[lojban] Re: Is Lojban a CFG? (was Re: [lojban-beginners] Re: Enumerating in Lojban)

[Robin Lee Powell <lojban-out@lojban.org>]

On Thu, Jul 13, 2006 at 11:09:49AM -0300, Jorge Llamb?as wrote:
> If I'm not mistaken, for any CFG, there is a PEG that will accept
> all and only the strings generated by that CFG. 

I don't think anyone's really sure about that.

> If that's correct, then what Jonathan wants (a CFG for Lojban) is
> compatible with what Robin wants (a PEG for Lojban). 

Not really.  What I want is something that parses Lojban as it's
currently defined.  I care not at all what formalism does so, as
long as there is a pure formalism for it.

What Jonathan wants is a CFG, which cannot parse Lojban as it is
currently defined.  In particular, there is probably not any pure
CFG that will reject "le nu le broda brode brodi".

> At least in principle, because maybe the required PEG might be way
> too complicated to write. Or maybe not, we can at least try.

Erm.  camxes is a PEG that parses Lojban as currently defined.  You
helped build it...  Perhaps you meant CFG there?

> I think this is, simplifying, the kind of thing we would need the
> PEG to do in order to handle the {li pa pa pa pa moi} example:

The PEG already handles it just fine: it barfs, as it should.  That
string isn't valid Lojban.

> Current PEG rules:
> 
> sentence <- sumti* selbri sumti*
> selbri <- number MOI
> sumti <- LI number LOhO?
> number <- PA+
> 
> Possible modification to make it context-free:
> 
> sentence <- sumti* selbri sumti*
> selbri <- moi-number MOI
> sumti <- LI li-number LOhO?
> li-number <- PA (PA !MOI)*
> moi-number <- PA+

I don't know what language that encodes, but it's not Lojban as I
currently understand Lojban to be defined.  My reading of it is that
it makes {li pa pa pa pa moi} equivalent to {li pa pa pa boi pa
moi}.

> But I don't know how hard it would be to modify the whole PEG so
> that it fully corresponds to a CFG.

I don't see the point of doing so; if we're going to make a CFG, we
should use CFGs to do so, no make a CFG-alike PEG.  I just don't see
the point of butchering Lojban to make a CFG that encodes a language
that is almost, but not quite, Lojban.

-Robin

-- 
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Reason #237 To Learn Lojban: "Homonyms: Their Grate!"
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