for-of-with proposal

0-generator.js

// Given the following generator that will walk a tree structure depth-first
// for each subtree it will go deeper unless the consumer returns `false`

function* depthFirst(tree, prefix=''){
  for(const [key, value] of Object.entries(tree)){
    const path = `${prefix}/${key}`;
    if(typeof value === 'string'){
      yield {path, value}
    }else{
      if(false !== (yield {path})){
        yield* depthFirst(value, path);
      }
    }
  }
}

1-proposed-syntax.js

// Run the depth-first walker on this tree
const tree = {
  'a': 'A',
  'b': {
    'c': 'C',
    'd': {
      'e': 'E'
    }
  },
  'f': 'F'
}


// notice the for-of-with statement below. 
// The expression after `with` is evaluated each round, and the 
// resulting value is passed to the generator in the `next()` call
let deeper = true;
for(const {path, value} of depthFirst(tree) with deeper){
  console.log(path)
  deeper = path !== '/b/d'
}

2-today.js

// Run the depth-first walker on this tree
const tree = {
  'a': 'A',
  'b': {
    'c': 'C',
    'd': {
      'e': 'E'
    }
  },
  'f': 'F'
}


// Wrap the generator in a call to `withFeedback()`, where the second parameter is a 
// function that returns the vaule to pass to the generator in the `next()` call
let deeper = true;
for(const {path, value} of withFeedback(depthFirst(tree), () => deeper)){
  console.log(path)
  deeper = path !== '/b/d'
}

function *withFeedback(iterator, feedback){
  try{
    while(true){
      const next = iterator.next(feedback());
      if(next.done){
        if(typeof iterator.return === 'Function') iterator.return();
        return;
      }
      yield next.value;
    }
  }finally{
    if(typeof iterator.return === 'Function') iterator.return();
  }
}

3-expected-output

/a
/b
/b/c
/b/d
/f

// notice that it skips `/b/d/e`