(define (make-stream m state)
  (delay
    (let ((o (m state)))
      (if (eq? #f o)
	  '()
	  (cons (car o)
		(make-stream m (cdr o)))))))

(define (stream-cons elem stream)
  (delay
    (cons elem stream)))

(define (stream-car stream)
  (car (force stream)))

(define (stream-cdr stream)
  (cdr (force stream)))

(define (stream-empty? stream)
  (null? (force stream)))

(define (stream-append s1 s2)
  (if (stream-empty? s1)
      s2
      (delay
	(cons (stream-car s1) (stream-append (stream-cdr s1) s2)))))
  
(define (list->stream l)
  (make-stream
   (lambda (l)
     (if (not (null? l))
         l                              ; it just works out that way     
         #f))
   l))

(define (stream->list stream)
  (if (stream-empty? stream)
      '()
      (cons (stream-car stream) (stream->list (stream-cdr stream)))))

(define (stream-map f stream)
  (make-stream
   (lambda (s)
     (if (stream-empty? s)
         #f
         (cons (f (stream-car s)) (stream-cdr s))))
   stream))

(define (make-stream-from-file filename)
  (let ((port (open-input-file filename)))
    (make-stream (lambda (p)
		   (let ((n (read-char p)))
		     (if (eof-object? n)
			 (begin (close-input-port p)
				#f)
			 (cons n p))))
		 port)))