DouKaku?

#!/usr/bin/env gosh
(define-class <b> ()
  ((b :init-keyword :b :accessor b-b)))

(define-method write-object ((b <b>) port)
  (let* ((j (b-b b))
         (k 1000.0)
         (l '(byte k M G T P E Z Y))
         (m (min (truncate->exact (/ (log j) (log k)))
                 (- (length l) 1))))
    (if (< j k)
        (display j)
        (display (/ (truncate (* (/ j (expt k m)) 10)) 10)))
    (print (list-ref l m))))

(define (main args)
  (display "input: ")(flush)
  (print (make <b> :b (x->number(read)))))