;; Streams mit Platzhaltern

(define-record-type :stream-nil
  (make-stream-nil)
  stream-nil?)

(define the-stream-nil (make-stream-nil))

(define-record-type :stream-head
  (make-stream-head first next-rv)
  stream-head?
  (first stream-head-first)
  (next-rv stream-head-next-rv))

(define-record-type :stream
  (really-make-stream rv)
  stream?
  (rv stream-rv))

(define (stream-take stream n)
  (let loop ((i n)
	     (stream-head (sync (stream-rv stream)))
	     (reverse-result '()))
    (if (or (zero? i)
	    (stream-nil? stream-head))
	(reverse reverse-result)
	(loop (- i 1)
	      (sync (stream-head-next-rv stream-head))
	      (cons (stream-head-first stream-head)
		    reverse-result)))))

(define (make-stream)
  (let ((placeholder (make-placeholder)))
    (values placeholder
	    (really-make-stream 
	     (placeholder-value-rv placeholder)))))

(define (extend-stream! placeholder value)
  (let ((new-placeholder (make-placeholder)))
    (placeholder-set! placeholder
		      (make-stream-head
		       value
		       (placeholder-value-rv new-placeholder)))
    new-placeholder))

(define (terminate-stream! placeholder)
  (placeholder-set! placeholder the-stream-nil))

(define (stream-from-to i j)
  (call-with-values
   make-stream
   (lambda (placeholder stream)
     (spawn
      (lambda ()
	(let loop ((placeholder placeholder)
		   (i i))
	  (if (< i j)
	      (loop (extend-stream! placeholder i) (+ 1 i))
	      (terminate-stream! placeholder)))))
     stream)))