antlr rule boolean parameter showing up in syntactic predicate code one level higher, causing compilation errors

Question

I have a grammar that can parse expressions like 1+2-4 or 1+x-y, creating an appropriate structure on the fly which later, given a Map<String, Integer> with appropriate content, can be evaluated numerically (after parsing is complete, i.e. for x or y only known later).

Inside the grammar, there are also places where an expression that can be evaluated on the spot, i.e. does not contain variables, should occur. I figured I could parse these with the same logic, adding a boolean parameter variablesAllowed to the rule, like so:

grammar MiniExprParser;
INT : ('0'..'9')+;
ID : ('a'..'z'| 'A'..'Z')('a'..'z'| 'A'..'Z'| '0'..'9')*;
PLUS  : '+';
MINUS : '-';

numexpr returns [Double val]:
  expr[false] {$val = /* some evaluation code */ 0.;};

varexpr /*...*/:
  expr[true] {/*...*/};

expr[boolean varsAllowed] /*...*/:  
  e=atomNode[varsAllowed]  {/*...*/}
        (PLUS e2=atomNode[varsAllowed] {/*...*/}
        |MINUS e2=atomNode[varsAllowed] {/*...*/}
        )* ;

atomNode[boolean varsAllowed] /*...*/:
   (n=INT {/*...*/})
  |{varsAllowed}?=> ID {/*...*/}  
  ;

result: 
  (numexpr) => numexpr {System.out.println("Numeric result: " + $numexpr.val);}
 |varexpr {System.out.println("Variable expression: " + $varexpr.text);};

However, the generated Java code does not compile. In the part apparently responsible for the final rule's syntactic predicate, varsAllowed occurs even although the variable is never defined at this level.

        /* ... */
        else if ( (LA3_0==ID) && ((varsAllowed))) {
            int LA3_2 = input.LA(2);

            if ( ((synpred1_MiniExprParser()&&(varsAllowed))) ) {
                alt3=1;
            }
            else if ( ((varsAllowed)) ) {
                alt3=2;
            }
        /* ... */

Am I using it wrong? (I am using Eclipse' AntlrIDE 2.1.2 with Antlr 3.5.2.)

I'd recommend against writing actions. If you want to do optimizations, do an extra pass in the parse tree walker, maybe generating intermediate code. — Mephy

Mike Lischke Mike Lischke · Accepted Answer · 2014-10-23T07:35:48

This problem is part of the hoisting process the parser uses for prediction. I encountered the same problem and ended up with a member var (or static var for the C target) instead of a parameter.

antlr rule boolean parameter showing up in syntactic predicate code one level higher, causing compilation errors

1 Answers