[lojban] Re: parsing with error detection and recovery

[John Leuner <lojban-out@lojban.org>]

> Rules failing would never be an error. There would be a group of rules
> that are flagged in the grammar (interpreted by the user of the parse
> tree, not actually affecting the parser) as being error conditions
> when they *suceed*. Actively matching different ways to mess up the
> input. This would require a lot of rules.

So if these error rules don't succeed, they will never consume any
input?

> Well, there are a couple ways in which PEG parsers themselves can
> produce error messages, but the parsers are extremely simple, so
> there's really not much room there. I think most of the work has to be
> done to the grammar itself. On the other hand, I *am* planning on
> doing a lot of experimenting with simple PEG grammars to prove the
> general concept, and to teach myself how PEG grammars are written,
> before I do anything with Lojban.

Do you think simple rules will be enough to control the error
detection/recovery process? Maybe it will be necessary to write some
logic in a programming language too.

> > My parser just tells you that the parse failed and the
> > point at which it failed.
> 
> What are your criteria for this? Just that the input stream wasn't
> fully eaten? I think that ideally, you'd be able to be confident
> enough in your grammar that you could pick a starting rule that isn't
> supposed to parse to the end of the stream, and use it to repeatedly
> apply to an input stream. And that any errors would result in error
> rules in the grammar rather than an incompletely parsed input. This
> could be a great efficiency optimization (for memory usage) for
> suitable input, becuase the memoization cache can be cleared after
> each substructure is parsed.

If the top-level rule ("text") parses successfully, the parser will
return the start-index and end-index of the matched text. There is also
a convenience method to check whether all the input was matched or not.

I don't see how you could just repeatedly apply a rule, you would have
to skip over stuff that was causing it to fail. Finding out what "is
causing it to fail" is very difficult.

> For Lojban, this basically amounts to calling the parser with the
> 'bridi' rule instead of the 'text' rule.

I'm not sure what the implications are of using a rule like "sentence"
instead of "text".

John




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