scala - 如何制作也可以以非尾递归方式引用自身的尾递归方法

Question

假设我有一个长时间运行的计算机制，可以暂停自己稍后恢复:

sealed trait LongRunning[+R];
case class Result[+R](result: R) extends LongRunning[R];
case class Suspend[+R](cont: () => LongRunning[R]) extends LongRunning[R];

运行它们的最简单方法是

@annotation.tailrec
def repeat[R](body: LongRunning[R]): R =
  body match {
    case Result(r)   => r
    case Suspend(c)  => {
      // perhaps do some other processing here
      println("Continuing suspended computation");
      repeat(c());
    }
  }

问题是创建这样的计算。假设我们要实现每 10 个周期暂停一次计算的尾递归阶乘:

@annotation.tailrec
def factorial(n: Int, acc: BigInt): LongRunning[BigInt] = {
  if (n <= 1)
    Result(acc);
  else if (n % 10 == 0)
    Suspend(() => factorial(n - 1, acc * n))
  else
    factorial(n - 1, acc * n)
}

但这不编译:

error: could not optimize @tailrec annotated method factorial: it contains a recursive call not in tail position

Suspend(() => factorial(n - 1, acc * n))

如何在非挂起调用上保留尾递归？

Answer 1

我找到了一个可能的答案。我们可以将尾递归部分移动到内部函数中，并在需要时引用外部函数，非尾递归:

def factorial(n: Int, acc: BigInt): LongRunning[BigInt] = {
  @annotation.tailrec
  def f(n: Int, acc: BigInt): LongRunning[BigInt] =
    if (n <= 1)
      Result(acc);
    else if (n % 10 == 0)
      Suspend(() => factorial(n - 1, acc * n))
    else
      f(n - 1, acc * n)
  f(n, acc)
}