scala - 确保尾递归函数得到优化的 Scala 注释是什么？

Question

我认为有@tailrec注解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 Scala 在脚本模式下使用(例如在 REPL 下使用 :load <file>)，它也可以工作吗？

Answer 1

来自“Tail calls, @tailrec and trampolines”博客文章:

• 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.

• 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。

示例:

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(来自 Scala REPL tips and tricks 的示例):

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 = {
                    ^