scala - 什么时候不能用 Future 代替 Promise？

Question

正在阅读本文article关于 promise ，作者使用了以下示例:

def redeemCampaignPledge(): Future[TaxCut] = {
    val p = Promise[TaxCut]()
    Future {
      println("Starting the new legislative period.")
      Thread.sleep(2000)
      //p.success(TaxCut(20))
      //println("We reduced the taxes! You must reelect us 2018!")
      p.failure(LameExcuse("global economy crisis"))
      println("We didn't fullfil our promises, so what?")
    }
    p.future
}

val taxCutF: Future[TaxCut] = redeemCampaignPledge()
  println("Now they're elected, let's see if they remember their promise.")
  taxCutF.onComplete {
    case Success(TaxCut(reduction)) =>
      println(s"Miracle! Taxes cut by $reduction percent.")
    case Failure(ex) =>
    println(s"They broke the promise again. Because of a ${ex.getMessage}")
  }

我的问题是，我不能去掉 Promise 并将其重写为:

def redeem(): Future[TaxCut] = Future {
    println("Starting legislative period...!!!!")
    Thread.sleep(2000)

    println("We were successful")
    TaxCut(25)
}

第二个版本缺少什么？我没有完全理解 promise 带来的值(value)。

Answer 1

是的，你完全正确。这是一个非常common anti pattern使用 JavaScript Promises(其中，Futures 称为 Promises，Promises 称为 deferred)。

基本上，它不是使用 future 提供的延续，而是以类似时尚的回调方式围绕它们构建新的延续。