Re: Backtracking yacc

Gary Merrill <sasghm@unx.sas.com> writes:
>Such tests can, of course, be performed in the lexical analyzer and you
>can use the results of these tests to drive the invocations of your
>parser.


With all due respect, this is not correct. Throwing the problem to the
lexical analyzer presupposes that the conlfict involves a lexeme; this, of
course, is not the general case.


Attaching conditional tests to productions instead of lexemes allows a
greater degree of freedom with respect to the definition of the language.
In particular, with tests on productions, you can introduce a
differentiation at any point in the language. For example, a production
which contains only nonterminals on the right hand side might be
conditioned on synthesized attributes of the right-hand-side elements,
inherited attributes, or arbitrary global variables or computations on any
of the above.


Note that this has nothing to do with LR vs. LL; any parser scheme can add
this capability.


Another source of disambiguation that has not been discussed is the use of
regular expression grammars. Because a regular expression collects more
syntax into a single production, it can, by virtue of the LR parsing
method, defer certain choices until a larger part of the input has been
processed. The result is a more natural, and less ambiguous grammar.


The LADE C++/AT utilizes all of these methods to parse C++.
--

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