无涯教程-F# - 联合(Unions)

联合(Unions)或有区别的联合(Discriminated Unions)使您可以创建明确的选择集的复杂数据结构。如，您需要构建 choice 变量的实现，该变量具有两个值yes和no，使用联合(Unions)工具，可以对此进行设计。

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

简单实现如下所示-

type choice =
   | Yes
   | No

以下示例使用类型选择-

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，将产生以下输出-

x: Yes
y: No

示例1

以下示例显示了将状态设置为高(High)或低(Low)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，将产生以下输出-

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

示例2

type Shape =
   //这里存储一个圆的半径
   | Circle of float
//这里存储边长。
| Square of float
//这里存储高度和宽度。
| 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，将产生以下输出-

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

参考链接

www.learnfk.com/fsharp/fsha…