Re: coupling LALR with a scanner?

"Paul B Mann" a icrit dans le message de groupe de discussion :
>> IELR was exactly made for that reason, as a first step to PSLR: some
>IELR(1) does not solve this problem. It solves the problem of reduce-
>reduce conflicts in an LR(0) [...]


OK I did not sentenced that right, this is PSLR which does all the
lexer/parser integration (hence Pseudo Scannerless LR).


>What you described with Javascript is a little language inside of
>another language. Why not switch parsers when a slash ('/') is
>encountered if necessary?


The problem is that / is used as a normal token AND as the start for regular
expression.
For example these are two valid sequences:
var regexpr = / b + a /
                                        2;
var numeric_var = 1 / b + a /
                                        2;


The 2; on the following lines will have two different meanings, in case
'regexpr', it is an instruction setting the expression value (as used in
eval() ), in the case second case, 2 is the second operand to a / operator.


I insist a bit on this to show there is no 'right side' heuristic to
determine / b + a / is a regular expression. On the other hand the '1'
expression could be as complex as you can imagine.


Of course in the presence of something like expr / expr the parser knows the
/ is for a division, and if the / is found where expr is expected it knows
there is a regular expression coming.
I don't see how I could hack this into a scanner only. maybe some specific
preceding tokens. I have pass again some time on that question.


Armel

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