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Phil Bewig <pbewig@xxxxxxxxxx> writes: > On Wednesday, February 12, 2003 10:17 AM, Richard Kelsey [SMTP:kelsey@xxxxxxx] wrote: > > Everyone who has seen a preliminary draft of this SRFI had the same > > comment that it should be split into "essential" and "library" pieces, > > but all had different ideas of where to make the split. > > > > I don't care too much about where the split happens as long as the > > core part isn't too big. A dozen or so procedures and macros should > > be enough to be useful while not being overwhelming. > > 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))) Other stream generators are then easily defined using unfold, as e.g. (define (stream-from n) (stream-unfold (lambda (s) (cons s (+ s 1))) n)) (define (stream-from-to n m) (stream-unfold (lambda (s) (and (<= s m) (cons s (+ s 1)))) n)) (define (stream-iterate f a) (stream-unfold (lambda (s) (cons s (f s))) a)) (define (list->stream l) (stream-unfold (lambda (l) (and (not (null? l)) l)) l)) ... Cheers Matthias -- Matthias Neubauer | Universität Freiburg, Institut für Informatik | tel +49 761 203 8060 Georges-Köhler-Allee 79, 79110 Freiburg i. Br., Germany | fax +49 761 203 8052