F# Quick Guides: Object Oriented Programming

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Hello! As I mentioned in my earlier post, the F# team is tremendously proud of being a part of Visual Studio 2010. We have worked long and hard to get to this point, and we are very excited with the release of the first fully supported F# language.

To kick off the release of F#, I wanted to share with you a few quick guides to F#. The point of the quick guides is not to be a comprehensive language reference, but more of a “I know how to do this in C#, but how do I do it in F#” guide. As a result, the guides will be organized in table format, with F# code in the left, and corresponding C# code on the right.

The first guide will be about Object Oriented Programming in F#. You can download a copy of the F# Object Oriented Programming Quick Guide to make it easy for offline browsing. In addition, I’ve provided the F# source code for all the examples on the F# Quick Guides Code Gallery page.

Without further ado, here is the F# Quick Guide for Object Oriented Programming.

Language Feature

F#

C#

Classes with properties and default constructor

type Vector(x : float, y : float) =

    member this.X = x

    member this.Y = y

 

// Usage:

let v = Vector(10., 10.)

let x = v.X

let y = v.Y

public class Vector

{

    double x;

    double y;

 

    public Vector(double x, double y)

    {

        this.x = x;

        this.y = y;

    }

    public double X

    {

        get { return this.x; }

    }

    public double Y

    {

        get { return this.y; }

    }

}

 

// Usage:

Vector v = new Vector(10, 10);

double x = v.X;

double y = v.Y;

Structs with properties

[<Struct>]

type Vector(x : float, y : float) =

    member this.X = x

    member this.Y = y

 

// Usage:

let v = Vector(10., 10.)

let x = v.X

let y = v.Y

public struct Vector

{

    double x;

    double y;

 

    public Vector(double x, double y)

    {

        this.x = x;

        this.y = y;

    }

    public double X

    {

        get { return this.x; }

    }

    public double Y

    {

        get { return this.y; }

    }

}

 

// Usage:

Vector v = new Vector(10, 10);

double x = v.X;

double y = v.Y;

Multiple constructors

type Vector(x : float, y : float) =

    member this.X = x

    member this.Y = y

    new(v : Vector, s) = Vector(v.X * s, v.Y * s)

 

// Usage:

let v = Vector(10., 10.)

let w = new Vector(v, 0.5)

public class Vector

{

    double x;

    double y;

    public Vector(double x, double y)

    {

        this.x = x;

        this.y = y;

    }

    public Vector(Vector v, double s) :

        this(v.x * s, v.y * s)

    {

    }

}

 

// Usage:

Vector v = new Vector(10, 10);

Vector w = new Vector(v, 0.5);

Member functions

type Vector(x : float, y : float) =

    member this.Scale(s : float) =

        Vector(x * s, y * s)

 

// Usage:

let v = Vector(10., 10.)

let v2 = v.Scale(0.5)

public class Vector

{

    double x;

    double y;

 

    public Vector(double x, double y)

    {

        this.x = x;

        this.y = y;

    }

    public double Scale(double s)

    {

        return new Vector(this.x * s,

            this.y * s);

    }

}

 

// Usage:

Vector v = new Vector(10, 10);

Vector v2 = v.Scale(0.5);

Operators

type Vector(x, y) =

    member this.X = x

    member this.Y = y

    static member (*) (a : Vector, b : Vector) =

        a.X * b.X + a.Y + b.Y

 

// Usage:

let x = Vector(2., 2.)

let y = Vector(3., 3.)

let dp = x * y

 

public class Vector

{

    double x;

    double y;

    public Vector(double x, double y)

    {

        this.x = x;

        this.y = y;

    }

    public double X

    {

        get { return this.x; }

    }

    public double Y

    {

        get { return this.y; }

    }

    public static double operator * (

        Vector v1, Vector v2)

    {

        return v1.x * v2.x + v1.y * v2.y;

    }

}

 

// Usage:

Vector x = new Vector(2, 2);

Vector y = new Vector(3, 3);

double dp = x * y;

Static members and properties

type Vector(x, y) =

    member this.X = x

    member this.Y = y

    static member Dot(a : Vector, b : Vector) =

        a.X * b.X + a.Y + b.Y

    static member NormX = Vector(1., 0.)

 

// Usage:

let x = Vector(2., 2.)

let y = Vector.NormX

let dp = Vector.Dot(x, y)

public class Vector

{

    double x;

    double y;

    public Vector(double x, double y)

    {

        this.x = x;

        this.y = y;

    }

    public double X

    {

        get { return this.x; }

    }

    public double Y

    {

        get { return this.y; }

    }

    public static double Dot(Vector v1,

        Vector v2)

    {

        return v1.x * v2.x + v1.y * v2.y;

    }

    public static Vector NormX

    {

        get { return new Vector(1, 0); }

    }

}

 

// Usage:

Vector x = new Vector(2, 2);

Vector y = Vector.NormX;

double dp = Vector.Dot(x, y);

Class properties that use let value computations in the constructor

type Vector(x, y) =

    let mag = sqrt(x * x + y * y)

    let rad = if x = 0. && y = 0. then

                  0.

              else if x >= 0. then

                  asin(y / mag)

              else

                  (-1. * asin(y / mag)) +

                      Math.PI

    member this.Mag = mag

    member this.Rad = rad

 

// Usage:

let v = Vector(10., 10.)

let mag = v.Mag

let rad = v.Rad

 

public class Vector

{

    double mag = 0.0;

    double rad = 0.0;

    public Vector(double x, double y)

    {

        this.mag = Math.Sqrt(x * x + y * y);

        if (x == 0.0 && y == 0.0) rad = 0.0;

        else if (x >= 0.0)

            rad = Math.Asin(y / mag);

        else

            rad = (-1.0 * Math.Asin(y / mag)) + Math.PI;

    }

    public double Mag

    {

        get { return this.mag; }

    }

    public double Rad

    {

        get { return this.rad; }

    }

}

 

// Usage:

Vector v = new Vector(10, 10);

double mag = v.Mag;

double rad = v.Rad;

Class members that use private function values

type Vector(x, y) =

    let rotate a =

        let x' = x * cos a – y * sin a

        let y' = y * sin a + y * cos a

        new Vector(x', y')

    member this.RotateByDegrees(d) =

        rotate (d * Math.PI / 180.)

    member this.RotateByRadians(r) =

        rotate r

 

// Usage:

let v = Vector(10., 0.)

let x = v.RotateByDegrees(90.)

let y = v.RotateByRadians(Math.PI / 6.)

 

public class Vector

{

    double x = 0.0;

    double y = 0.0;

    Vector rotate(double a)

    {

        double xx = this.x * Math.Cos(a) –

                    this.y * Math.Sin(a);

        double yy = this.y * Math.Sin(a) +

                    this.y * Math.Cos(a);

        return new Vector(xx, yy);

    }

    public Vector(double x, double y)

    {

        this.x = x;

        this.y = y;

    }

    public Vector RotateByDegrees(double d)

    {

        return rotate(d * Math.PI / 180);

    }

    public Vector RotateByRadians(double r)

    {

        return rotate(r);

    }

}

 

// Usage:

Vector v = new Vector(10, 10);

Vector x = v.RotateByDegrees(90.0);

Vector y = v.RotateByRadians(Math.PI / 6.0);

 

Overloading members

type Car() =

    member this.Drive() =

        this.Drive(10)

        ()

    member this.Drive(mph : int) =

        // Do something

        ()

 

// Usage:

let c = Car()

c.Drive()

c.Drive(10)

public class Car

{

    public void Drive()

    {

        Drive(10);

    }

    public void Drive(int mph)

    {

        // Do something

    }

}

 

// Usage:

Car c = new Car();

c.Drive();

c.Drive(10);

Mutable fields in a class with get/set properties

type MutableVector(x : float, y : float) =

    let mutable cx = x

    let mutable cy = y

    member this.X with get() = cx and

                       set(v) = cx <- v

    member this.Y with get() = cy and

                       set(v) = cy <- v

    member this.Length = sqrt(x * x + y * y)

 

// Usage:

let v = MutableVector(2., 2.)

let len1 = v.Length

v.X <- 3.

v.Y <- 3.

let len2 = v.Length

public class MutableVector

{

    public MutableVector(double x, double y)

    {

        this.X = x;

        this.Y = y;

    }

    public double X { get; set; }

    public double Y { get; set; }

    public double Length

    {

        get { return Math.Sqrt(

                  this.X * this.X +

                  this.Y * this.Y); }

    }

}

 

// Usage:

MutableVector v = new MutableVector(2.0, 2.0);

double len1 = v.Length;

v.X = 3.0;

v.Y = 3.0;

double len2 = v.Length;

Generic classes and function arguments

type Factory<'T>(f : unit -> 'T) =

    member this.Create() =

        f()

 

// Usage:

let strings = Factory<string>(

    fun () -> "Hello!")

let res = strings.Create()

let ints = Factory<int>(fun () -> 10)

let res = ints.Create()

 

public class Factory<T>

{

    Func<T> creator;

    public Factory(Func<T> f)

    {

        this.creator = f;

    }

    public T Create()

    {

        return this.creator();

    }

}

 

// Usage:

Factory<string> strings = new

    Factory<string>(() => "Hello");

string res1 = strings.Create();

Factory<int> ints = new Factory<int>(() => 10);

int res2 = ints.Create();

Generic classes and methods

type Container<'T>(a : 'T) =

    member this.Convert<'U>(f : 'T -> 'U) =

        f a

 

// Usage:

let c = new Container<int>(10)

let b = c.Convert(fun a -> a.ToString())

public class Container<T>

{

    private T value;

    public Container(T t)

    {

        this.value = t;

    }

    public U Convert<U>(Func<T, U> f)

    {

        return f(this.value);

    }

}

 

// Usage:

Container<int> c = new Container<int>(10);

string result = c.Convert(n => n.ToString());

Extension methods

type List<'T> with

    member this.MyExtensionMethod() =

        this |> Seq.map (fun a -> a.ToString())

 

// Usage:

let c = [1; 2; 3]

let d = c.MyExtensionMethod()

public static class ExtensionMethods

{

    public static IEnumerable<string>

        MyExtensionMethod<T>(this List<T> a)

    {

        return a.Select(s => s.ToString());

    }

}

 

// Usage:

List<int> l = new List<int> { 1, 2, 3 };

IEnumerable<string> res =

    l.MyExtensionMethod();

 

 

Extension properties

type List<'T> with

    member this.MyExtensionProp =

        this |> Seq.map (fun a -> a.ToString())

 

// Usage:

let c = [1; 2; 3]

let d = c.MyExtensionProp

N/A. C# does not support this feature.

Indexers

type Table() =

    member this.Item

        with get(key : string) = int key

 

// Usage:

let tab = Table()

let x = tab.["10"]

let y = tab.["12"]

public class Table

{

    public int this[string key]

    {

        get { return Convert.ToInt32(key); }

    }

}

 

// Usage:

Table tab = new Table();

int x = tab["10"];

int y = tab["12"];

Indexed Properties

type Table() =

    member this.Values

        with get(key : string) = int key

    member this.MultipleValues

        with get(key1, key2) = key1 + key2

// Usage:

let tab = Table()

let x = tab.Values("10")

let y = tab.Values("12")

let a = tab.MultipleValues(10, 5)

N/A. C# does not support this feature.

Abstract classes

[<AbstractClass>]

type Shape() =

    abstract Name : string with get

    abstract Scale : float -> Shape

public abstract class Shape

{

    public abstract string Name { get; }

    public abstract Shape Scale(double scale);

}

Derive from a base class and overriding base methods with generics

[<AbstractClass>]

type Shape<'T>() =

    abstract Name : string with get

    abstract Scale : float -> 'T

   

type Vector(angle, mag) =

    inherit Shape<Vector>()

    member this.Angle = angle

    member this.Mag = makg

    override this.Name = "Vector"

    override this.Scale(factor) =

        Vector(angle, mag * factor)

 

// Usage:

let v = Vector(45., 10.)

let v2 = v.Scale(0.5)

public abstract class Shape<T>

{

    public abstract string Name { get; }

    public abstract T Scale(double scale);

}

 

public class Vector : Shape<Vector>

{

    double angle;

    double mag;

    public double Angle {get{return angle;}}

    public double Mag {get{return mag;}}

    public Vector(double angle, double mag)

    {

        this.angle = angle;

        this.mag = mag;

    }

    public override string Name

    {

        get { return "Vector"; }

    }

    public override Vector Scale(double scale)

    {

        return new Vector(this.Angle,

            this.Mag * scale);

    }

}

// Usage:

Vector v = new Vector(45, 10);

Vector v2 = v.Scale(0.5);

Calling a base class method

type Animal() =

    member this.Rest() =

        // Rest for the animal

        ()

           

type Dog() =

    inherit Animal()

    member this.Run() =

        // Run

        base.Rest()

// Usage:

let brian = new Dog()

brian.Run()

public class Animal

{

    public void Rest()

    {

        // Rest for the animal

    }

}

public class Dog : Animal

{

    public void Run()

    {

        // Run

        base.Rest();

    }

}

// Usage:

Dog brian = new Dog();

brian.Run();

Implementing an interface

type IVector =

    abstract Mag : double with get

    abstract Scale : float -> IVector

       

type Vector(x, y) =

    interface IVector with

        member this.Mag = sqrt(x * x + y * y)

        member this.Scale(s) =

            Vector(x * s, y * s) :> IVector

    member this.X = x

    member this.Y = y

// Usage:

let v = new Vector(1., 2.) :> IVector

let w = v.Scale(0.5)

let mag = w.Mag

 

interface IVector

{

    double Mag { get; }

    IVector Scale(double s);

}

class Vector : IVector

{

    double x;

    double y;

    public Vector(double x, double y)

    {

        this.x = x;

        this.y = y;

    }

    public double X

    {

        get { return this.x; }

    }

    public double Y

    {

        get { return this.y; }

    }

    public double Mag

    {

        get { return Math.Sqrt( this.x * this.x +

                this.y * this.y); }

    }

    public IVector Scale(double s)

    {

        return new Vector(this.x * s,

            this.y * s);

    }

}

// Usage:

IVector v = new Vector(1, 2);

IVector w = v.Scale(0.5);

double mag = w.Mag;

Implementing an interface with object expressions

type ICustomer =

    abstract Name : string with get

    abstract Age : int with get

       

let CreateCustomer name age =

    { new ICustomer with

        member this.Name = name

        member this.Age = age

    }

// Usage:

let c = CreateCustomer "Snoopy" 16

let d = CreateCustomer "Garfield" 5

N/A. C# does not support creating object expressions.

Events

type BarkArgs(msg:string) =

    inherit EventArgs()

    member this.Message = msg

 

type BarkDelegate =

    delegate of obj * BarkArgs -> unit

 

type Dog() =

    let ev = new Event<BarkDelegate, BarkArgs>()

    member this.Bark(msg) =

        ev.Trigger(this, new BarkArgs(msg))

    [<CLIEvent>]

    member this.OnBark = ev.Publish

// Usage:

let snoopy = new Dog()

snoopy.OnBark.Add(

    fun ba -> printfn "%s" (ba.Message))

snoopy.Bark("Hello")

public class BarkArgs : EventArgs

{

    private string msg;

    public BarkArgs(string msg)

    {

        this.msg = msg;

    }

    public string Message

    {

        get { return this.msg; }

    }

}

 

public delegate void BarkDelegate(

    Object sender, BarkArgs args);

 

class Dog

{

    public event BarkDelegate OnBark;

    public void Bark(string msg)

    {

        OnBark(this, new BarkArgs(msg));

    }

}

// Usage:

Dog snoopy = new Dog();

snoopy.OnBark += new BarkDelegate(

    (sender, msg) =>

        Console.WriteLine(msg.Message));

snoopy.Bark("Hello");

Explicit class constructor

type Vector =

    val mutable x : float

    val mutable y : float

    new() = {x = 0.; y = 0.}

       

// Usage:

let v = Vector()

v.x <- 10.

v.y <- 10.

 

Explicit public fields

type Vector() =

    [<DefaultValue()>]

    val mutable x : int

    [<DefaultValue()>]

    val mutable y : int

 

// Usage:

let v = Vector()

v.x <- 10

v.y <- 10

public class Vector

{

    public int x;

    public int y;

}

 

// Usage:

Vector v = new Vector();

v.x = 10;

v.y = 10;

Explicit struct definition

[<Struct>]

type Vector =

    val mutable public x : int

    val mutable public y : int

 

// Usage:

let mutable v = Vector()

v.x <- 10

v.y <- 10

public struct Vector

{

    public int x;

    public int y;

}

 

// Usage:

Vector v = new Vector();

v.x = 10;

v.y = 10;