Attached to the end of this note is yet another reference
implementation of the core stream syntax and functions.
This is the third implementation I have posted:

    1  based on R5RS delay/force, and including
        both the core and the library

    2  based on Richard Kelsey's delay/force

    3  based on Andre von Tonder's most recent
        lazy primitives (even later than his Sept 6th
        posting to c.l.scheme, and considerably
        simpler and faster)

I would appreciate experimentation with this version of
the core stream syntax and functions, especially with
regard to its safe-for-space properties.  Hopefully Andre's
lazy primitives make it safe-for-space in all implementations
of Scheme.

Phil

;;; STREAM -- LIBRARY OF SYNTAX AND FUNCTIONS TO MANIPULATE STREAMS

;;; A stream is a new data type, disjoint from all other data types, that
;;; contains a promise that, when forced, is either nil (a single object
;;; distinguishable from all other objects) or consists of an object (the
;;; stream element) followed by a stream.  Each stream element is evaluated
;;; exactly once, when it is first retrieved (not when it is created); once
;;; evaluated its value is saved to be returned by subsequent retrievals
;;; without being evaluated again.

;; STREAM-TYPE -- type of streams
;; STREAM? object -- #t if object is a stream, #f otherwise
(define-record-type stream-type
  (make-stream promise)
  stream?
  (promise stream-promise))

;;; UTILITY FUNCTIONS

;; STREAM-ERROR message -- print message then abort execution
;  replace this with a call to the native error handler
;  if stream-error returns, so will the stream library function that called it
(define (stream-error message) (display message) (newline) (car '()))

;; ANY pred? list -- first non-#f (pred? list-item), else #f
(define (any pred? lst)
  (cond ((null? lst) #f)
        ((null? (cdr lst)) (pred? (car lst)))
        (else (or (pred? (car lst)) (any pred? (cdr lst))))))

;; ALL pred? list -- #f if any (pred? list-item) is #f, or last pred?
(define (all pred? lst)
  (cond ((null? lst) #t)
        ((null? (cdr lst)) (pred? (car lst)))
        (else (and (pred? (car lst)) (all pred? (cdr lst))))))

;;; LOW-LEVEL STREAM FUNCTIONS by Andre von Tonder (private e-mail 13-SEP-2003)

;; STREAM-LOW-LEVEL-LAZY -- an "atomic" (delay (force ...))
(define-syntax stream-low-level-lazy
  (syntax-rules ()
    ((stream-low-level-lazy exp)
        (cons 'suspension (lambda () exp)))))

;; STREAM-LOW-LEVEL-STRICT -- make a value into a low-level promise
(define (stream-low-level-strict x)
  (make-stream (cons 'value x)))

;; STREAM-LOW-LEVEL-DELAY -- make an expression into a low-level promise
(define-syntax stream-low-level-delay
  (syntax-rules ()
    ((stream-low-level-delay exp)
      (stream-low-level-lazy (stream-low-level-strict exp)))))

;; STREAM-LOW-LEVEL-FORCE -- force the value from a low-level promise
(define (stream-low-level-force prom)
    (case (car prom)
      ((value)      (cdr prom))
      ((suspension) (let ((val (stream-promise ((cdr prom)))))
                      (set-car! prom (car val))
                      (set-cdr! prom (cdr val))
                      (stream-low-level-force prom)))))

;;; STREAM SYNTAX AND FUNCTIONS

;; STREAM-NULL -- the distinguished nil stream
(define stream-null (make-stream (stream-low-level-delay '())))

;; STREAM-CONS object stream -- primitive constructor of streams
(define-syntax stream-cons
  (syntax-rules ()
    ((stream-cons obj strm)
      (make-stream
        (stream-low-level-delay
          (if (not (stream? strm))
              (stream-error "attempt to stream-cons onto non-stream")
              (cons obj strm)))))))

;; STREAM-NULL? object -- #t if object is the null stream, #f otherwise
(define (stream-null? obj)
  (and (stream? obj) (null? (stream-low-level-force (stream-promise obj)))))

;; STREAM-PAIR? object -- #t if object is a non-null stream, #f otherwise
(define (stream-pair? obj)
  (and (stream? obj) (not (null? (stream-low-level-force (stream-promise obj))))))

;; STREAM-CAR stream -- first element of stream
(define (stream-car strm)
  (cond ((not (stream? strm)) (stream-error "attempt to take stream-car of non-stream"))
        ((stream-null? strm) (stream-error "attempt to take stream-car of null stream"))
        (else (car (stream-low-level-force (stream-promise strm))))))

;; STREAM-CDR stream -- remaining elements of stream after first
(define (stream-cdr strm)
  (cond ((not (stream? strm)) (stream-error "attempt to take stream-cdr of non-stream"))
        ((stream-null? strm) (stream-error "attempt to take stream-cdr of null stream"))
        (else (cdr (stream-low-level-force (stream-promise strm))))))

;; STREAM-DELAY object -- the essential stream mechanism
(define-syntax stream-delay
  (syntax-rules ()
    ((stream-delay expr)
      (make-stream
        (stream-low-level-lazy expr)))))

;; STREAM object ... -- new stream whose elements are object ...
(define (stream . objs)
  (let loop ((objs objs))
    (stream-delay
      (if (null? objs)
          stream-null
          (stream-cons (car objs) (loop (cdr objs)))))))

;; STREAM-UNFOLDN generator seed n -- n+1 streams from (generator seed)
(define (stream-unfoldn gen seed n)
  (define (unfold-result-stream gen seed)
    (let loop ((seed seed))
      (stream-delay
        (call-with-values
          (lambda () (gen seed))
          (lambda (next . results)
            (stream-cons results (loop next)))))))
  (define (result-stream->output-stream result-stream i)
    (stream-delay
      (let ((result (list-ref (stream-car result-stream) i)))
        (cond ((pair? result)
                (stream-cons (car result)
                             (result-stream->output-stream
                               (stream-cdr result-stream) i)))
              ((not result)
                (result-stream->output-stream (stream-cdr result-stream) i))
              ((null? result) stream-null)
              (else (stream-error "can't happen"))))))
  (define (result-stream->output-streams result-stream n)
    (let loop ((i 0) (outputs '()))
      (if (= i n)
        (apply values (reverse outputs))
        (loop (+ i 1)
              (cons (result-stream->output-stream result-stream i)
                    outputs)))))
  (result-stream->output-streams (unfold-result-stream gen seed) n))

;; STREAM-MAP func stream ... -- stream produced by applying func element-wise
(define (stream-map func . strms)
  (cond ((not (procedure? func)) (stream-error "non-functional argument to stream-map"))
        ((null? strms) (stream-error "no stream arguments to stream-map"))
        ((not (all stream? strms)) (stream-error "non-stream argument to stream-map"))
        (else (let loop ((strms strms))
                (stream-delay
                  (if (any stream-null? strms)
                      stream-null
                      (stream-cons (apply func (map stream-car strms))
                                   (loop (map stream-cdr strms)))))))))

;; STREAM-FOR-EACH proc stream ... -- apply proc element-wise for side-effects
(define (stream-for-each proc . strms)
  (cond ((not (procedure? proc)) (stream-error "non-functional argument to stream-for-each"))
        ((null? strms) (stream-error "no stream arguments to stream-for-each"))
        ((not (all stream? strms)) (stream-error "non-stream argument to stream-for-each"))
        (else (let loop ((strms strms))
                (if (not (any stream-null? strms))
                    (begin (apply proc (map stream-car strms))
                           (loop (map stream-cdr strms))))))))

;; STREAM-FILTER pred? stream -- new stream including only items passing pred?
(define (stream-filter pred? strm)
  (cond ((not (procedure? pred?)) (stream-error "non-functional argument to stream-filter"))
        ((not (stream? strm)) (stream-error "attempt to apply stream-filter to non-stream"))
        (else (stream-unfoldn
                (lambda (s)
                  (values
                    (stream-cdr s)
                    (cond ((stream-null? s) '())
                          ((pred? (stream-car s)) (list (stream-car s)))
                          (else #f))))
                strm
                1))))