(use srfi-1)
(define (collatz n)
  (unfold (lambda (s)
            (= s 1))
          (lambda (s)
            s)
          (lambda (s)
            (cond ((even? s)(/ s 2))
                  ((odd? s)(+ (* s 3) 1))
                  (else s)))
          n
          (lambda (s)
            (cons s '()))))
(collatz 10)
; -> (10 5 16 8 4 2 1)