如何制作一个尾递归方法,它也可以以非尾递归的方式引用自己 [英] How to make a tail-recursive method that can also refer to itself in a non-tail-recursive way
问题描述
假设我有一个长时间运行计算的机制,可以暂停自己以便稍后恢复:
Suppose I have a mechanism for long-running computations that can suspend themselves to be resumed later:
sealed trait LongRunning[+R];
case class Result[+R](result: R) extends LongRunning[R];
case class Suspend[+R](cont: () => LongRunning[R]) extends LongRunning[R];
最简单的运行方式是
@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());
}
}
问题在于创建这样的计算.假设我们想要实现尾递归阶乘,每 10 个周期暂停一次计算:
The problem is creating such computations. Let's say we want to implement tail-recursive factorial that suspends its computation every 10 cycles:
@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)
}
但这不会编译:
错误:无法优化@tailrec
注释方法factorial
:它包含不在尾部位置的递归调用
error: could not optimize
@tailrec
annotated methodfactorial
: 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?
How to retain tail recursion on the non-suspending calls?
推荐答案
我找到了一个可能的答案.我们可以将尾递归部分移到内部函数中,并在需要时引用外部非尾递归部分:
I found one possible answer. We can move the tail-recursive part into an inner function, and refer to the outer one, non-tail-recursive, when we need:
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)
}
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