How to use call/cc for non-local exit in Scheme -

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currently, studying scheme language. i'm confused in how use call-with-current-continuation.(call/cc) better understanding it, wrote example code non-local exits. doesn't work properly.

does know why? appreciated. in advance

[example code]

(define (product ls)   (call/cc    (lambda (return)      (cond       ((null? ls ) =>        (begin          (display "list end")          (newline)          1)) ;; ng          ;;(return 1)) ;; ok        ((not (number? (car ls))) =>        (begin          (display "not number")          (newline)          (return 0)))       (else =>             (begin               (display (car ls))               (newline)               (* (car ls) (product (cdr ls)))))))))

[repl output]

gosh> (product '(1 2 3)) ; works expected. ==> 1 ==> 2 ==> not number ==> 0 (return)  gosh> (product '(1 2 3)) ;; doesn't work expected. expect 6 return value. ==> 1 ==> 2 ==> 3 ==> list end *** error: invalid application: (1 #t)

there couple of things going on here.

first , foremost, looks => you're inserting cond clauses causing problem. in scheme, => has special meaning... don't want. take them out, , think you'll see code behaves expect.

but: use of call/cc not causing non-local exit, believe you're intending. is, guess want 0 bypass of waiting multiplies, , it's not. see this, change 0 can't multiplied---say, string "not number"--and watch fail.

this because you're re-binding return on each call function. think want this:

(define (product ls)   (call/cc    (lambda (return)      (letrec ([loop               (lambda (ls)                 (cond                   ((null? ls )                    (begin                      (display "list end")                      (newline)                      1)) ;; ng                   ;;(return 1)) ;; ok                    ((not (number? (car ls)))                    (begin                      (display "not number")                      (newline)                      (return "not number")))                   (else                     (begin                      (display (car ls))                      (newline)                      (* (car ls) (loop (cdr ls)))))))])        (loop ls)))))  (product '(1 2 3))

... produces output:

1 2 not number "not number" >

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