antlr - ANTLR3: Parameters and semantic predicates ("cannot find symbol", "illegal start of type") -

i realize "branch" in antlr3.

i figured using

branch[boolean is_a]     : ({ $is_a}? => a)     | ({!$is_a}? => b); 

would trick, compiling errors "cannot find symbol" , "illegal start of type", because in in generated source i.e. dfa45.specialstatetransition(...) not have parameter is_a.

i tried omitting =>¹, and/or omitting $ of $is_a.

the first sets of a , b not disjoint.

in fact b of type ((c) => c) | a.

¹) don't understand difference between {...}? => ... , {...}? ...

i'm not 100% sure why error: i'd need see entire grammar that. anyway, there no need check both is_a and !is_a. , both $is_a , is_a valid.

let's you're parsing list of numbers, , every 4th number, want handle through different "branch". grammar like:

grammar t;  parse @init{int n = 1;}   :  (number[n\%4 == 0] {n++;})+ eof   ;   number [boolean multipleof4]   :  {multipleof4}?=> int {system.out.println("branch -> " + $int.text);}   |                   int {system.out.println("branch b :: " + $int.text);}   ;  int   :  '0'..'9'+   ;  space   :  (' ' | '\t' | '\r' | '\n') {skip();}   ; 

(note % reserved character inside antlr grammars (not inside string literals , comments though), needs escaping backslash)

and can tested class:

import org.antlr.runtime.*;  public class main {     public static void main(string[] args) throws exception {         antlrstringstream in = new antlrstringstream("11 22 33 44 55 66 77 88 99");         tlexer lexer = new tlexer(in);         commontokenstream tokens = new commontokenstream(lexer);         tparser parser = new tparser(tokens);         parser.parse();     } } 

now generate parser/lexer (a), compile source files (b) , run main class (c):

 java -cp antlr-3.2.jar org.antlr.tool t.g // javac -cp antlr-3.2.jar *.java            // b java -cp .:antlr-3.2.jar main             // c 

(on windows, run doing java -cp .;antlr-3.2.jar main)

which produces following output:

 branch b :: 11 branch b :: 22 branch b :: 33 branch -> 44 branch b :: 55 branch b :: 66 branch b :: 77 branch -> 88 branch b :: 99 

so, yes, needed "gated semantic predicate" ({boolean}?=>) in case, not "validating semantic predicate" ({boolean}?). difference between 2 predicates explained in previous q&a: what 'semantic predicate' in antlr?