;;; do optimize fx< fx<= fx= fx> fx>=
;;; do optimize fxzero?
;;; do optimize fxadd1 fxsub1 fx+ fx- 
(module (s-car s-cdr (s-cons make-stream) stream? s-head)
  (import scheme)
  (define-record stream (car cdr))
  (define (s-car x)
    (if (stream? x)
        ((stream-car x))
        (error 's-car "~s is not a stream" x)))
  (define (s-cdr x)
    (if (stream? x)
        ((stream-cdr x))
        (error 's-cdr "~s is not a stream" x)))
  (define-syntax s-cons
    (syntax-rules ()
      [(_ a d) (make-stream (lambda () a) (lambda () d))]))
  (define (s-head n s)
    (unless (and (fixnum? n) (fx>= n 0))
      (error 's-head 
             "length must be a non-negative fixnum, got ~s"
             n))
    (unless (stream? s)
      (error 's-head "~s is not a stream" s))
    (let f ([n n] [s s] [ac '()])
      (cond
        [(fxzero? n) (reverse ac)]
        [else 
         (f (fx- n 1) (s-cdr s) (cons (s-car s) ac))]))))


  (define (going-up x lim)
    (cond ((= x lim) (s-cons x (going-down (- x 1) (+ lim 1))))
          (else (s-cons x (going-up (+ x 1) lim)))))
  (define (going-down x lim)
    (cond ((= x 0) (going-up 1 lim))
          (else (s-cons x (going-down (- x 1) lim)))))
  (define s (going-up 1 1))

#!eof

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Subject: Re: Stream problem
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From: Andru Luvisi <luv...@andru.sonoma.edu>
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Several of the answers that have been posted involve using reverse,
which has the disadvantage that the later you go in the stream, the
more of it you will need to hold in ram if you are processing and
discarding elements as you go.

  (define (going-up x lim)
    (cond ((= x lim) (s-cons x (going-down (- x 1) (+ lim 1))))
          (else (s-cons x (going-up (+ x 1) lim)))))
  (define (going-down x lim)
    (cond ((= x 0) (going-up 1 lim))
          (else (s-cons x (going-down (- x 1) lim)))))
  (define s (going-up 1 1))

Andru
-- 
Andru Luvisi

Quote Of The Moment:
  Quidquid latine dictum sit, altum viditur.
  ( Whatever is said in Latin sounds profound. )