F# Snippets 01

This week I had the pleasure of writing some of our test automation infrastructure in F#. Not only does this give us a chance to dogfood the language, but it also helps us find pain points in the developer experience. For me at least, the biggest pain point right now is a lack of F# examples surrounding core syntax.

But I would like to help combat this, so I’ve written a very simple app to show how to do basic object inheritance as well as implement an interface. If you have any suggestions or comments about language constructs that seem difficult to use or could use more examples please let me know.



open System


(* Define a simple class with private field *)

type TimelessPerson = class

    val m_name : string


    // This syntax is to specify a public constructor which takes an

    // argument. It then uses the Record Initialization syntax.

    public new (name) = { m_name = name }


    // Define a public member function ‘SayHello’. F# doesn’t use a

    // ‘this’ keyword, rather you specify the ‘this’ variable yourself.

    // In this case ‘x’ is refering to the this pointer.

    member public x.SayHello() =

        Console.WriteLine(x.m_name ^ @” says, ‘Hello'”)



(* Defines a simple interface *)

type IAge = interface

    // in C# void GrowOlder(TimeSpan elapsedTime)

    abstract GrowOlder : TimeSpan -> unit

    // A property. In C# int Age { get; }

    abstract Age : int



(* Define a derived class which also implements an interface *)

type AgingPerson = class

    // Inherit from TimelessPerson and define the ‘baseclass’ as

    // ‘base’

    inherit TimelessPerson as base


    interface IAge with

        // Implement the GrowOlder method.

        member public x.GrowOlder elapsed =

            x.m_age <- x.m_age.Add(elapsed)

            Console.WriteLine(base.m_name ^ ” has aged “ ^ elapsed.ToString())

        // Implement the Age property, notice the ‘getter’ syntax. If

        // the property weren’t read only we would write:

        // … with get() = … and set(x) = …

        member public x.Age with get() = int_of_float (x.m_age.TotalDays / 365.0)


    val mutable m_age : TimeSpan


    // In order to call the base class’s constructor you need the

    // ‘inherit BaseClassName()’ structure.

    public new(name, age) = { inherit TimelessPerson(name); m_age = age}




(* Using a basic class *)

let santa = new TimelessPerson(“Chris Kringle”)



(* Using a derived class *)

let me = new AgingPerson(“Chris Smith”, (new TimeSpan(365 * 25,0,0,0)))



(* Casting me to the IAge interface, and calling methods/properties *)

let myIAgeInterface = me :> IAge

myIAgeInterface.GrowOlder(new TimeSpan(12,0,0))

Console.WriteLine(“My age = “ + myIAgeInterface.Age.ToString())




Note: this code snippet is “AS IS” and provides no warranties or rights, expressed or implied.

Comments (6)

  1. mattman206 says:

    Nice post!  Very helpful.  Thanks 🙂

    One "pain point" that I’ve run into is how to call a method from a class constructor.  For example, in your first class, say that you wanted to call SayHello() right after setting m_name = name?

    This is the typical C# paradigm for WinForms — call the InitializeComponent() method from the constructor to set everything up.

    I still haven’t figured it out.

  2. d2bg says:


    You can use the "then" syntax like

    type TimelessPerson = class

       val m_name : string

       new (name) as self = { m_name = name } then self.SayHello()

       member public x.SayHello() = print_endline (x.m_name ^ @" says, ‘Hello’")



    I’m not saying I have better ideas, but the duplicated "inherit" inside the definition of AgingPerson looks so ugly.

  3. Chance says:

    I think if you want to get rid of the 2nd inherit statement you can do so with …

    type AgingPerson(name,age) = class …

       inherit TimelessPerson(name) as base


    and then setting the default value attribute on m_age and giving it a value.

  4. d2bg says:


    Could you give the full code pls? IMHO, implicit class construction has so many restrictions for the moment and can’t mix with explicit class construction at all.

  5. jeremy says:

    hi chris,

    i am trying to do something even simpler than your example above. i have an interface ICommand which defines one single method Execute and want to implement a class fulfilling that interface. i tried this:

    type Command = class

     interface ICommand with

       member x.Execute( commandData ) =

         show_message "Kilroy was here"



    when i examine the resulting assembly using reflector, the method name is not Execute, which is what i need and expect, but has been decorated and become ICommand-Execute … hoever, i need the undecorated name Execute. what is causing this decoration? how can i suppress than?

    thank you and cheers,


  6. ChrSmith says:

    Hey Jeremy,

    You cannot change the way F# emits compiler-generated function names. So unfortunately you are stick with ICommand-Execute, ICommand@Execute, etc.

    Do you mind if I asked what is the scenario you are trying to achieve? Do you want the type to have a method named Execute as well as implement the Execute interface method?