;; naive implementation
(define (make-generator transformer start #!optional (retriever identity) #!rest args)
  (let ((next start))
    (lambda ()
      (let ((yield next))
        (set! next (apply transformer (cons next args)))
        (retriever yield)))))

(define (list->generator ls)
  (make-generator
   (lambda (x)
     (if (pair? x)
         (cdr x)
         #!eof))
   ls
   (lambda (x)
     (if (pair? x)
         (car x)
         #!eof))))

;; naive usage
(define listgen
  (list->generator
   '(a b c d)))

(define iotagen
  (make-generator add1 0))