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On Thursday, February 13, 2003 11:26 AM, Matthias Neubauer [SMTP:neubauer@xxxxxxxxxxxxxxxxxxxxxxxxxx] wrote:
> Phil Bewig <pbewig@xxxxxxxxxx> writes:
>
> > Certainly NULL, CONS, CAR, CDR, NULL?, PAIR? and LAMBDA must be
> > in the core, since they all necessarily touch the underlying representation of
> > streams. You probably want STREAM to build finite streams and ITERATE
> > to build infinite streams, and REF to access items other than the first.
> > MAP, FOR-EACH, FILTER and FOLD-LEFT are useful in their own right and
> > also as building blocks for numerous other functions. That's fourteen. Does
> > that make a useful core?
>
> To my mind, the most intrinsic stream abstraction ever is still
> missing in this srfi: UNFOLDR. UNFOLDR is for streams, what FOLDR is
> for lists. Whereas it is a highly common task to consume a list to a
> single value using a consumer function---typically done by using
> FOLDR---, the common pattern for streams is generating a stream
> starting from an initial seed value using a generator function.
>
> And that's what UNFOLDR is all about!
>
> So I vote for putting UNFOLDR in the core srfi:
> (Be warned: the following code is completely untested und unoptimized!)
>
> ;; stream-unfold : (b -> (union (cons a b) #f)) b -> stream(a)
> (stream-define (stream-unfold gen-fun seed)
> (let ((res (gen-fun seed)))
> (if res
> (stream-cons (car res)
> (stream-unfold gen-fun (cdr res)))
> stream-null)))
I think the STREAM-ITERATE in the core does exactly what
your STREAM-UNFOLD does, and certainly your STREAM-FROM
and STREAM-FROM-TO examples can be written based on
STREAM-ITERATE. And, given that STREAM is in the core, your
LIST->STREAM can be written
(define (list->stream lst) (apply stream lst))
Phil