[antlr-interest] Customizing errors in antlr3

[Benoit Miller]

What is the best way to customize reported errors in antlr3?

For example, in the following snippet:

start
     :   forwardDecl* objectDecl*
     ;

forwardDecl
     :   classOrStruct IDENTIFIER ';'
         -> ^(classOrStruct IDENTIFIER)
     ;

objectDecl
     :   attrList? classOrStruct IDENTIFIER '{' property* '}' ';'
         -> ^(classOrStruct IDENTIFIER attrList? property*)
     ;

[unedited snippet, so this time it should be representative :)]

Given an input such as "[foo] class Bar;", I want to report to the tool 
user something like "cannot specify attributes on forward declarations", 
but it's unclear to me how to do this.

I tried using a syntactic predicate to throw my error from the parser:

objectDecl
     :   attrList? classOrStruct IDENTIFIER
         ( {input.LA(1)==';'}? { throw new Exception("foo"); }
         | '{' property* '}' ';'
           -> ^(classOrStruct IDENTIFIER attrList? property*)
         )
     ;

..but that didn't work, the parser still throws a NoViableAlt exception 
instead of my own:

[objectDecl]: line 1:20  state 0 (decision=5) no viable alt; 
token=[@7,20:20=';',<30>,1:20]

Any suggestions?

Thanks,
Ben.