scheme - Racket - lang plai - 定义类型和类型案例解释

Question

有人可以尝试解释这两个函数: Racket PLAI 方案中的“define-type”和“type-case”吗？我是一个菜鸟程序员，我不太了解 Racket 网站上的文档。如果有人可以提供示例，将不胜感激。谢谢。

Answer 1

这是一个如何使用define-type 和type-case 的小例子:

#lang plai

; A ListOfNumbers are either
; is either an empty list of numbers
; or is constructed to two things a, and, d,
; where a is a number and d is a list of numbers.

(define-type ListOfNumbers
  (Empty)
  (Cons (a number?) (d ListOfNumbers?)))

; construct a list of numbers as an example
(define a-list (Cons 42 (Cons 43 (Empty))))    
a-list   ; prints: (Cons 42 (Cons 43 (Empty)))

(type-case ListOfNumbers a-list
  (Empty ()     "the list is empty")
  (Cons (a d)  (~a "the first number in the list is " a)))
; prints: "the first number in the list is 42"