Type

F# programs are built around types. With types, we group related pieces of data together. We add behavior (in functions and properties) to our types.

When specifying a type, we can add initialization logic. This ensures each instance of the type is valid. We can specify members of a type.

First example

Here we introduce an Animal type. To create an animal we must specify an int for its size (this is like its constructor).

Info The "mutable size" is a field on the Animal type. It is initialized to the value passed to the Animal at creation time.

Note When initialized, the Animal will execute its "do" statements. Here it prints a helpful message.

Note 2 The Weight is a method on the Animal. It returns an int. It returns the size times 10.

type Animal(s : int) =
    // Store the argument in the object.
    let mutable size = s

    // Write a helpful message.
    do printfn "Animal created, size = %A" size

    // For Weight, return size times 10, an int.
    member x.Weight : int = size * 10

// Create an Animal with size 5.
let animal = Animal 5

// Get the Weight of the animal.
let weight = animal.Weight
printfn "%A" weightAnimal created, size = 5
50

Get, set

Properties help us control how a type is used. Here we introduce a type Box that has a Color property. With the get() and set() keywords we add accessors.

Info We use the "with" keyword to introduce the get() accessor. This returns the value of the "color" mutable field.

Detail In F# we introduce a setter with "and set." The "and" is important. We store the argument "c" in the color field.

type Box() =
    // A mutable color field.
    let mutable color : string = null

    // A Box has a color property with get and set.
    member x.Color
        with get() = color
        and set(c) = color <- c

// Create a Box instance.
let x = Box()

// Set and get color values.
x.Color <- "blue"
printfn "%A" x.Color

// Set another value.
x.Color <- "red"
printfn "%A" x.Color
"blue"
"red"

Typeof

This is an operator that returns the System.Type instance for the specified type. We must place a type name within the angle brackets.

Here We create a simple type called Color with one field. With typeof we get the System.Type.

// A type with one field.
type Color() =
    let mutable Value = null

// Get typeof Color type.
// ... This is an instance of System.Type.
let t = typeof<Color>
printfn "%A" t
Program+Color

In F# we can use tuples and records to store data. But for important, often-used information, we use types. This gives us a way to add helpful methods and validation.