#define ONE 2

Adam Hinz: The Blog

anything python can do, scheme can do better

On 2007-12-16 at 12/16/2007 10:07:00 PM...

Jens Lekman - A Man Walks Into a Bar

I started learning Python today just for the hell of it, and I found out you can access the last items in a list with a negative index. I thought, "shit, I bet Scheme can do that just fine," and here's my solution:

(define list-ref-a
  (lambda (ols n)
    (letrec ([list-ref-n-help
              (lambda (ls count k)
                (cond
                  [(and (null? ls) (> (- n) count))
                   (error 'list-ref "index ~s is out of range for list ~s"
                          n ols)]
                  [(null? ls) 0]
                  [else (let ([res
                               (add1
                                (list-ref-n-help (cdr ls) (add1 count) k))])
                          (cond
                            [(= n (- res)) (k (car ls))]
                            [else res]))]))])
      (cond
        [(>= n 0) (list-ref ols n)]
        [else (call/cc (lambda (k) (list-ref-n-help ols 0 k)))]))))

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