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>>>>> "Matze" == Matthias Neubauer <neubauer@xxxxxxxxxxxxxxxxxxxxxxxxxx> writes: [snip] Matze> Another example is the definition of STREAM-FILTER. Suppose you have a Matze> non-leaking implementation of STREAM-DROP-UNTIL at hand , the Matze> following simple definition of STREAM-FILTER should (hopfully) also Matze> make sense: Matze> (define (stream-filter pred? stream) Matze> (stream-unfold (lambda (stream) Matze> (let ((stream-1 (stream-drop-until pred? stream))) Matze> (cons (stream-car stream-1) Matze> (stream-cdr stream-1)))) Matze> stream)) Matze> Here the "state" that is passed from step to step is the remainder of Matze> the original stream that still has to be processed for the next Matze> element. The actual processing in each step is then performed by Matze> calling STREAM-DROP-UNTIL. [snip] Matze>  Which isn't the case with the current reference implementation I Matze> just realized. Your STREAM-DROP-UNTIL bares the same space problems as Matze> the naive STREAM-FILTER implementation does. Your code above leaks space even if STREAM-DROP-UNTIL doesn't: the closure handed out to STREAM-UNFOLD holds a reference to STREAM. This isn't just a nitpick: I think it shows that using STREAM-UNFOLD doesn't gain you as much simplicity as you might would expect. This doesn't add anything to the STREAM-UNFOLD vs. STREAM-ITERATE debate of course. -- Martin