同处理 gcd 一样，我们也只需要处理 make-rat 就可以让分数处理正负数变得规范。

;;; a better version of make-rat that can handle sign: normalize the
;;; sign that if the rational number is positive, both the numerator
;;; and denominator are positive, and if the rational number is
;;; negative, only the numerator is negative.

(define (make-rat n d)
  (define (gcd a b)
    (if (= b 0)
        a
        (gcd b (remainder a b))))
  (let* ((n-and-d
          (cond ((and (< n 0) (< d 0)) (cons (- 0  n) (- 0  d)))
                ((< d 0) (cons (- 0 n) (- 0 d)))
                (else (cons n d))))
         (n (car n-and-d))
         (d (cdr n-and-d))
         (g (abs (gcd n d))))
    (cons (/ n g)
          (/ d g))))

(define (numer x) (car x))

(define (denom x) (cdr x))

然后我们对输出进行测试

(load "../testframe.scm")

(define (print-rat x)
  (newline)
  (display (numer x))
  (display "/")
  (display (denom x)))

(assert-output-equal "\n1/3" (print-rat (make-rat -2 -6)))
(assert-output-equal "\n-1/3" (print-rat (make-rat -2 6)))
(assert-output-equal "\n-1/3" (print-rat (make-rat 2 -6)))
(assert-output-equal "\n1/3" (print-rat (make-rat 2 6)))