Scala for-yield控制流程

Question

我想知道for-yield块中的scala中的控制流。我看到所有元素都首先通过＆＃39; for＆＃39;部分后跟＆＃39; yield＆＃39;部分。这是为什么？如果它不是for-yield for element1，for-yield for element2 ......

scala> val list = List(1,2,3,4,5,6)
list: List[Int] = List(1, 2, 3, 4, 5, 6)

scala> :paste
// Entering paste mode (ctrl-D to finish)

val t = for {
  i <- list
  log = println("processing " + i)
} yield {
  println("In yield for " + i)
  i
}

// Exiting paste mode, now interpreting.

processing 1   //All of them first go through the for block
processing 2
processing 3
processing 4
processing 5
processing 6
In yield for 1    // yield comes after all
In yield for 2
In yield for 3
In yield for 4
In yield for 5
In yield for 6
t: List[Int] = List(1, 2, 3, 4, 5, 6)

Answer 1

您的代码

val t = for {
  i <- list
  log = println("processing " + i)
} yield {
  println("In yield for " + i)
  i
}

粗略地贬低

val t = list.map { i =>
    val log = println("processing " + i)
    (i, log)
}.map { x =>
    x match {
        case (i, log) =>
            println("In yield for " + i)
    }
}

从这里你可以看到为什么你首先得到所有“处理...”消息。