The Nothing Type

fun fail(message: String): Nothing {
    throw IllegalStateException(message)
}

fun main() {
    try {
        fail("Something went wrong")
    } catch (e: IllegalStateException) {
        println("Caught: ${e.message}")
    }
}

fun getOrDefault(value: Int?): Int {
    return value ?: throw IllegalArgumentException("value was null")
}

fun main() {
    println(getOrDefault(5))
}

fun plannedFeature(): Int {
    return try {
        TODO("implement this later")
    } catch (e: NotImplementedError) {
        -1
    }
}

fun main() {
    println("Placeholder result: ${plannedFeature()}")
}

1. Confusing Nothing with null or with Unit; Nothing means a function never completes normally, while Unit means it completes but returns no useful value.
2. Not realizing that a function like TODO() returns Nothing, which is why it can be used anywhere a value is expected without a type error.
3. Trying to create an instance of Nothing; it has no instances -- it exists only to describe code that never returns.

• Nothing is a type with no instances, used to represent an expression that never completes normally.
• Functions that always throw an exception, like ones ending in throw, have an inferred return type of Nothing.
• Because Nothing is a subtype of every other type, it can be used anywhere any type is expected, such as after ?? in the Elvis operator.
• TODO() from the standard library returns Nothing, letting you stub out unfinished code that still type-checks.