The Nature of Nothing in Kotlin

For someone who comes from the Java world the concept of Nothing might be confusing. In this article, I’ll try to clean that up with some practical examples to boot.

What is `Nothing`?

Simply put, Nothing is a bottom type:

In type theory, a theory within mathematical logic, the bottom type is the type that has no values. A function whose return type is bottom cannot return any value.

So this means that Nothing is something like Void in Java, and the opposite of Any in Kotlin, no? Well, not quite. In Java, when a method has no return value we can declare its return type as void:

public void someMethod() {
    // do nothing
}

In Kotlin, however, we already have a type which is returned from functions which don’t declare a return type: Unit.

fun fnWithNoExplicitReturnType() {
    // returns Unit implicitly.
}

fun fnWithExplicitReturnType(): Unit {
    // same as above, with implicit return of Unit
}

fun fnWithExplicitReturn(): Unit {
    return Unit // same as above
}

// can be called like this
val unit: Unit = fnWithNoExplicitReturnType()

Although the documentation says it corresponds to void there is an important difference. Unit has an actual instance, that’s why we can assign it to a variable, like in the example above:

/**
 * The type with only one value: the Unit object. This type corresponds to the `void` type in Java.
 */
public object Unit {
    override fun toString() = "kotlin.Unit"
}

So what good Nothing does for us? Let’s consider this function which does not return at all:

fun throwException() {
    throw RuntimeException("Oops...")
}  

This function would have returned Unit if it hadn’t thrown an Exception. Now if you guess that this is exactly the case of returning Nothing you are right. Let’s look at the actual implementation of it:

/**
 * Nothing has no instances. You can use Nothing to represent "a value that never exists": for example,
 * if a function has the return type of Nothing, it means that it never returns (always throws an exception).
 */
public class Nothing private constructor()

Since it has a private constructor and it is not called from the class itself this means that there can’t be any instances of Nothing. Nothing is also a bottom type which means that it is a subtype of every other type (including Unit).

Signaling exceptional conditions with `Nothing`

Now you might begin to wonder why all this stuff is useful? Let’s say that we have a function which interoperates with Java classes which might return null:

fun interact(): String? {
    // ...
}

If we call this in our code we have to do something about the nullability if we want a simple non-nullable String out of this:

val result: String = interact() ?: throwException()

But wait… This doesn’t compile because throwException returns Unit and we want to get a String. Let’s try redefining throwException in a more useful way:

fun throwException(): Nothing {
    throw RuntimeException("Oops...")
}

// this suddenly compiles
val result: String = interact() ?: throwException()

Our code now compiles. Why is that? The answer is that since Nothing is a subtype of every other class in Kotlin, the compiler can accept it, and because throwException is never going to return successfully, its assignment will never happen. Better yet now you can start using it to explicitly state that a function will always throw an Exception.

Understanding how Nothing works might be a bit hard but you can develop a nice intuition for it if you imagine Nothing as something which is the opposite of Any. Any is the supertype of everything (just like Object in Java) and Nothing is the subtype of everything.

Representing `null`

Now that you know how Nothing works you might start feeling warm and fuzzy…until you try writing a function like this:

fun throwException(): Nothing? {
    throw RuntimeException("Oops...")
}

If you try this with the previous example it no longer compiles. Why is that? Let’s see if there is a value which can be returned from a function which has the Nothing? return type:

fun returnNull(): Nothing? = null

val nullValue : Nothing? = returnNull()

Yes. The only value which can be returned from this method is null. This also means that null has the type of Nothing?. Although this is hardly useful in our day-to-day work now at least we understand it.

Algebraic data types

Algebraic data types are sometimes very useful and you might already be using them even if you don’t know it (a linked list for example). In Kotlin we can use Nothing if we want to implement them to great effect. Let’s take a look at a good example, Either:

sealed class Either<out L, out R> {

    data class Left<out L>(val value: L) : Either<L, Nothing>()
    
    data class Right<out R>(val value: R) : Either<Nothing, R>()

    fun isRight() = this is Right<R>

    fun isLeft() = this is Left<L>
}

fun <L> left(a: L) = Either.Left(a)
fun <R> right(b: R) = Either.Right(b)

Either represents a value which might be either Left or Right. Left traditionally means an exceptional condition and Right usually is just a normal value but Either can’t hold both values. We represent this concept with Nothing. The right type of Left and the left type of Right is Nothing. We can then use Either for example as a return type instead of throwing exceptions:

fun parseInt(str: String) : Either<Exception, Int> {
    return str.toIntOrNull()?.let {
        right(it)
    } ?: left(NumberFormatException())
}

val result: Either<Exception, Int> = parseInt("xul")
println(result)

// Left(value=java.lang.NumberFormatException)

Note that if you replace Nothing with Unit or any other type it simply won’t compile, but here it represents what it was made for: nothingness.

Another fine example of using Nothing is a tree structure:

sealed class Tree<out T> {
    class Node<T>(val left: Tree<T>, val right: Tree<T>) : Tree<T>()
    class Leaf<T>(val value: T) : Tree<T>()
    object Empty : Tree<Nothing>()
}

val tree = Node(
              Leaf("Foo"),
              Node(Leaf("baz"), Empty))

Here Nothing represents a missing left or right Node in a Tree.

Conclusion

Although Nothing might not be intuitive at first, once we learn how it works it quickly becomes a useful asset in our toolkit. We can use it to indicate nothingness in data structures, to signal the inevitability of Exceptions being thrown or even null.

Now that we have all this under our belt let’s go forth and Kode on!

What is Nothing?

Signaling exceptional conditions with Nothing

Representing null

Algebraic data types

Conclusion

What is `Nothing`?

Signaling exceptional conditions with `Nothing`

Representing `null`