ANTLR Propositional Logic Evaluator

Question

I'm trying to create a grammar in ANTLR that evaluates propositional logic formulas. So for the input (1 & 0) | 1, it should return true.

I have constructed the following:

code returns[boolean value]
  : formula EOF {$value = $formula.value;}
  ; 

formula returns [boolean value]
  : equiv {$value = $equiv.value;}
  ; 

equiv returns [boolean value]
  : a=implies  {$value = $a.value;} 
  ( '#' b=implies {$value = $value == $b.value;}  
  )* 
  ;

implies returns [boolean value]
  : a=or   {$value = $a.value;} 
  ( '>' b=or {$value = !$value || $b.value;}
  )* 
  ;  

or returns [boolean value]
  : a=and {$value = $a.value;}
  ( '|' b=and {$value ||= $b.value;}
  )* 
  ;

and returns [boolean value]
  :  a=term {$value = $a.value;}
  ( '&' b=term {$value &&= $b.value;}
  )* 
  ; 

term returns [boolean value]
  : '(' formula ')' {$value = $formula.value;}
  | '0' {$value = false;}
  | '1' {$value = true;}
  | '¬' term {$value = !$term.value;}
  ;

WHITESPACE: (' '|'\t'|'\r'|'\f'|'\n')+{$channel = HIDDEN;} ; 

However I keep getting the error java.lang.NoSuchFieldError: offendingToken. Is there anyway to find out where the error is or how to fix it?

Answer 1

There are 3 issues:

• {$value ||= $b.value;} should be {$value = $value || $b.value;}
• {$value &&= $b.value;} should be {$value = $value && $b.value;}
• the label $term in the fourth alternative of the term rule is ambiguous: is could refer to the rule itself, or the term followed by '¬'

I.e., the following:

term returns [boolean value]
  : ...
  | '¬' term {$value = !$term.value;}
  ;

should be:

term returns [boolean value]
  : ...
  | '¬' t=term {$value = !$t.value;}
  ;

Having made those changes, the following test class:

import org.antlr.runtime.*;

public class Main {
  public static void main(String[] args) throws Exception {
    String source = "(1 & 0) | 1";
    TestLexer lexer = new TestLexer(new ANTLRStringStream(source));
    TestParser parser = new TestParser(new CommonTokenStream(lexer));
    System.out.println(source + " = " + parser.code());
  }
}

produces the desired output:

java -cp antlr-3.3.jar org.antlr.Tool Test.g
javac -cp antlr-3.3.jar *.java
java -cp .:antlr-3.3.jar Main

(1 & 0) | 1 = true