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 d

Answer 1

The main reason why Unit exists is because of Generic reasons. Let's use the example from the Kotlin docs.

class Box(t: T) {
    var value = t
}

We can have

var box = Box(Unit)

This is why Unit returns a value so the Kotlin can infer it from the type passed into class initialization. Of course, you could also explicitly write it like this,

var box = Box(Unit)

but all the same, it must have a return value. Now, the void keyword in Java in Kotlin is Nothing. Nothing is the last but one type in the type hierarchy in Kotlin with the last one being Nothing? (Nullable Nothing). This does not return any value at all. Because it doesn't return any value at all, we can't pass it as a type in the above code.

var box = Box(Nothing) //This will return an Error