Re: LR(1) Parsing : Error Handling & Recovery

Stefan Monnier schrieb:
>> This sort of comment has appeared frequently in the past and to some
>> extent I find it puzzing. A recursive descent translation of an LL(k)
>> (with k small) grammar is straightforward, and should readilly match the
>> grammar, and give you the parse you desire. Is the problem that error
>
> This is misleading. Take your BNF and pass it to a PEG parser: a PEG
> parser can accept pretty much the same input syntax as a BNF parser, and
> can be considered as a kind of RD parser, so you'd think the result is
> necessarily correct.


Sorry, that's ambiguous :-]


You miss to define what is "passing a grammar to an parser". Should the
PEG parser
- parse the grammar
- create an parser from the grammar
- interpret the grammar in parsing further input


In general the result cannot be more correct than is your grammar.


If your grammar is unambiguous, the results should be the same
(equivalent at least) in either case. But if your grammar is ambiguous,
what "result" do you consider possibly incorrect?


> Well, not so, because the semantics of the language is not the same:


That's the case with every grammar: who knows whether it really reflects
the semantics of the language? A parser generator only can verify that a
grammar is *valid*, but not that it matches a specific language.


> The PEG/RD parser will "silently resolve ambiguities".


IMO a correct PEG parser generator should warn about ambiguities in the
grammar, just as every other parser generator does. In such cases the
disambiguation is *not* performed by the parser generator, but by the
grammar writer, who arranges the alternatives in the correct (intended)
order.


I only can hope that you know how to use your programming tools, and
don't belong to those coders who say: "it compiles, so let's ship it" ;-)


DoDi

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