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(define get-first
  (lambda (l)
    (call-with-current-continuation
      (lambda (here)
        (set! leave here)
        (waddle l)
        (leave (quote ()))))))

(define get-first
  (lambda (l)
    (call-with-current-continuation
      (lambda (here)
        (set! leave here)
        (leave (waddle l))))))

对于任何不熟悉“The Seasoned Schemer”一书的人来说get-first，get-next、 和waddle（最后两个未在此处定义）显然是对协程建模以遍历传递给仅产生叶子的树的过程。waddle就在waddle倒数第二次重新进入的 y​​ield 之前，它将重新进入点设置为它只会返回纯值的位置，即is的实际值而'()不是yield ，就好像它是一直都是纯函数。 '()waddle '()

考虑到这一点，我们可以看到get-first设置了什么......当waddle返回“真实”时，它将在call/ccin内部get-first，然后(leave (quote ()))是get-first（并且，反过来，这leave旨在返回到get-next最后一次迭代，因此，它是)get-next的“实际”回报'()。

那么为什么第二个版本不等价，其中waddle的值'()将是 的参数leave？

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1 回答 1

4

The confusion is because "leave" is not the function I want it to be, but the function it evaluates to when it's evaluated, which appears to be left-to-right and thus before "waddle". That means it evaluates to what it was just set to, in the statement prior.

Moral: beware when using functions that are subject to redefining WITHIN the call to the function! If this was on a right-to-left interpreter, waddle would be evaluated before the symbol leave was looked up as the function that "leaves" to wherever, during which time it would be set to a DIFFERENT function.

于 2012-06-19T03:42:58.740 回答