cs15100: A Racket repository from caocao485

类型签名：(: name : type) （:print-type expression）
1. 匿名函数的标识和struct的标识与一般的不同
```
(lambda ([param-1 : ty-1] ... [param-n : ty-n]) expr)
```
基本类型：
1. Natural
2. Integer
3. Exact-Rational
4. Real
5. Number
6. Boolean
7. String
8. Symbol
作用：减少错误、性能优化、利于阅读
词法作用域：静态作用域
union type： (: U T_1 T_2 ... T_k)

single type：枚举类型

(: R : 'red)
(define R 'red)

This kind of type is called a singleton type, since it is a type that has 
exactly one value. By combining multiple singleton types in a union, 
we can describe enumeration types. For example, our color type is 
the union of 'red, 'green, and 'blue. Two other useful singleton 
types are #t and #f (as one might guess, Boolean is the union of #t and #f).

(: color-to-string : (U 'red 'green 'blue) -> String)
;; convert a color to a string
(define (color-to-string c)
  (cond
    [(symbol=? c 'red) "red"]
    [(symbol=? c 'blue) "blue"]
    [else "green"]))

Type definitions：

;; A color is one of 'red, 'blue, or 'green.
we can give an explicit definition using the define-type construct

(define-type Color (U 'red 'blue 'green))

Subtyping
```
使用U
```
Pattern-matching gotchas：

Patterns and pattern matching
递归类型

多态函数和多态类型

(All (A B) (Two-Things A B) -> (Two-Things B A))
is called universal quantification and it means 
"for all types A and B a function from (Two-Things A B) to (Two-Things B A)". 
If we apply swap to a value of type (Two-Things Integer Boolean)

高阶函数
Listof和Vectorof
命令式：

(void)：
ctrl + i 格式化代码
optional，要么空，要么有值。maybe，maybe monad
将现存类型命名成新类型
match*：模式匹配多个值
match：== 相等扩展

https://blog.csdn.net/yeswenqian/article/details/22291675

boxOf

cast

caocao485/cs15100