Unit and Nothing

Unit

Example

A function that returns no useful value returns Unit (like void in Java). You normally do not write it. Unit is a real object, so it can be printed.

fun logEvent(name: String): Unit {
    println("Event: $name")
}

fun logEvent2(name: String) {     // Unit is implied
    println("Event: $name")
}

fun main() {
    logEvent("app_open")
    val result = logEvent2("add_to_cart")
    println(result)
}

Output

Event: app_open
Event: add_to_cart
kotlin.Unit

Nothing

Nothing is the return type of a function that never returns normally: it always throws an exception or loops forever. The compiler knows the code after it cannot run. TODO() and error() in the standard library return Nothing.

Example

Because fail() returns Nothing, it can be used on the right side of ?:, and user becomes a non-null String.

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

fun main() {
    val loggedInUser: String? = "asha"
    val user = loggedInUser ?: fail("Not logged in")
    println("User: $user")

    val guest: String? = null
    val name = guest ?: fail("Session expired, please log in")
    println("Never printed: $name")
}

Output

User: asha
Exception: java.lang.IllegalStateException: Session expired, please log in

Unit vs Nothing vs void

Type	Meaning	Example
`Unit`	function finishes, returns no useful value	`fun log(msg: String)`
`Nothing`	function never finishes normally	`fun fail(msg: String): Nothing`, `TODO()`
Java `void`	no return value (not a type you can store)	`void log(String msg)`