Recursive Generators in JavaScript

问题:

I am trying to write a recursive generator for an in order traversal.

class Tree {
  *inOrderTraversal() {
    function* helper(node) {
      if (node.left !== null) {
        // this line is executed, but helper is not being called
        helper(node.left); 
      }
      yield node.value;
      if (node.right !== null) {
        helper(node.right);
      }
    }

    for (let i of helper(this.root)) {
      yield i;
    }
  }
  // other methods omitted
}

And I am calling the generator like so:

const tree = new Tree();
tree.add(2);
tree.add(1);
tree.add(3);

for (let i of tree.inOrderTraversal()) {
    console.log(i); // only prints 2
}

Why is the generator only yielding 2? Why is it at least not yielding 1 before 2?

How can I fix this?

If it helps, I am transpiling the code using babel.

babel --optional runtime test.js | node

回答1:

The problem was not with the recursion. Your function did call itself recursively, it just didn't yield values outside. When you call helper(), you get an iterator as a return value, but you wanted the iterated values of that iterator to be yielded. If you want to yield recursively, you need to yield *. Try like this:

  * inOrderTraversal() {
    function* helper(node) {
      if (node.left !== null) {
        // this line is executed, but helper is not being called
        yield * helper(node.left); 
      }
      yield node.value;
      if (node.right !== null) {
        yield * helper(node.right);
      }
    }

    for (let i of helper(this.root)) {
      yield i;
    }
  }

And while you're at it, you can replace the for loop with:

yield * helper(this.root)

回答2:

helper(node.left); does call the function and create the generator, but the generator function body is never executed because the generator is never advanced. To forward all of its values to the current generator, you can use the yield* keyword, which works just like the

for (let i of helper(this.root))
    yield i;

you've used in your inOrderTraversal method. And indeed, that should've been a yield* just as well - or even better, there is no reason to make inOrderTraversal a generator function when it can just be a normal method that returns a generator:

class Tree {
  inOrderTraversal() {
    function* helper(node) {
      if (node.left !== null)
        yield* helper(node.left);
      yield node.value;
      if (node.right !== null)
        yield* helper(node.right);
    }

    return helper(this.root);
  }
  … // other methods
}