[antlr-interest] resolving ambiguity

Stephen Siegel siegel at udel.edu
Mon May 21 08:50:20 PDT 2012

In the following grammar:

grammar g1;
options { output=AST; }
tokens { B_CHAIN; }
r	:	(A | b_chain)*;
b_chain	:	B+ -> ^(B_CHAIN B+);
A 	:	'A';
B	:	'B';

// BB should yield ^(BCHAIN B B)

there is an ambiguity since input "BB" could be parsed as (1) two "b_chain"s (each with one "B") or as (2) one b_chain (with two "B"s).  I want to tell ANTLR to choose option (2).

Right now, I get this warning:

[11:45:02] Checking Grammar g1.g...
[11:45:02] warning(200): g1.g:5:11: 
Decision can match input such as "B" using multiple alternatives: 1, 2

As a result, alternative(s) 2 were disabled for that input

Does anyone know how to resolve the ambiguity?


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