Scheme Kata のつづき。多値でクイックソート。filter からは逃れられず。

(define (quicksort/values ls)
  (if (null? ls)
      '()
      (call-with-values
          (lambda ()
            (values
             (filter (lambda (x) (> (car ls) x)) (cdr ls))
             (list (car ls))
             (filter (lambda (x) (<= (car ls) x)) (cdr ls))))
        (lambda (low x high)
          (append (quicksort/values low)
                  x
                  (quicksort/values high))))))