;;; $Id: nth-element-mod.scm,v 1.2 2006/01/05 22:24:09 leavens Exp $
;;; AUTHOR: Dalei Li and Gary T. Leavens

(module nth-element-mod (lib "typedscm.ss" "typedscm")

(provide nth-element)

(deftype nth-element (forall (t) (-> ((list-of t) number) t)))
(define nth-element
  (lambda (lst n)
    ;; ENSURES: Result is the nth, zero-based, element of lst
    (if (null? lst)
        (eopl:error 'nth-element
                    "List too short by ~s elements" (+ n 1))
        (if (zero? n)
            (car lst)
            (nth-element (cdr lst) (- n 1))))))

) ;; end module