map.scm

;; map, the naive way
(define (naive-map f xs)
  (cond
   ((null? xs) '())
   (else (cons (f (car xs)) (map f (cdr xs))))))

(define (tr-map f xs)
  (define (recurse f xs accum)
    (cond
     ((null? xs) (reverse accum))
     (else
      (recurse f (cdr xs) (cons (f (car xs)) accum)))))
  (recurse f xs '()))

(define (tr-inline-map f xs)
  (define head (list #t)) ;; need a non-null to set-cdr! on
  (define (recurse f xs end)
    (cond
     ((null? xs) (cdr head))
     (else
      (set-cdr! end (list (f (car xs))))
      (recurse f (cdr xs) (cdr end)))))
  (recurse f xs head))

(define f (lambda (x) (* 2 x)))
(define known (naive-map f '(1 2 3 4 5)))
(display (equal? known (tr-map f '(1 2 3 4 5))))
(display (equal? known (tr-inline-map f '(1 2 3 4 5))))