"Top-down parsing is guessing": new blog post.

[Jeffrey Kegler]

My newest post is second in a series about top-down parsing, and Perl 6 grammars.

This first post in the series was "Grammar reuse".   I forgot to announce the first one to this group when it came out.  Sorry!

[davidnm...@gmail.com]

Hi Jeffrey,

I was intrigued by these two posts, but left wondering if Marpa does support grammar reuse / extension?

I'd love to be able to declare a grammar as a class containing a series of tokens and be able to inherit and override individual tokens ala Perl 6.

If it doesn't support it now, could Marpa support it? I've been working on a Perl 6 Grammar for Pod (https://github.com/dnmfarrell/Pod-Perl5), and would love to try parsing Pod with Marpa. But I want to be able to extend the base Pod grammar with new syntax without having to copy and paste the original grammar into a new class.

Many thanks

David

[Jeffrey Kegler]

"Support" is being used in two different senses here.
1.) "have the horsepower in the grammar engine to allow grammar reuse".
2.) "have a syntax that makes it convenient to specify, without respect to whether it works".

I will plead guilty to having given 2.) the lower priority.  And, yes, Marpa would support 2.) better than any alternative out there.

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[Ron Savage]

There are various modules which patch the BNF on the fly, as a way of combining parts of a grammar. See:

https://metacpan.org/pod/Text::Balanced::Marpa

https://metacpan.org/pod/Text::Delimited::Marpa

Also, in this list, http://savage.net.au/Marpa.html#Perl_Packages, J Durand has a number of modules, some of which likewise patch the BNF.

[Jeffrey Kegler]

Re your specific problem -- POD for Perl 6.

POD is a hackish grammar, designed to be parsed by RE's and/or top-down methods.  It's something Perl 6 grammars should be able to do reasonably well.

There is an effort to port Marpa to Perl 6 underway, but it has only ported the lower layers, and what you want requires the upper ones.  I hear hints the Perl 6 team may look harder at Marpa once the current release stabilizes.  Previously I think they may have expected top-down methods could support things like language-oriented programming and nothave seen the need for something like Marpa.

So, in your place, if your intent is creating a Perl 6-based POD parser, you might as well take a first crack at it using Perl 6 grammars.  They should get the basic job done, and a Marpa infrastructure inside Perl 6 is just not ready.

[David Farrell]

Thanks Jeffrey and Ron for the replies, very informative.

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[Durand Jean-Damien]

Also, in this list, http://savage.net.au/Marpa.html#Perl_Packages, J Durand has a number of modules, some of which likewise patch the BNF.

All are "hacks" in the sense that there is no meta-package describing the BNF itself. Once I had a look to Grammar::Formal though, that looked promising in such direction 

[Ron Savage]

Well, yes. All treat the BNF as a string, not as an object with attributes and methods.

I had never heard of Grammar::Formal before you mentioned it. The MetaCPAN reverse dependencies list is empty, meaning - apparently - there are no modules using it.