On Sun, Nov 3, 2013 at 1:13 AM, Philippe Sigaud <philippe.sigaud@gmail.com> wrote:

On Sun, Nov 3, 2013 at 2:45 AM, Timothee Cour <thelastmammoth@gmail.com> wrote:

1)
The main issue I see with pegged is PEG grammars don't support left recursion, so for example will fail on foo[1].bar(2).fun().

Unless there's a plan to accomodate those, I sense a dead end.

One can eliminate left recursion but this has issues.

Indeed. Eliminating left-recursion can be done, but it might disrupt the parse tree.
There is no discarding the fact that PEG are intrinsically made to go with top-down recursive parsers.

2)
There is some material on extending PEG to support those, eg "Left Recursion in Parsing Expression Grammars", or code https://github.com/orlandohill/peg-left-recursion but I don't know how well they work in practice.

I'll have a look, thanks. The idea seems similar to what I wanted to do, with a growing seed.

Great, please let us know how that works.

3)
Finally, a parser should be as fast as possible; I'm not sure how well pegged performs compared to dmd frontend.

Oh, for D it works (it's even the biggest grammar I know), but it's too slow to be of practical interest. But that just means the generated parser is not top-notch, which is reasonable: I'm not a parsing pro, just a guy doing this during his free time :)

Other promising options are using lemon, a LALR(1) parser generator.

My current plan is to write different engines, and letting either the user select them at compile-time, or to have the parser decide which one to use, depending on the grammar. I'm pretty sure the 'Type 3' parts of a grammar (regular expressions) could be bone by using std.regex, for example.

even lexing can't be done with regex, eg nesting comments : /+ ... +/

Also, although it may seem cleaner at first to combine lexing and parsing in 1 big grammar (as done in pegged), it usually is faster do feed a (separate) lexer output into parser.

I guess I'll have to write a CT-compatible LALR(1) engine...

Growler:

I'm using it for parsing C and Markup in two different projects.
I've also just started working on an Octave (well ok, Matlab) M-file
parser. It will probably never see the light of day but with
Pegged is great fun to play with.

Please don't lose interest in it !!

OK guys, I'm hearing you. Thanks for the nice words growler! I tried to have Pegged simple to use, compared to other generator I know and I'm pleased to see it seems to work. If you have new grammars, you can send them to me, I'll put them in the examples.

Yes. Probably the most significant D project at this time.

Andrei

That's nothing compared to vide.d! But I guess it indeed demonstrates what can be done with the generative capabilities of D.

Thanks for the kind words.