Hi, Apparently, Sérgio Medeiros spent some time on semantics for left-recursive PEGs. The link he refers to is dead, though. http://lua-users.org/lists/lua-l/2010-03/msg00021.html
A few others have pointed out flaws in Warth's algorithm. http://tratt.net/laurie/research/publications/papers/tratt__direct_left_recursive_parsing_expression_grammars.pdf Maybe, it's no better but, I still have a feeling the algorithm I hand-wavingly suggested, three years ago, actually works. It really seemed like it handled indirect left-recursion, allowed left-associative trees and maintained a linear runtime. In hindsight, I could have got help in trying to formalize and publish it. Anyway, I'm interested to hear what progress you've made. Orlando. On Fri, Nov 12, 2010 at 6:33 PM, Repain Alex <alex.rep...@gmail.com> wrote: > Hi PEG list, > > As an intern, I worked this summer in a japanese laboratory, on > left-recursion support for packrat parsers. One of the questions we had a > hard time dealing with was the following : > > Since the original definition of PEG grammars doesn't take in account the > grammars with left-recursive rules (directly or indirectly), what is the > behaviour we expect from a PEG parser, when dealing with those grammars ? > > The first and intuitive solution is to avoid these rules, but a lot of work > has been produced on the subject, including Wrath et al.'s paper [1] (which > might be the most acknowledged), and today it seems we want to be able to > handle left-recursive grammars. But this can reveal itself quite tricky. > Take for instance the following grammar : > > S <- A | B > A <- S a | a > B <- S b | b > > which is an indirectly left-recursive grammar. In a CFG context, this > grammar would represent the langage (a|b)+ . What about this in the PEG > context ? > > My guess is that, despites the fact that PEGs impose the concept of ordered > choice, the expected behaviour of this grammar is to recognize the very same > (a|b)+ langage, through a PEG parser - say for instance a packrat parser. > Still, I would like to be sure of it, and if anybody has a CLEAR idea of > what SHOULD happen with a correct support for left-recursion, I'm eager to > hear about it. Is there only a real convention for a correct behaviour ? > > My actual situation is the following : during my internship this summer, I > started to doubt about the capacity for Wrath et al.'s algorithm to handle > every type of left-recursive grammars. For instance, the above grammar, when > passed to a Packrat parser with Wrath et al.'s enhancement, doesn't > recognize (a|b)+, but only a subset of this langage. That is not the > behaviour I expected, and thus I started working on a new algorithm able to > take into account complex left-recursion cases. Yet, if my vision of how a > parser behaves "correctly" is altered in some way, my work here could be > just good for the trash bin. > > Thanks for your help. > Alex > > P.S. : if someone is interested in my work, or in examples of strange > behaviour with Wrath et al.'s, I can provide them, one-to-one (to avoid > attached files on the mailing lists). > > [1] Packrat Parsers Can Support Left Recursion, Alessandro Warth, James R. > Douglass, and Todd Millstein (2008) > > > _______________________________________________ > PEG mailing list > PEG@lists.csail.mit.edu > https://lists.csail.mit.edu/mailman/listinfo/peg > > _______________________________________________ PEG mailing list PEG@lists.csail.mit.edu https://lists.csail.mit.edu/mailman/listinfo/peg
