sicp每日一题[2.83]

Exercise 2.83

Suppose you are designing a generic arithmetic system for dealing with the tower of types shown in Figure 2.25: integer, rational, real, complex. For each type (except complex),
design a procedure that raises objects of that type one level in the tower. Show how to install a generic raise operation that will work for each type (except complex).

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这道题挺简单的，只要在每个包里用下一级的构造方法生成符合下一级规则的数就行。因为有理数本身就可以直接看成实数，所以我没有写有理数到实数和实数到复数的转换过程，这样直接在 2.81 的基础上修改就行了。

(define (raise x) (apply-generic 'raise x))

; interger to rational，在 scheme-number 包添加如下代码
(put 'raise '(scheme-number) (lambda (x) make-rational x 1))

; rational to complex，在 rational 包添加如下代码
(put 'raise '(rational) (lambda (x) (make-complex-from-real-imag x 0)))

; 如果要实现有理数到实数的转换，只要将有理数转换为分子分母相除再乘以 1.0 即可