Re: Is it the job of a parser to validate the input data?

On Thursday, August 12, 2021 at 10:52:15 AM UTC-5, Christopher F Clark wrote:
> It is almost always done in the AST creation routines, not only do you
> as our insightful moderator mentioned generally get better error
> messages that way, but curiously, the features of extract a number,
> turn it into a count, and apply that count (and yes those might be 3
> distinct operations) to be how many items a list involves has not been
> implemented in any parser generator or lexer generator that I have
> ever seen. That's a bizarre omission, particularly since it is a
> common feature in many languages like networking protocols. Doing
> fixed counts isn't rare, but doing a count held in a "register" or
> "variable" seems to not be done.
>


I think the omission comes from difficulty in the formalization. Having to
apply such a dynamic count moves the parser from context-free to
context-sensitive, jumping to a new level in the Chomsky hierarchy.
So all the training wheels come off and it all gets scary.

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