Seeking contrived example code: continuations!

Question

So I believe I understand continuations now, at least on some level, thanks to the community scheme wiki and Learn Scheme in Fixnum Days .

But I'd like more practice -- that is, more example code I can work through in my head (preferably contrived, so there's not extraneous stuff to distract from the concept).

Specifically , I'd like to work through more problems with continuations that resume and/or coroutines, as opposed to just using them to exit a loop or whatever (which is fairly straightforward).

Anyway, if you know of good tutorials besides the ones I linked above, or if you'd care to post something you've written that would be a good exercise, I'd be very appreciative!

Answer 1

Yeah, continuations can be pretty mind-bending. Here's a good puzzle I found a while back - try to figure out what's printed and why:

(define (mondo-bizarro)
  (let ((k (call/cc (lambda (c) c)))) ; A
    (write 1)
    (call/cc (lambda (c) (k c))) ; B 
    (write 2)
    (call/cc (lambda (c) (k c))) ; C
    (write 3)))

(mondo-bizarro)

---

Explanation of how this works (contains spoilers!):

1. The first call/cc stores returns it's own continuation and stores it in k .
2. The number 1 is written to the screen.
3. The current continuation, which is to continue at point B, is returned to k, which returns to A
4. This time, k is now bound to the continuation we got at B
5. The number 1 is written again to the screen
6. The current continuation, which is to continue at point B, is returned to k, which is another (but different) continuation to another point B
7. Once we're back in the original continuation, it's important to note that here k is still bound to A
8. The number 2 is written to the screen
9. The current continuation, which is to continue at point C, is returned to k, which returns to A
10. This time, k is now bound to the continuation we got at C
11. The number 1 is written again to the screen
12. The current continuation, which is to continue at point B, is returned to k, which returns to C
13. The number 3 is written to the screen
14. And you're done

Therefore, the correct output is 11213 . The most common sticking point I've put in bold text - it's important to note that when you use continuations to 'reset' the value of k that it doesn't affect the value of k back in the original continuation. Once you know that it becomes easier to understand.

Answer 2

布朗大学的编程语言课程在公开提供的延续问题上存在问题 。