Re: Grammar for optional elements

torbenm@app-6.diku.dk (Torben Fgidius Mogensen) writes:


> If the order is fixed, i.e., if attribute1 must be before attribute2
> (if both appear) and so on, it takes only a grammar of size N:


Yes, but even in this case, I would advise not encoding the order
requirement, nor the no-duplicates requirement, in the grammar.
It will be easier to give good error messages if these rules
are checked in a later semantic analysis phase. And I think
it makes things simpler overall.


The user would prefer things like "duplicate attr1 (see line 123)",
or "attr1 must appear before attr3", rather than "ATTR2 or ATTR3
expected here", or "end of file expected".


In general, I think it's not wise to encode too much in the (context
free) grammar, even when it is feasible to do so.


- Bob

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