SICP 习题2.1 make-rat 处理整数和负数版本

(define make-rat
  (lambda (n d) 
    (let ((n1 (if (or (and (> n 0) (< d 0)) (and (< n 0) (> d 0))) -1 1))
      (d1 1)
      (n2 (if (> n 0) n (* -1 n)))
      (d2 (if (> d 0) d (* -1 d))))
        (let ((g (gcd n2 d2)))
      (cons (* n1 (/ n2 g)) (* d1 (/ d2 g)))))))

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

(define numer car)
(define denom cdr)

(define one-half (make-rat 1 2))

(print-rat one-half)
(print-rat (make-rat 2 -3))
(print-rat (make-rat -2 3))
(print-rat (make-rat -2 -3))