真正有用的事情(一般来说)是能够将未来的错误转化为正确的价值.或者换句话说，将一个 Future[T] 转换成一个 Future[Try[T]](成功的返回值变成了一个 Success[T]code> 而失败案例变成了 Failure[T]).下面是我们如何实现它:

A genuinely useful thing (generally speaking) would be to be able to promote the error of a future into a proper value. Or in other words, transform a Future[T] into a Future[Try[T]] (the succesful return value becomes a Success[T] while the failure case becomes a Failure[T]). Here is how we might implement it:

// Can also be done more concisely (but less efficiently) as:
// f.map(Success(_)).recover{ case t: Throwable => Failure( t ) }
// NOTE: you might also want to move this into an enrichment class
def mapValue[T]( f: Future[T] ): Future[Try[T]] = {
  val prom = Promise[Try[T]]()
  f onComplete prom.success
  prom.future
}

现在，如果您执行以下操作:

Now, if you do the following:

Future.traverse(seq)( f andThen mapValue )

您将获得一个成功的 Future[Seq[Try[A]]]，其最终值包含每个成功未来的 Success 实例，以及一个 Failure 每个失败的未来实例.如果需要，您可以在此序列上使用 collect 删除 Failure 实例并仅保留成功的值.

You'll obtain a succesful Future[Seq[Try[A]]], whose eventual value contains a Success instance for each successful future, and a Failure instance for each failed future. If needed, you can then use collect on this seq to drop the Failure instances and keep only the sucessful values.

换句话说，您可以按如下方式重写您的辅助方法:

In other words, you can rewrite your helper method as follows:

def traverseFilteringErrors[A, B](seq: Seq[A])(f: A => Future[B]): Future[Seq[B]] = {
  Future.traverse( seq )( f andThen mapValue ) map ( _ collect{ case Success( x ) => x } )
}