使用scala.concurrent.Future与EitherT时缺少Functor和Monad实例

我正在尝试使用Scalaz EitherT和scala.concurrent.Future.当试图在for-comprehension中使用它时:

import scalaz._
import Scalaz._

val et1:EitherT[Future, String, Int] = EitherT(Future.successful(1.right))

val et2:EitherT[Future, String, String] = EitherT(Future.successful("done".right))

val r:EitherT[Future, String, String] = for {
    a <- et1
    b <- et2
} yield (s"$a $b")

我得到以下缺少的Functor和Monad实例错误:

could not find implicit value for parameter F: scalaz.Functor[scala.concurrent.Future]
b <- et2
  ^
could not find implicit value for parameter F: scalaz.Monad[scala.concurrent.Future]
a <- et1

scalaz是否定义了Functor和Monad for Future的实例？如果没有提供这些实例的其他库或我是否需要编写它们？

你需要一个隐含ExecutionContext的范围.import ExecutionContext.Implicits.global将为您提供全局执行上下文.

完整示例:

  import scala.concurrent.ExecutionContext.Implicits.global

  import scalaz._
  import Scalaz._

  val et1:EitherT[Future, String, Int] = EitherT(Future.successful(1.right))

  val et2:EitherT[Future, String, String] = EitherT(Future.successful("done".right))

  val r:EitherT[Future, String, String] = for {
    a <- et1
    b <- et2
  } yield s"$a $b"

  val foo = Await.result(r.run, 1 seconds)
  // => \/-("1 done")