Notably, a future may be defined without specifying which specific promise will set its value, and different possible promises may set the value of a given future, though this can be done only once for a given future.
值得注意的是，无须指定可以设置其值的promise就可以定义future，并且不同的promise可以设置同一个future的值，尽管对于给定的future仅可以执行一次。