05.6 结构类型的生成性

5.6 结构类型的生成性

每次对一个struct表求值时，它就生成一个与所有现有结构类型不同的结构类型，即使某些其他结构类型具有相同的名称和字段。

这种生成性对执行抽象和执行程序是有用的，就像口译员，但小心放置struct表被多次求值的位置。

例如：

(define (add-bigger-fish lst)
  (struct fish (size) #:transparent) ;每次都生成新的
  (cond
   [(null? lst) (list (fish 1))]
   [else (cons (fish (* 2 (fish-size (car lst))))
               lst)]))

> (add-bigger-fish null)
(list (fish 1))

> (add-bigger-fish (add-bigger-fish null))
fish-size: contract violation;
 given value instantiates a different structure type with
the same name
  expected: fish?
  given: (fish 1)

(struct fish (size) #:transparent)
(define (add-bigger-fish lst)
  (cond
   [(null? lst) (list (fish 1))]
   [else (cons (fish (* 2 (fish-size (car lst))))
               lst)]))

> (add-bigger-fish (add-bigger-fish null))
(list (fish 2) (fish 1))