;; IN: list, a function
;; OUT: list
;; BEHAVIOR: destructively applies the function onto every element of the list
;;           preserves original pointer

(define (map! fn ls)
  (if (null? ls) '()
      (begin
	(set-car! ls (fn (car ls)))
	(map! fn (cdr ls))
	ls)))


;; constructive version

(define (map fn ls)
  (if (null? ls) '()
      (cons (fn (car ls))
	    (map fn (cdr ls)))))