scala - Scala中的自定义 "let"表达式

Question

我很想拥有let构造类似于 Scala 中的 Haskell 中的构造。我尝试了几种方法，但似乎没有一个是好的。这是一些代码:

object CustomLet extends App {
  val data = for (i <- 1 to 1024; j <- 1 to 512) yield (i % j) * i * (i + 1) - 1

  def heavyCalc() = { println("heavyCalc called"); data.sum }

  def doSomethingWithRes(res: Int) = {
    println(s"${res * res}")
    1
  }

  def cond(value: Int): Boolean = value > 256

  // not really usable, even though it's an expression (2x heavyCalc calls)
  def withoutLet() = if (cond(heavyCalc())) doSomethingWithRes(heavyCalc()) else 0

  // not an expression
  def letWithVal(): Int = {
    val res = heavyCalc()
    if (cond(res)) doSomethingWithRes(res)
    else 0
  }

  // a lot of code to simulate "let", at least it is an expression
  def letWithMatch(): Int = heavyCalc() match {
    case res => if (cond(res)) doSomethingWithRes(res) else 0
  }

  // not perfect solution from
  // http://stackoverflow.com/questions/3241101/with-statement-equivalent-for-scala/3241249#3241249
  def let[A, B](param: A)(body: A => B): B = body(param)

  // not bad, but I'm not sure if it could handle more bindings at once
  def letWithApp(): Int = let(heavyCalc()) {res => if (cond(res)) doSomethingWithRes(res) else 0}

  List[(String, () => Int)](
    ("withoutLet", withoutLet),
    ("letWithVal", letWithVal),
    ("letWithMatch", letWithMatch),
    ("letWithApp", letWithApp)
  ).foreach(
    item => item match {
      case (title, func) => {
        println(s"executing $title")
        val ret = func()
        println(s"$title finished with $ret")
        println()
      }
    }
  )
}

这是它的理想外观(只有一个绑定(bind)，更多可以用 , 分隔；不确定 in 关键字):

  // desired look
  def letTest(): Int =
    let res = heavyCalc() in
      if (cond(res)) doSomethingWithRes(res) else 0

我不确定这是否可能，但我没有使用大多数高级 Scala 东西(如宏)的经验，所以我无法确定。

EDIT1:需要明确的是，我对它的主要期望是:成为 表达式 和相对 简单语法 (就像上面概述的那样)。

Answer 1

您可以使用正向管道:

object ForwardPipeContainer {
  implicit class ForwardPipe[A](val value: A) extends AnyVal {
    def |>[B](f: A => B): B = f(value)
  }
}

像这样使用:

import ForwardPipeContainer._

def f(i: Int) = i * i

println( f(3) |> (x => x * x) )

您可以将多个参数放在一个元组中:

println( (f(2), f(3)) |> (x => x._1 * x._2) )

如果与部分函数 synatx 结合使用，效果会更好:

println( (f(2), f(3)) |> { case (x, y) => x * y } )

这个答案是 What is a good way of reusing function result in Scala 的变体, 两者都基于 Cache an intermediate variable in an one-liner我从哪里得到最初的想法。