Scala future 基础知识

Question

我在 Scala 中有以下代码:

case class Water(temp: Int)

case class Milk(temp: Int)

def heatWaterFor(minutes: Int, water: Water) = Future {
  Thread.sleep(1000)
  Water(82)
}

def boilMilkFor(minutes: Int, milk: Milk) = Future {
  Thread.sleep(1000)
  Milk(90)
}

def frothMilk(hotwater: Water, hotmilk: Milk) = Future {
  Thread.sleep(1000)
  hotmilk
}

val start = System.currentTimeMillis()

val milkMaker = for {
   water <- heatWaterFor(10, Water(10))
   milk <- boilMilkFor(5, Milk(10))
   frothed = frothMilk(water, milk)
   hotMilk <- frothed
 } yield (hotMilk)

Await.ready(milkMaker, Duration.Inf)
val end = System.currentTimeMillis() - start
println(milkMaker.value + " , Time taken: "+((end/1000))+" seconds.")

我的目的是并行化 heatWaterFor(...)和 boilMilkFor(...)因为他们是独立的。但我觉得上面的代码是连续的，根本没有利用 future 的力量。显然，运行需要 3000 毫秒(这是一个额外的证明)。

我在这里缺少的基本东西是什么？

Answer 1

A for 表达式简化为一系列 map , flatMap和 withFilter操作。你的特定表达被简化为这样的:

heatWaterFor(10, Water(10))
        .flatMap(water => boilMilkFor(5, Milk(10))
                             .flatMap(milk => frothMilk(water, milk))

正如你在这里看到的，下一个 future 的执行是在上一个完成时开始的。因此，如果要并行执行它们，则需要执行以下操作:

val heatWater = heatWaterFor(10, Water(10))
val boilMilk = boilMilkFor(5, Milk(10))

val milkMaker = for {
   water <- heatWater
   milk <- boilMilk
   hotMilk <- frothMilk(water, milk)
} yield (hotMilk)

这将开始 heatWaterFor和 boilMilkFor同时，然后开始 frothMilk当这两个都完成时(因为它取决于其他两个 future 的结果)。