3.2 Maybe

8.6

3.2 Maybe

Nil的安全类型，在类型上避免空指针错误。

3.2.1 Maybe类型定义

procedure
(Maybe/c a) → contract?
a : any/c

“泛型”maybe容器，检测Just是否为a。

procedure
(Just a) → (Maybe/c any/c)
a : any/c
(Nothing) → (Maybe/c any/c)

Maybe构造器。同时可用于match。

value
nothing : (Maybe/c any/c)

已确定的Maybe的值。无须再调用生成。

3.2.2 操作

procedure
(maybe? x) → boolean?
x : any/c

检测是否Maybe。

procedure
(->maybe x) → (Maybe/c x)
x : any/c

将任意一个值转化成Maybe。Racket不像其他语言有个特殊的Nil，它用#f表示“空”，因此该函数仅会将#f转化为nothing，其他一切都为Just。

这引来一个问题，那么我们该如何得到Just #f呢？我的答案是直接调用(Just #f)。

Examples:

> (->maybe 1)
(Just 1)
> (->maybe 0)
(Just 0)
> (->maybe #f)
(Nothing)
> (Just #f)
(Just #f)
> (->maybe #t)
(Just #t)

procedure
(maybe-map f a) → (Maybe/c any/c)
f : (-> any/c any/c)
a : (Maybe/c any/c)

Functor映射。

Examples:

> (maybe-map add1 (Just 1))
(Just 2)
> (maybe-map add1 nothing)
(Nothing)

procedure
(maybe-then f a) → (Maybe/c any/c)
f : (-> any/c (Maybe/c any/c))
a : (Maybe/c any/c)

Monad的binding。

Examples:

> (define (f x)
(Just (add1 x)))
> (maybe-then f (Just 1))
(Just 2)
> (maybe-then f nothing)
(Nothing)

procedure
(maybe-filter f ma) → (Maybe/c a)
f : (-> a boolean?)
ma : (Maybe/c a)

类似于filter，f返回#f，整个结果就成了nothing。

Examples:

> (maybe-filter even? (Just 2))
(Just 2)
> (define (gt xs)
(> (length xs) 5))
> (maybe-filter gt (Just (list 1 2 3)))
(Nothing)
> (maybe-filter gt (Just (list 1 2 3 4 5 6)))
(Just '(1 2 3 4 5 6))

procedure
(maybe-replace x ma) → (Maybe/c)
x : any/c
ma : (Maybe/c any/c)

替换Just的值，遇到nothing则不变。

Examples:

> (maybe-replace 1 (Just 2))
(Just 1)
> (maybe-replace "hello" (Just 1))
(Just "hello")
> (maybe-replace "hello" nothing)
(Nothing)

procedure
(maybe-and ma mb) → (Maybe/c)
ma : (Maybe/c any/c)
mb : (Maybe/c)

类似于and。ma、mb其中之一是nothing，结果就是nothing，反之返回mb。

Examples:

> (maybe-and (Just 1) nothing)
(Nothing)
> (maybe-and (Nothing) (Just 2))
(Nothing)
> (maybe-and (Just 1) (Just 2))
(Just 2)

procedure
(maybe-or ma mb) → (Maybe/c any/c)
ma : (Maybe/c any/c)
mb : (Maybe/c any/c)

类似于or。见maybe-and。

Examples:

> (maybe-or (Just 1) nothing)
(Nothing)
> (maybe-or (Nothing) (Just 2))
(Nothing)
> (maybe-or (Just 1) (Just 2))
(Just 1)

procedure
(maybe-alt ma mb) → (Maybe/c any/c)
ma : (Maybe/c any/c)
mb : (Maybe/c any/c)

只有当ma、mb都为nothing，结果才为nothing。

Examples:

> (maybe-alt (Just 1) (Just 2))
(Just 1)
> (maybe-alt (Just 1) (Nothing))
(Just 1)
> (maybe-alt nothing (Just 2))
(Just 2)

procedure
(maybe-else a f ma) → any/c
  a : any/c
  f : (-> any/c any/c)
  ma : (Maybe/c any/c)

根据条件选择对应的回调函数：Just调用f；nothing则返回a。

Examples:

> (maybe-else 10 add1 (Just 1))
2
> (maybe-else 10 add1 nothing)
10

procedure
(maybe-> x f) → (or/c x any/c)
x : any/c
f : (Maybe/c any/c)

同maybe，接受一个默认值x，如果f为Just，则直接获取Just内容，反之以x代之。

Examples:

> (maybe-> 1 nothing)
1
> (maybe-> 1 (Just 2))
2

procedure
(maybe-unwrap x) → y
x : (Maybe/c y)

解包Maybe，遇到nothing直接抛异常。

Examples:

> (maybe-unwrap (Just 1))
1
> (maybe-unwrap (Just #f))
#f
> (maybe-unwrap nothing)
maybe-unwrap: 试图解包nothing！

语法
(maybe-catch expr)

expr = 任何表达式

自动捕获expr错误，如果expr无任何错误，返回"(Just expr)"，反之返回nothing。

Examples:

> (maybe-catch (* 1 0))
(Just 0)
> (maybe-catch (/ 1 0))
(Nothing)

1	简介
2	新朝雅语
3	数据结构
4	标准库

3.1	JSON
3.2	Maybe
3.3	列表
3.4	函数