Short introduction to Discriminated Unions in F#

Cover photo by Ilija Boshkov on Unsplash

The last few months I've been trying to get my head around F#. If you'd like to start learning F# I can highly recommend Get Programming with F# by Isaac Abraham, especially when you have a background in C# like I do.
Today I've learned something about Discriminated Unions in F#. You use a DU when you have to model an is-a relationship.
But first things first, how does such a relationship look like in an object oriented language like C#?

public abstract class Animal
{
    public abstract string Talk();
}

public class Dog : Animal
{
    public override string Talk()
    {
        return "Woof!";
    }
}

public class Cat : Animal
{
    public bool IsAggressive { get; set; }

    public override string Talk()
    {
        return IsAggressive
            ? "MEEEEEEEEOOOOOOOW!!!"
            : "Meow";
    }
}

// usage
var dog = new Dog();
dog.Talk(); // Woof!

var cat = new Cat();
cat.Talk(); // Meow

var aggressiveCat = new Cat { IsAggressive = true };
aggressiveCat.Talk(); // MEEEEEEEEOOOOOOOW!!!

And now let's see the equivalent in F#:

type Animal = // base type
    | Dog // no custom fields
    | Cat of IsAggressive:bool // add custom field

// add function with the help of pattern matching
let talk animal =
    match animal with
    | Dog -> "Woof!"
    | Cat aggressive when aggressive = true -> "MEEEEEEEEOOOOOOOW!!!"
    | Cat _ -> "Meow"

// usage

let dog = Dog
dog |> talk // Woof!

let cat = Cat false
cat |> talk // Meow

let aggressiveCat = Cat true
aggressiveCat |> talk // MEEEEEEEEOOOOOOOW!!!

Pretty neat, isn't it?
What I really like about the F# approach is that I'm able to see the whole type hierarchy at a glance. Of course it's technically possible to put all C# classes into one file. But then you would certainly follow the "One class per file" convention ;-)

You can read more about Discriminated Unions on F# for fun and profit or Microsoft Docs.