什么是 Scala 注释以确保优化尾递归函数? [英] What is the Scala annotation to ensure a tail recursive function is optimized?

问题描述

我认为有 @tailrec 注释以确保编译器将优化尾递归函数.你只是把它放在声明前面吗?如果在脚本模式下使用 Scala(例如在 REPL 下使用 :load )，它是否也有效?

I think there is @tailrec annotation to ensure the compiler will optimize a tail recursive function. Do you just put it in front of the declaration? Does it also work if Scala is used in scripting mode (for instance using :load <file> under REPL)?

推荐答案

来自尾调用、@tailrec 和蹦床" 博客文章:

• 在 Scala 2.8 中，您还可以使用新的 @tailrec 注释来获取有关优化了哪些方法的信息.
  此注释可让您标记希望编译器优化的特定方法.
  如果编译器未优化它们，您将收到警告.
• 在 Scala 2.7 或更早版本中，您需要依靠手动测试或字节码检查来确定方法是否已优化.

• In Scala 2.8, you will also be able to use the new @tailrec annotation to get information about which methods are optimised.
  This annotation lets you mark specific methods that you hope the compiler will optimise.
  You will then get a warning if they are not optimised by the compiler.
• In Scala 2.7 or earlier, you will need to rely on manual testing, or inspection of the bytecode, to work out whether a method has been optimised.

示例:

您可以添加一个 @tailrec 注释，以确保您的更改有效.

you could add a @tailrec annotation so that you can be sure that your changes have worked.

import scala.annotation.tailrec

class Factorial2 {
  def factorial(n: Int): Int = {
    @tailrec def factorialAcc(acc: Int, n: Int): Int = {
      if (n <= 1) acc
      else factorialAcc(n * acc, n - 1)
    }
    factorialAcc(1, n)
  }
}

<小时>

它在 REPL 中工作(例如来自 Scala REPL 提示和技巧):

C:\Prog\Scala\tests>scala
Welcome to Scala version 2.8.0.RC5 (Java HotSpot(TM) 64-Bit Server VM, Java 1.6.0_18).
Type in expressions to have them evaluated.
Type :help for more information.

scala> import scala.annotation.tailrec
import scala.annotation.tailrec

scala> class Tails {
     | @tailrec def boom(x: Int): Int = {
     | if (x == 0) throw new Exception("boom!")
     | else boom(x-1)+ 1
     | }
     | @tailrec def bang(x: Int): Int = {
     | if (x == 0) throw new Exception("bang!")
     | else bang(x-1)
     | }
     | }
<console>:9: error: could not optimize @tailrec annotated method: it contains a recursive call not in tail position
       @tailrec def boom(x: Int): Int = {
                    ^
<console>:13: error: could not optimize @tailrec annotated method: it is neither private nor final so can be overridden
       @tailrec def bang(x: Int): Int = {
                    ^

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