F# Quick Guides: Object Oriented Programming

Article
04/06/2010

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

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

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;

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F# Quick Guides: Object Oriented Programming

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