方案继续：顶层的“ call / cc”呼叫与非顶层的“ call / cc”呼叫有什么区别？

Question

This code works as expected:

(define saved #f)
(cons 'wo (call/cc (lambda (k) (set! saved k) '())))
(saved 'ca!)

output (Racket console):

'(wo)
'(wo . ca!)

But when I wrap it in a function and call it, the program never stops. Why?

(define (test)
    (define saved #f)
    (cons 'wo (call/cc (lambda (k) (set! saved k) '())))
    (saved 'ca!))

(test)

Answer 1

A continuation is all that's left to be done in the execution context where it's saved.

In the first case, the continuation is saved when calling cons , so it's just to cons 'wo to something and return to the REPL.

In the second case, you call procedure test , so the continuation is both

1. cons 'wo to something
2. call the procedure bound to saved (ie the continuation) with 'ca!

so the continuation calls itself, hence the loop.