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Re: some suggestions

This page is part of the web mail archives of SRFI 40 from before July 7th, 2015. The new archives for SRFI 40 contain all messages, not just those from before July 7th, 2015.

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))