kotlin - What is the purpose of Unit-returning in functions

Question

From the Kotlin documentation:

If a function does not return any useful value, its return type is Unit. Unit is a type with only one value — Unit.VALUE. This value does not have to be returned explicitly:

fun printHello(name : String?) : Unit { 
   if (name != null) 
     print("Hello, $name!") 
   else 
     print("Hi there!") 
   // We don't need to write 'return Unit.VALUE' or 'return', although we could 
}

What is the purpose of Unit-returning in functions? Why is VALUE there? What is this VALUE?

Answer 1

The purpose is the same as C's or Java's void. Only Unit is a proper type, so it can be passed as a generic argument etc.

1. Why we don't call it "Void": because the word "void" means "nothing", and there's another type, Nothing, that means just "no value at all", i.e. the computation did not complete normally (looped forever or threw an exception). We could not afford the clash of meanings.

2. Why Unit has a value (i.e. is not the same as Nothing): because generic code can work smoothly then. If you pass Unit for a generic parameter T, the code written for any T will expect an object, and there must be an object, the sole value of Unit.

3. How to access that value of Unit: since it's a singleton object, just say Unit