Exercise 3.81.  Exercise 3.6 discussed generalizing the random-number 
generator to allow one to reset the random-number sequence so as to 
produce repeatable sequences of ``random'' numbers. Produce a stream 
formulation of this same generator that operates on an input stream of 
requests to generate a new random number or to reset the sequence to a 
specified value and that produces the desired stream of random numbers. 
Don't use assignment in your solution.

————————————————————————————————————————————————————————————————————————

3.6 solution:

(define rand
  (let ((x random-init))
    (lambda (m)
      (cond ((eq? m 'reset) (lambda (y) (set! x y)))
            ((eq? m 'generate) (set! x (rand-update x)) x)
            (else (error "hm..."))))))

stream-based:

(define (rand request-stream)
  (define (f last-result last-request)
    (cond ((eq? last-request 'generate) (rand-update last-result))
          ((pair? last-request) (rand-update (cadr last-request)))))
  (define results
    (cons-stream random-init          ; defined elsewhere, as above
                 (stream-map f results request-stream)))
  results)

The stream of requests should contain requests that are either the 
symbol "generate" or a list ('reset x) where x is the value to set.