Better implementation of `fold`.

fold.rkt

(define fold
  (lambda (l result fun)
    (cond
      ((null? l) result)
      (else
        (fold (cdr l) (fun result (car l)) fun)))))

(define fold
  (lambda (l init fun)
    (cond
      ((null? (cdr l)) (fun init (car l)))
      (else
        (fun (fold (cdr l) init fun) (car l))))))

(define sum
  (lambda (l)
    (fold l 0 (lambda (col x) (+ x col)))))

(define map
  (lambda (l fun)
    (fold l '() (lambda (col x) (cons (fun x) col)))))