Question

In ANTLR I want to define a rule like this:

rule : ( a b c | a c b | b a c | b c a | c a b | c b a );

But in my case I have 10 rules instead of three, that I want to permute so it gets very impractical.
Is there any way of expressing this in ANTLR without having to write all the permutations?

Answer 1

I would just match any a, b or c once or more:

rule
 : ( a | b | c )+
 ;

and then, after parsing, traversing the parse tree and checking if a, b and c all matched exactly once.

But Yes, it is possible in the grammar itself by using predicates where needed.

A demo:

grammar Permutation;

parse
  :  permutation[5] {System.out.println("parsed: " + $permutation.text);} EOF
  ;

permutation[final int n]
@init{
  java.util.Set set = new java.util.HashSet();
  int counter = n;
}
  :  (
       {counter > 0}?=> token   // keep matching a `token` as long as `counter > 0`
       {                        //
         set.add($token.text);  // add the contents of `token` to `set`
         counter--;             // decrease `counter`
       }                        //
     )+ 
     {set.size() == n}?         // if `set.size() != n`, an exception is thrown
  ;

token
  :  A
  |  B
  |  C
  |  D
  |  E
  ;

A : 'A';
B : 'B';
C : 'C';
D : 'D';
E : 'E';

Space : ' ' {skip();};

The demo grammar above uses 2 different types of predicates: 1) a gated semantic predicate ⁱ to make sure that the permutation rule matches no more than the parameter final int n tokens, and 2) a validating semantic predicate ⁱ to ensure that the set holds exactly the final int n elements to ensure that it’s a proper permutation of the 5 tokens.

More info about semantic predicates can be found here: What is a 'semantic predicate' in ANTLR?

You can test the grammar with the following class:

import org.antlr.runtime.*;

public class Main {
  public static void main(String[] args) throws Exception {
    PermutationLexer lexer = new PermutationLexer(new ANTLRStringStream(args[0]));
    PermutationParser parser = new PermutationParser(new CommonTokenStream(lexer));
    parser.parse();
  }
}

java -cp antlr-3.3.jar org.antlr.Tool Permutation.g 
javac -cp antlr-3.3.jar *.java

java -cp .:antlr-3.3.jar Main "A B C D E"
parsed: ABCDE

java -cp .:antlr-3.3.jar Main "B D C E A"
parsed: BDCEA

java -cp .:antlr-3.3.jar Main "A B C D B"
line 1:9 rule permutation failed predicate: {set.size() == n}?
parsed: null