On Fri, 22 Apr 2005, Matthew D Swank wrote:

> ...
> there is the following construct:
>
> 1. (let* ((promise* (stream-promise ((s:promise-content content))))
>                    (content  (unbox promise)))
>
> however, since 'content' is being bound to (unbox promise), and not
> (unbox promise*),
> the 'let*' in 1. is superfluous.

No, because the evaluation of ((s:promise-content content)) may have a side
effect that changes the original promise.  I believe this also answers your
subsequent questions.  All this is explained in the SRFI-45 "Primitives for
Expressing Iterative Lazy Algorithms" document.  In particular, the following
code there should have the indicated behaviour:

(define q
   (let ((count 5))
     (define (get-count) count)
     (define p (delay (if (<= count 0)
                          count
                          (begin (set! count (- count 1))
                                 (force p)
                                 (set! count (+ count 2))
                                 count))))
     (list get-count p)))
(define get-count (car q))
(define p (cadr q))

(get-count)  ; =>   5
(force p)    ; =>   0
(get-count)  ; =>   10

The alternative you suggest was in fact
what appeared in initial versions of SRFI-45, found to be incorrect, and
corrected.

Regards
Andre van Tonder