sicp每日一题[2.80]

Exercise 2.80

Define a generic predicate =zero? that tests if its argument is zero, and install it in the generic arithmetic package. This operation should work for ordinary numbers, rational numbers, and complex numbers.

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这道题更简单，直接与0相比较就行，以下代码在 2.79 的基础上修改

(define (=zero? x) (apply-generic '=zero? x))

; 一般数字
(put '=zero? '(scheme-number) (lambda (x) (= x 0)))

; 有理数
(put '=zero? '(rational) (lambda (x) (= (numer x) 0)))

; 复数，这里我有点奇怪，用 (= (magnitude z) 0) 实现居然检查不出来，我也没找到原因是啥
(put '=zero? '(complex) (lambda (z) (= (real-part z) (imag-part z) 0)))

(install-scheme-number-package)
(define n1 (make-scheme-number 0))
(define n2 (make-scheme-number 10))
(=zero? n1)
(=zero? n2)

(newline)
(install-rational-package)
(define r1 (make-rational 5 10))
(define r2 (make-rational 0 20))
(=zero? r1)
(=zero? r2)

(newline)
(install-rectangular-package)
(install-polar-package)
(install-complex-package)
(define z1 (make-complex-from-real-imag 3 4))
(define z2 (make-complex-from-real-imag 0 0))
(=zero? z1)
(=zero? z2)

; 执行结果
'done
#t
#f

'done
#f
#t

'done
'done
'done
#f
#t