My version of stream-maker from PL class. This function works under the assumption that there is some state that is passed along from thunk to thunk as context. The current stream output value can be computed using val-fn. The state to pass to the next thunk is computed using the next-state function. I think it's much easier to understand, thoug…

stream-maker.rkt

(define (my-stream-maker val-fn next-state seed)
  (define (stream-thunk current-state)
    (lambda () (cons (val-fn current-state)
                     (stream-thunk (next-state current-state)))))
  (stream-thunk seed))


(define ones
  (my-stream-maker identity
                   identity
                   1))

(define (increment n) (+ n 1))
(define nats
  (my-stream-maker identity
                   increment
                   1))

(define powers-of-two
  (my-stream-maker (lambda (n) (expt 2 n))
                   increment
                   0))

(define powers-of-two-another-way
  (my-stream-maker identity
                   (lambda (n) (* n 2))
                   1))

(define (rotate-text-left text)
  (string-append (substring text 1) (substring text 0 1)))

(define marquee
  (my-stream-maker identity
                   rotate-text-left
                   "Hello, World!"))

(define (random-increment-or-decrement n)
  (if (= 0 (random 2)) (+ n 1) (- n 1)))

(define random-walk
  (my-stream-maker identity
                   random-increment-or-decrement
                   0))