Aug 1, 2023 2 min read fsharp

Demystifying F#'s Unit Type: The One-Value Wonder

The unit type is a type that represents the lack of a specific value; it has only one value that works as a placeholder when no other value exists or is required.

Photo by Adi Goldstein / Unsplash

Introduction

Coming from a C# background as a developer, the first time I saw unit type in F#. In my interpretation, I was dealing with the voidkeyword.

Moreover, it isn't a direct equivalent but the closest one when a function does not return any meaningful value (this is also true in other programming languages).

That's why, in this post, we'll try to explore a unit type in F# and give examples of it.

Okay, then, let's get started.

What's a Unit Type?

In F#, a "unit type" is a type that represents a value with only one possible instance, denoted as ().

In other words, unit type indicates the absence of a specific value but describes a set with only a single element denoted by ().

Usefulness of Unit Type

I think that for about a year of working with F# language, unit type has been helpful in F# to ensure a consistent and type-safe approach when dealing with functions that don't produce intended results or are just used for side effects.

Example

The example below is a console application.

But let's try to play around before looking at the console application.

(* I created a small function to check if the type is a unit.  *)

let isUnitType (value: 'a) : bool = 
    match value with 
        | () -> true
        | _ -> false

From the function above, it checks if the passed value is a unit.

let myUnit = ()

let result: bool = myUnit |> isUnitType 

Console.WriteLine("Test myUnit using the func isUnitType: {0}", result)

Just a quick note: If we're going to execute the let myUnit = () run on the interactive; it will result into t myUnit = ();; val myUnit: unit = ().

Okay, going back to the example, the last line Console.WriteLine("Test myUnit using the func isUnitType: {0}", result) will result in: "Test myUnit using the func isUnitType: True."

Let's try another one.

let saySomething() :unit  =
    printfn "I love F#"
    ()

let result1: bool = saySomething() |> isUnitType 

Console.WriteLine("Test saySomething using the func isUnitType:  {0}", result1)

This will be just the same. I hope you'll try it on yourself.

Okay, now for the entire code sample, please see it below.

open System

(* I created a small function to check if the type is a unit.  *)

let isUnitType (value: 'a) : bool = 
    match value with 
        | () -> true
        | _ -> false


let myUnit = ()

let result: bool = myUnit |> isUnitType 

Console.WriteLine("Test myUnit using the func isUnitType: {0}", result)

let saySomething() :unit  =
    printfn "I love F#"
    ()

let result1: bool = saySomething() |> isUnitType 

Console.WriteLine("Test saySomething using the func isUnitType:  {0}", result1)

let saySomethingAgain() :unit = 
    printfn "I love C#"


let result2 = saySomethingAgain() |> isUnitType
Console.WriteLine("Test saySomethingAgain using the func isUnitType:  {0}", result2)

0

For the sample output, see the screenshot below.