#lang racket
(require rackunit)
(provide get)

; --------------------------------------------------------------------------
; EXERCISE 1: get                             (implements Racket's list-ref)
; --------------------------------------------------------------------------

(define get
  (lambda (los n)
    (if (null? los)
        (error 'nth "no such position")       ; "nth: no such position" 
        (if (zero? n)
            (first los)
            (get (rest los) (sub1 n))))))

(check-equal? (get '(a b c)     2) 'c)
(check-equal? (get '(d c b a z) 1) 'c)

; The following test asserts that a certain call to nth should cause
; an error.  Notice that we wrap the call in a lambda expression.
; 
(check-exn  exn:fail?  (lambda () (get '(d c b a z) 5)))

; ---------------------------------------------------------------------------