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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