to create a Github issue.
>
> -- jeffrey
>
> On 05/14/2014 08:32 AM, Jeffrey Kegler wrote:
>
> The ? quantifier is harder than it may seem, because it pairs a nulling
> rule and a regular rule, and there are all sort of tricky aspects about
> this. This is already done in star rules (A ::= B*), but most of the
> tricky stuff is handled at a low-level -- in the Libmarpa C code. So, to
> do this, I either hack the inner part of Marpa, or else execute the same
> logic twice, once at the Perl level and once at the C level, with all the
> interaction issues that implies.
>
> It would be straight-forward to do this in a front-end/wrapper to the
> SLIF, and several people have aired the idea of doing this, but so far
> nobody has taken it on.
>
> -- jeffrey
>
> On 05/13/2014 07:50 AM, Steven Haryanto wrote:
>
> 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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