Question

我想在Futures上试验foldleft。我从一个简单/愚蠢的例子开始作为工作表：

import scala.concurrent._
import ExecutionContext.Implicits.global

val list = (1 to 10).toList
def doubleFuture(i: Int) = Future { println(i);i }

val twoFutures = list map doubleFuture //returns List[Future[Int]]
val res = (twoFutures foldLeft(List[Int]())
    ((theList:List[Int], aFuture:Future[Int]) =>
    {
      theList :+ 1
    }))

编译器不满意并指出：

Error:(11, 48) type mismatch;
 found   : (List[Int], scala.concurrent.Future[Int]) => List[Int]
 required: Int
    ((theList:List[Int], aFuture:Future[Int]) =>
                                          ^

我不明白为什么foldleft函数的第二个参数不是Future [Int]类型，因为twoFutures的类型为List [Future [Int]]。你能解释一下是什么问题吗？

谢谢！

Answer 1

您需要在列表后面使用句点（.）告诉编译器将括号后面的块或括号绑定到foldLeft而不是twoFutures：

import scala.concurrent._
import ExecutionContext.Implicits.global

object FutureList {
  def main(args: Array[String]) {
    val list = (1 to 10).toList
    def doubleFuture(i: Int) = Future { println(i); i }

    val twoFutures = list map doubleFuture //returns List[Future[Int]]

    val res = twoFutures.foldLeft(List[Int]())(
      (theList, aFuture) => theList :+ 1)

    println(res)

    // Uncomment these lines to unfold the mystery
    //    val theList = List[Int]()
    //    val aFuture = Future[Int](0)
    //    twoFutures((theList: List[Int], aFuture: Future[Int]))

  }
}

要解释它的含义，您可以取消注释上面的三个注释行。您将看到与twoFutures列表之后没有句点的情况相同的编译错误：

 Multiple markers at this line
    - type mismatch; found : (List[Int], scala.concurrent.Future[Int]) required: 
     Int
    - type mismatch; found : (List[Int], scala.concurrent.Future[Int]) required: 
     Int

输出

2
4
1
3
5
6
8
10
7
9
List(1, 1, 1, 1, 1, 1, 1, 1, 1, 1)

Answer 2

这个解决方案在左侧折叠处使用点符号，即

val res = (twoFutures.foldLeft(List[Int]())
    ((theList:List[Int], aFuture:Future[Int]) =>
    {
      theList :+ 1
    }))

期货清单上的折叠列的参数

2 个答案:

输出