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Re: [lojban] rp-expression-tail ordered choice with empty grouping

On Tue, Nov 23, 2010 at 11:43:52AM -0700, .alyn.post. wrote:
> What does the following production in the lojban grammar mean?:
> 
> rp-expression-tail <- rp-expression operator rp-expression-tail /
> ()
> 
> Do I understand that to mean 'the sequence of rp-expression,
> operator, and rp-expression-tail OR nothing?'  Could the above
> also be expressed:
> 
> rp-expression-tail <- (rp-expression operator rp-expression-tail)?
> 
> And if so, why the odd way of specifying it?  I'm weak on pitfalls
> of recursive behavior in grammars, am I looking at a workaround
> related to that?

Yes, and it mostly doesn't work.  Lojban's RPN (which, if I have my
druthers, the next version of the language will not have, by the
way) is left-recursive, which is problematic.  See the peg mailing
list for various solutions, but IMO since a number of PEG parser
generators handle this on their own, I think we should just have it
be left-recursive and say that people have to use such a generator.

If you can figure out how to fix it in in the rules so the right
behaviour pops out, though, *please* do so.  There are links to
papers on the peg mailing list; it came up just last week; "*[1]*
*Packrat Parsers Can Support Left Recursion*, Alessandro Warth,
James R. Douglass, and Todd Millstein (2008)" is the citation I'm
seeing at a brief glance.

-Robin

-- 
http://singinst.org/ :  Our last, best hope for a fantastic future.
Lojban (http://www.lojban.org/): The language in which "this parrot
is dead" is "ti poi spitaki cu morsi", but "this sentence is false"
is "na nei".   My personal page: http://www.digitalkingdom.org/rlp/

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