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).
如果我们要设计这样的系统的话，在 integer 中定义
(define (raise x)
(make-rational x 1))
(put 'raise 'integer raise)
而在 rational 中定义
(define (raise x)
(make-real (/ (numer x) (denom x))))
(put 'raise 'rational raise)
在 real 中定义
(define (raise x)
(make-complex-from-real-imag x 0))
(put 'raise 'real raise)
最终的泛型 raise 可以如下定义：
(define (raise x)
((get 'raise (type-tag x)) (contents x)))