如何使尾recusive方法也可以指本身在非尾递归的方式

Question

想我必须在今后恢复长时间运行的计算，可以挂起自身的机制：

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

如何运行它们最简单的方法是

@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。比方说，我们要实现尾递归阶乘是暂停其计算每10个循环：

@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) 
} 

但是，这并不编译：

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

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

如何留住在非暂停通话尾递归？

Answer 1

我找到了一个可能的答案。我们可以将尾递归部分进入内部函数，并参考外一个非尾递归，当我们需要：

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) 
}