定制“出租"Scala 中的表达式 [英] Custom "let" expression in Scala

问题描述

我希望 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:

I'd love to have let construct similar to the one in Haskell in Scala. I tried a few ways, but none seems to be good. Here's some code:

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，更多的可以用 , 分隔；不确定 in 关键字):

This is the ideal look of it (with only one binding, more could be separated by ,; not sure about the in keyword):

  // 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，但我没有使用宏等大多数高级 Scala 东西的经验，所以我无法确定.

I'm not sure if it's possible, but I have no experience with most of advanced Scala stuff like macros, so I can't really tell.

明确地说，我对它的主要期望是:表达式和相对简单的语法(如上面概述的那样).

To be clear, the main things I'm expecting from it are: being expression and relatively simple syntax (like the one outlined above).

推荐答案

您可以使用前向管道:

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) )

<小时>

你可以在一个元组中放置多个参数:

---

You can put multiple arguments in a tuple:

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

如果与部分函数 synatx 结合看起来更好:

which looks better if combined with partial function synatx:

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

<小时>

此答案是 在 Scala 中重用函数结果的好方法是什么 的变体，两者均基于在单行中缓存一个中间变量，我从那里得到了最初的想法.

---

This answer is a variation of What is a good way of reusing function result in Scala, and both are based on Cache an intermediate variable in an one-liner where I got the initial idea from.

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