Hi everyone, I was reading through the wikipedia article on call/cc, and I thought that playing around with some of the provided examples in DrRacket would give me a better understanding of the feature. The second example was how a python style generator could be created using call/cc (http://en.wikipedia.org/wiki/Call-with-current-continuation#Examples).
I was looking how set! was used to update control-state's return argument to the continuation referencing the most recent call to the generator. I figured that if I removed the set! and called the generator twice in a row, it would create an infinite loop of sorts since control-state's return argument would forever hold a continuation originating from the first call. However, this is not what happens when I remove the set!. Instead it works exactly the same way it did before. Any ideas why? Thanks in advance for your help, Stephen Halter Below is the code that I where I don't (re)set! control-state's return argument: https://gist.github.com/1129202 #lang racket ;; Since the control-state blows itself away after the first call by ;; set!ing itself to a continuation, I would expect the return argument to ;; control-state to retain the value from it's first call. Therefore I ;; expected that calls subsequent to the first call to control-state would ;; "return" execution back to the first call creating an infinite loop of ;; sorts, but in DrRacket 5.1 it outputs 0 1 and finishes execution. ;; [LISTOF X] -> ( -> X u 'you-fell-off-the-end) (define (generate-one-element-at-a-time lst) ;; Hand the next item from a-list to "return" or an end-of-list marker (define (control-state return) (for-each (lambda (element) (call-with-current-continuation (lambda (resume-here) ;; Grab the current continuation (set! control-state resume-here) (return element)))) lst) (return 'you-fell-off-the-end)) ;; (-> X u 'you-fell-off-the-end) ;; This is the actual generator, producing one item from a-list at a time (lambda () (call-with-current-continuation control-state))) (define generate-digit (generate-one-element-at-a-time '(0 1 2 3 4 5 6 7 8 9))) (generate-digit) (generate-digit)
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