Implement Future.sequence for case class CanBuildFrom Traver

Question

>我有这个：

case class Sides[A](east: A, west: A)

我想将此Sides[Future[Boolean]]转换为此Future[Sides[Boolean]]

如何使用Future.sequence？

这是文档：

def
sequence[A, M[X] <: TraversableOnce[X]](in: M[Future[A]])(implicit cbf: CanBuildFrom[M[Future[A]], A, M[A]], executor: ExecutionContext): Future[M[A]]
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Simple version of Future.traverse. Transforms a TraversableOnce[Future[A]] into a Future[TraversableOnce[A]]. Useful for reducing many Futures into a single Future

Answer 1

编辑：由于这个问题有scalaz标签，我假设有问题的Future来自scalaz。在问题被编辑之后，很明显OP已经从Scala标准库中Future了。

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我正在扩展我的评论作为答案。您需要一个用于Sides的Traverse实例，而不是Future 。（Future不存在Traverse实例，除非你愿意急切地评估Future。

import scalaz.{Applicative, Traverse}
import scalaz.concurrent.Future
import scalaz.syntax.traverse._
case class Sides[A](east: A, west: A)
implicit def traverseSides: Traverse[Sides] = new Traverse[Sides] {
  def traverseImpl[G[_], A, B](s: Sides[A])(f: A => G[B])(implicit G: Applicative[G]): G[Sides[B]] =
    G.apply2(f(s.east), f(s.west))(Sides(_, _))
}
val sf: Sides[Future[Boolean]] = ???
vaf fs: Future[Sides[Boolean]] = sf.sequence