Kotlin Programming: The Big Nerd Ranch Guide, First Edition

Unit Functions

Not all functions return a value. Some use side effects instead to do their work, like modifying the state of a variable or calling other functions that yield system output. Think about your player status and aura display code, or the castFireball function, for example. They define no return type and have no return statement. They use println to do their work.

    private fun castFireball(numFireballs: Int = 2) =
        println("A glass of Fireball springs into existence. (x$numFireballs)")

In Kotlin, such functions are known as Unit functions, meaning their return type is Unit. Click on the castFireball function’s name and press Control-Shift-P (Ctrl-P). IntelliJ will display its return type information (Figure 4.9).