If you have promises OR you have coroutines, your life is good. Either way, you don't have to choose between writing fragile sequential code and writing error-p

If you have promises OR you have coroutines, your life is good. Either way, you don't have to choose between writing fragile sequential code and writing error-prone synchronization logic. If you don't have either, then you'll probably end up re-implementing one or the other eventually anyway.