iter.hs

iter [] step out = out
iter (x:xs) step out = do
  val <- x
  step val (iter xs out)

iter.js

function iter(list, step, out) {
  return loop(0);

  function loop(i) {
    if (i >= list.length) return out();

    return list[i].then(function(val) {
      return step(val, function() { return loop(i+1); })
    });
  }
}