[antlr-interest] Re: Local lookahead depth

[Tom Moog]

I remember that John Lilley required the use of actions inside
guess blocks to parse C++.  I believe that parsing some C++
declarations which make use of nested classes might require
symbol table additions that must be undone if the guess fails.
Reasonable people may disagree about whether C++ is a real
programming language or not (laugh).  He created a special
version of pccts for this purpose.

In pccts, one can execute actions in guess mode by exploiting
the way in which code is generated.  Since init-actions are
executed even in guess mode, one can put semantic actions
there - even though they are only to supposed to have
declarations.

The important thing to remember is that init-actions may be
executed multiple times as back-tracking takes place.  Even if
the first guess is correct, the init-action will be executed
twice: once in guess mode and once in non-guess mode after
the guess succeeds.  In the case where there is a guess
within a guess, its quite possible that an init-action
would be executed 4 times, etc.



 

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