import "dart:async";

typedef O FutureFunction<I, O>(I value);

class FutureQueue {
  Future _future;

  FutureQueue() : _future = new Future.value();

  void add(FutureFunction function) {
    _future = _future.then(function);
  }

  Future get done => _future;
}

void main() {
  var queue = new FutureQueue();

  queue.add((_) {
    print("Hello");
  });

  queue.add((_) {
    print("World");
  });

  queue.done.then((_) {
    print("Hello World");
  });
}