[antlr-interest] SLL(2) in "The Definitive ANTLR Reference"

Sam Harwell sharwell at pixelminegames.com
Wed Feb 2 11:21:54 PST 2011


The problem is the decision inside rule r. Since ANTLR uses lookahead only
(as opposed to lookbehind), the decision making doesn't know which instance
of r is being parsed. The following would resolve the issue:

s : X r1 A B
   | Y r2 B
   ;

r1 : A | ;
r2 : A | ;

-----Original Message-----
From: antlr-interest-bounces at antlr.org
[mailto:antlr-interest-bounces at antlr.org] On Behalf Of Alan D. Cabrera
Sent: Wednesday, February 02, 2011 11:08 AM
To: antlr-interest at antlr.org
Subject: [antlr-interest] SLL(2) in "The Definitive ANTLR Reference"

I was reading about the following grammar on page 287 of the PDF document

grammar t;
s : X r A B
  | Y r B
  ;

r : A
  |
  ;

I don't see where the problem is since the alternatives in s begin with two
different tokens X and Y.  I think that since these two tokens are different
I can easily construct a DFA that would unambiguously parse a stream of
tokens.

I sense that this example was supposed to bring out a finer point about how
ANTLR generates parsers but I'm afraid that it is being lost on me.


Regards,
Alan


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