It was omitted because it's a complex design issue. Someone using the future would assume that if they added a callback, it would run on the same thread that th

It was omitted because it's a complex design issue. Someone using the future would assume that if they added a callback, it would run on the same thread that they called it from. That's not possible in the general case: the thread they called it on may not have an event loop and so could not be directly notified. It's better to leave this to frameworks or libraries that have their own event loop mechanism to handle callbacks.