當使用scala.concurrent.Future與EitherT時缺少Functor和Monad實例

Question

我正在嘗試使用Scalaz EitherT與scala.concurrent.Future。當試圖用它換理解：

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") 

我得到以下缺失的函子和單子實例錯誤：

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 

是否scalaz定義函子和單子實例的未來？如果沒有，還有其他庫提供這些實例還是我需要編寫它們？

Answer 1

您需要在範圍內隱含ExecutionContext。 import ExecutionContext.Implicits.global會讓你的global execution context。

完整的示例：

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")