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2. 学习F＃
3. F＃ - 可区分联合( Discriminated Unions)

F＃ - 可区分联合( Discriminated Unions)

联合会或有区别的联合允许您构建代表明确定义的选择的复杂数据结构。 例如，您需要构建一个choice变量的实现，它有两个值yes和no。 使用联合工具，您可以设计它。

语法 (Syntax)

使用以下语法定义受歧视的联合 -

type type-name =
   | case-identifier1 [of [ fieldname1 : ] type1 [ * [ fieldname2 : ] 
type2 ...]
   | case-identifier2 [of [fieldname3 : ]type3 [ * [ fieldname4 : ]type4 ...]
...

我们的简单实现， choice,将如下所示 -

type choice =
   | Yes
   | No

以下示例使用类型选项 -

type choice =
   | Yes
   | No
let x = Yes (* creates an instance of choice *)
let y = No (* creates another instance of choice *)
let main() =
   printfn "x: %A" x
   printfn "y: %A" y
main()

编译并执行程序时，它会产生以下输出 -

x: Yes
y: No

例子1 (Example 1)

以下示例显示了将电压设置为高电平或低电平的电压状态的实现 -

type VoltageState =
   | High
   | Low
let toggleSwitch = function (* pattern matching input *)
   | High -> Low
   | Low -> High
let main() =
   let on = High
   let off = Low
   let change = toggleSwitch off
   printfn "Switch on state: %A" on
   printfn "Switch off state: %A" off
   printfn "Toggle off: %A" change
   printfn "Toggle the Changed state: %A" (toggleSwitch change)
main()

编译并执行程序时，它会产生以下输出 -

Switch on state: High
Switch off state: Low
Toggle off: High
Toggle the Changed state: Low

例子2 (Example 2)

type Shape =
   // here we store the radius of a circle
   | Circle of float
   // here we store the side length.
   | Square of float
   // here we store the height and width.
   | Rectangle of float * float
let pi = 3.141592654
let area myShape =
   match myShape with
   | Circle radius -> pi * radius * radius
   | Square s -> s * s
   | Rectangle (h, w) -> h * w
let radius = 12.0
let myCircle = Circle(radius)
printfn "Area of circle with radius %g: %g" radius (area myCircle)
let side = 15.0
let mySquare = Square(side)
printfn "Area of square that has side %g: %g" side (area mySquare)
let height, width = 5.0, 8.0
let myRectangle = Rectangle(height, width)
printfn "Area of rectangle with height %g and width %g is %g" height width (area myRectangle)

编译并执行程序时，它会产生以下输出 -

Area of circle with radius 12: 452.389
Area of square that has side 15: 225
Area of rectangle with height 5 and width 8 is 40