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> From: Stephen McCracken <samccpa@xxxxxxxxx> > The reference implementation of stream-partition > appears to evaluate the entire stream before returning > any results. As such, it has no advantages over a > function that operates on lists. > ;; Obvious quick fix: This tests each element twice, > ;; and it may have problems with space retention. > (define (stream-partition pred? strm) > (values > (stream-filter pred? strm) > (stream-filter (lambda (x) (not (pred? x))) > strm))) > It might be possible to write a stream-partition that > was incremental but still tested each element only > once. Here's one untested way to do so: (define (stream-partition pred? strm) ;; Filter2 returns a stream of all the elements of stream VALS for ;; which the corresponding element of stream BOOLS is not #f. (define (filter2 vals bools) (cond ((stream-null? vals) stream-null) ((stream-car bools) (stream-cons (stream-car vals) (filter2 (stream-cdr vals) (stream-cdr bools)))) (else (filter2 (stream-cdr vals) (stream-cdr bools))))) (let ((bools (stream-map pred? strm))) (values (filter2 strm bools) (filter2 strm (stream-map not bools))))) -al