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Unions, or discriminated unions allows you to build up complex data structures representing well-defined set of choices. For example, you need to build an implementation of a choice variable, which has two values yes and no. Using the Unions tool, you can design this.
Syntax
Discriminated unions are defined using the following syntax −
type type-name = | case-identifier1 [of [ fieldname1 : ] type1 [ * [ fieldname2 : ] type2 ...] | case-identifier2 [of [fieldname3 : ]type3 [ * [ fieldname4 : ]type4 ...] ...
Our simple implementation of ,choice, will look like the following −
type choice = | Yes | No
The following example uses the type choice −
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()
When you compile and execute the program, it yields the following output −
x: Yes y: No
Example 1
The following example shows the implementation of the voltage states that sets a bit on high or low −
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()
When you compile and execute the program, it yields the following output −
Switch on state: High Switch off state: Low Toggle off: High Toggle the Changed state: Low
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)
When you compile and execute the program, it yields the following output −
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
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