Kotlin made this explicit by calling the void type Unit, where it is a class that has a single instance with the same name. So when you define a function:

Kotlin made this explicit by calling the void type Unit, where it is a class that has a single instance with the same name. So when you define a function: fun foo(a: Int): Unit { ... } You can actually assign a value to the result: val x = foo() // x now contains the value Unit There is another type which does not have any instances, called Nothing. Declaring a function to return Nothing indicates that the function will never return. Other than that, Nothing is a regular type, but code that uses it will be unreachable (and flagged as such by the IDE) because you can never create an instance of it. fun foo(): Nothing { throw SomeException() } This leads me to the question. Unit is obviously the mathematical Unit type, but is there a way to model the Nothing type in mathematically?