Kotlin Koans BR: The "nothing" type (Nothing)

🔗 Task

Specify the Nothing return type for the failWithWrongAge function.

Without specifying the Nothing type, the checkAge function fails to compile, because the compiler assumes that age could be null.

Use case

Nothing represents a value that never exists, and you can’t have a value or object of this class because its constructor is private.

Nothing is used to mark functions that never return a value.

fun waitForever(): Nothing {
    while (true) {
        // I'm waiting...
    }
}

The special role of Nothing

In type theory, Nothing is considered the “bottom type” — in other words, it’s a subtype of every other type in Kotlin. This means a Nothing value can be assigned to variables of any type.

fun error(message: String): Nothing = throw IllegalStateException(message)

fun findSession(sessionId: Int): Session =
    sessionsInProgress
        .firstOrNull { it.id == sessionId }
        ?: error("Session not found!")

Even though the error() function returns Nothing, assigning its result to a variable of type Session is valid, because Nothing acts as a subtype of Session.

Advantages

• Clear, direct communication: a function that returns Nothing has no intention of returning a value, removing any ambiguity.
• Flexibility: it behaves like a “chameleon” in Kotlin’s type world, becoming useful in many different scenarios.
• Resource savings: thanks to the compiler’s smarts, we don’t waste memory allocating something that should never exist.
• Protection against errors: Nothing makes it clear: I will never return. This kind of guarantee can prevent unpleasant surprises at runtime.

Disadvantages

• Some adjustment needed: for people just getting started with Kotlin, Nothing can come across as a puzzling concept, with a bit of a learning curve.
• Overuse: it’s possible to fall into the trap of using Nothing in a confusing, excessive way, complicating your code instead of simplifying it.

Analogy

Imagine a colorful book with a cover showing the title, the authors, and the publisher. But when you open it, every page is blank. The book exists, it has weight, it has shape, but it has no content.

That’s how Nothing works in Kotlin. It has presence, it has a representation, yet it holds no intrinsic value or meaning.

In code, when a function returns Nothing, it’s as if we were opening a book expecting a story, but finding empty pages.