ANTLR syntactic pred

Question

I have the following ANTLR grammar which is given as an example by Terrence Parr.

grammar b;
backtrack
   :    (cast ';')=> cast ';'
   |   (e ';')=>    e ';'
   |                e '.'
   ;

cast:   '(' ID ')' ;

e   :   '(' e ')'
    |   ID
;

ID  :   'a'..'z'+ ;

However, when I try to interpret (a) with the backtrack rule, I get a MisMatchedToken exception. I ask this question because I have a much bigger grammar which I use for a compiler. I have the exact problem there. If I remove one of the rules everything works fine (except for the input parse by the rule removed, of course) but when I add the syntactic predicates I get an error when I parse, even though the grammar compiles(exactly like with the b grammar).

Any suggestions or ideas why this might be? Thank you.

Answer 1

The interpreter in ANTLRWorks is notoriously buggy, and doesn't handle any kind of predicate at all. So don't use it.

Note that you said you parsed "a()" , but that will not be parsed properly. The parser will complain it is missing a "." . I presume you meant you's parsing "a();"

If you run the following demo:

grammar b;

@parser::members {
  public static void main(String[] args) throws Exception {
    bLexer lexer = new bLexer(new ANTLRStringStream("(a);"));
    bParser parser = new bParser(new CommonTokenStream(lexer));
    parser.backtrack();
  }
}

backtrack
 : (cast ';')=> cast ';' {System.out.println("cast");}
 | (e ';')=>    e ';'    {System.out.println("e");}
 |              e '.'
 ;

cast
 : '(' ID ')' 
 ;

e
 : '(' e ')'
 | ID
 ;

ID : 'a'..'z'+ ;

by executing:

java -cp antlr-3.3.jar org.antlr.Tool b.g 
javac -cp antlr-3.3.jar *.java
java -cp .:antlr-3.3.jar bParser

(on Windows, the last command should look like: java -cp .;antlr-3.3.jar bParser )

you'll see that cast is being printed to the console without any kind of error or warning from ANTLR.

Answer 2

If you're getting this error in ANTLRWorks I would try loading the grammar in a sample program and see if you get the same error. ANTLRWorks can sometimes give errors on valid input.