I try to implement function that takes list of atoms and returns only sub part of given list. Sub part is everything after particular token. I try to use call-with-current-continuation for that purpose.
Here is my approach:
(define rest
(lambda (a lat)
(call-with-current-continuation
(lambda (res)
(letrec ((r (lambda (l)
(cond
((null? l) ())
((eq? (car l) a) (res (cdr l)))
(else (r (cdr l)))))))
(r lat))))))
as you can see I try to pass data to continuation
((eq? (car l) a) (res (cdr l))) but I always get back () using this approach. But if I hard-coded values like this ((eq? (car l) a) (apple tea) it works fine.
I am really stuck and I need help.
EDIT: I've found solution instead of (res (cdr l)) I have to pass (res (r (cdr l)))
Also real mistake was not with how I call continuation, but with (else (r (cdr l))))))) I should write as:
(else (cons (car l)
(r (cdr l))))))))
Your code works fine in MIT-Scheme 9.2:
MIT/GNU Scheme running under OS X
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1 ]=> (define rest
(lambda (a lat)
(call-with-current-continuation
(lambda (res)
(letrec ((r (lambda (l)
(cond
((null? l) ())
((eq? (car l) a) (res (cdr l)))
(else (r (cdr l)))))))
(r lat))))))
;Value: rest
1 ]=> (rest 'c '(a b c d))
;Value 2: (d)
1 ]=> (rest 'c '(a b c d e))
;Value 3: (d e)
1 ]=> (rest 'c ())
;Value: ()
My best guess is that you're working with objects where eq? is the inappropriate comparison procedure.
FWIW, here's a shorter implementation:
(define (rest a lat)
(call-with-current-continuation
(lambda (res)
(let r ((l lat))
(if (null? l)
'()
((if (eq? (car l) a) res r) (cdr l)))))))