Kotlin - Void vs. Unit vs. Nothing

Question

Kotlin has three types that are very similar in nature:

• Void
• Unit
• Nothing

It almost seems like they're making the JavaScript mistake:

• null
• undefined
• void(0)

Assuming that they haven't fallen into the same mistake, what are they all for, and how do they differ?

Answer 1

The Void type is from Java. You generally won't use this from Kotlin unless you're using some Java-library that uses it.

The Unit type is what you return from a function that doesn't return anything of interest. Such a function is usually performing some kind of side effect. The unit type has only one possible value, which is the Unit object. You use Unit as a return type in Kotlin when you would use void (lowercase v) in Java.

The Nothing type has no values. If a function has return type Nothing, then it cannot return normally. It either has to throw an exception, or enter an infinite loop. Code that follows a call to a function with return type Nothing will be marked as unreachable by the Kotlin compiler.

Because Nothing has no values, Nothing? is actually the type that captures only the null value in Kotlin.

Answer 2

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Unit

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Unit is like void

In Kotlin, when a function does not return any meaningful value, it is declared to return Unit, just like void in Java:

fun greet(): Unit { println("Good day!") }

It's a convention to skip writing Unit when a function returns Unit because Unit is considered the default return type by the compiler:

fun greet() { println("Good day!") }

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Unit is a Singleton

The Unit is a class with only a single object (singleton pattern) and that object is the Unit itself. It is declared in the kotlin package using an object declaration as shown below:

public object Unit {
    override fun toString() = "kotlin.Unit"
}

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Unit in Functional Programming

Kotlin has first-class support for functional programming. It's common to have a Unit in a functional programming language. It makes the function types more readable by enabling all the functions to be declared as having a return value, even when a function does not return a value:

val greet: () -> Unit = { println("Good day!") }

Here, () -> Unit is a function type and the Unit after the -> indicates that this function type does not return any meaningful value. Mentioning the Unit cannot be skipped in function types.

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Unit for Extending Generics

Every function has to return a value. Kotlin decided to represent this with a class rather than with a special type void as in Java. The reason for using a class is that the type system can be made more consistent by making it a part of the type hierarchy.

For example, let's say we have a generic interface called Worker<T> that performs some work. The doWork() function of this interface does some work and has to return a value T:

interface Worker<T> {
    fun doWork(): T
}

But sometimes, we might want to use this interface for some work where we don't need to return any value, for example, the work of logging, in the LogWorker class shown below that extends the Worker interface:

class LogWorker : Worker<Unit> {
    override fun doWork() {
        // Do the logging
    }
}

This is the magic of Unit where we are able to use the pre-existing interface that was originally designed to return a value. Here we make the doWork() function return the Unit value to serve our purpose in which we don't have anything to return. So, it's useful when you override a function that returns a generic parameter.

Notice that we have also skipped mentioning Unit return type for the doWork() function. There's no need to write a return statement either.

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Nothing

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Nothing's Value Never Exists

In Kotlin, the class Nothing represents a value that never exists. There can never be any value/object of this class because its constructor is kept private. It's defined in the kotlin package as follows:

public class Nothing private constructor()

Nothing is used for the return type of a function that never returns a value. For example, a function with an infinite loop or a function that always throws an exception. The error() function from Kotlin standard library is an example that always throws an exception and returns Nothing. Here is the code for it:

fun error(message: Any): Nothing = throw IllegalStateException(message.toString())

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Nothing is the Bottom Type

In type theory, the type that has no values is called a bottom type and it is a subtype of all other types. So, Nothing is the subtype of all types in Kotlin, just like Any? is the supertype of all types. So, the value(that never exists) of type Nothing is assignable to the variables of all types, for example:

val user: User = request.user ?: error("User not found")

Here, we are calling the error() function that we defined earlier, if the user is null, using the elvis operator(?:). The error() function returns the value of type Nothing but it can be assigned to the variable of type User because Nothing is a subtype of User, just like it is a subtype of any other type. The compiler allows this because it knows that the error() function will never return a value, so there is no harm.

Similarly, you can return Nothing from a function that has any other return type:

fun getUser(request: Request): User {
    return request.user ?: error("User not found")
}

Here, even though the getUser() function is declared to return a User, it may return Nothing, if the user is null.

---

Nothing in Null Object Pattern

Consider the following example of a function that deletes the files given in a list:

fun deleteFiles(files: List<File>? = null) {
    if (files != null) files.forEach { it.delete() }
}

The problem with the design of this function is that it doesn't convey whether the List<File> is empty or null or has elements. Also, we need to check whether the list is null before using it.

To solve this problem, we use the null object design pattern. In null object pattern, instead of using a null reference to convey the absence of an object, we use an object which implements the expected interface, but leaves the method body empty.

So, we define the object of the interface List<Nothing>:

// This function is already defined in the Kotlin standard library
fun emptyList() = object : List<Nothing> {
    override fun iterator(): Iterator<Nothing> = EmptyIterator
    ...
}

Now we use this null object in our deleteFiles() function as a default value of our parameter:

fun deleteFiles(files: List<File> = emptyList()) {
    files.forEach { it.delete() }
}

This removes the uncertainty of null or empty and makes the intent clearer. It also removes the null checks because the functions on the null object are empty, they will be called but they are no-ops (no operation in them, so they will do nothing).

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Nothing for Covariant Generics

In the example above, the compiler allows us to pass List<Nothing> where List<File> is expected. This is because the List interface in Kotlin is covariant since it's defined using the out keyword, that is, List<out T>. And as we learnt, Nothing is a subtype of all types, Nothing is a subtype of File too. And due to covariance, List<Nothing> is a subtype of List<File>, List<Int>, List<User> and so on... List<AllTypes>. This applies to any type with the covariant generics(out), not just List.

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Nothing for Better Performance

Just like the function emptyList() used in our example, there are predefined functions like emptyMap(), emptySet(), emptySequence() that return null objects. All these are defined using Nothing. You can define your own objects like this.

The advantage here is that these return singleton objects, for example, you can call the same emptyList() function for getting an empty instance, whether it is for assigning to List<File>, List<Int> and ... List<AllTypes> and in multiple places. Since the same object is returned every time, it saves the cost of object creation and memory allocation.

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Void

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Void for Extending Generics in Java

The Void class is from the java.lang package while the Unit and Nothing are from the kotlin package. Void is not intended to be used in Kotlin. Kotlin has its own class in the form of Unit.

Void is used in Java for extending generic interfaces like our Worker interface example written for Unit where we have to return a value. So for converting our Kotlin code to Java, we can use Void the same way we have used Unit for our Worker example and rewrite the code in Java as follows:

interface Worker<T> {
    T doWork();
}

class LogWorker implements Worker<Void> {
    @Override public Void doWork() {
        // Do the logging
        return null;
    }
}

Notice that when using Void, we have to use Void as a return type(can't skip) as well as need to write the return statement whereas for Unit we can skip both. This is another reason to avoid using Void in Kotlin code.

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Conclusion

So, Unit and Nothing are not a mistake by Kotlin designers in my opinion and are not as questionable as null, undefined and void(0) in Javascript. Unit and Nothing make the functional programming a breeze while providing other useful features mentioned. They are common in other functional programming languages too.

That's it! Hope that helps.

Answer 3

Void is uninstantiable type. It is a plain Java class and has no special meaning in Kotlin.

Unit type has only one value. Replaced Java void (notice: not Void). More info in Kotlin docs.

Nothing has no instances (just like Void). It represents "a value that never exists". In Kotlin if you throw an error it is a Nothing (see Kotlin docs).