Nil and Void

Disclaimer: The title plagiarised from GNU’s Sather programming language specification

This post is nothing investigative, nor any take away value from it. Consider this a meditation into the philosophy of code or something around the lines, so the following have some significance.

What is Void in Swift?

Void is simply a type alias on an empty tuple.

typealias Void = ()

According to the docs, it is the return type of function that doesn’t specify any return type.

func returnNothing() {
}

Is in reality

func returnNothing() -> Void {
    return Void()
}

This also gives rise to some useless experiments like

let x: [Void] = [Void(), Void(), Void()]

And the size of Void

MemoryLayout<Void>.size

Int = 0

Why is it there?

Void is Swift’s representation of a unit type. And unit type is a concept in type theory holding only one value and thus no information.

What is nil in Swift?

nil is one of the two different variations value possible for an Optional type.

var x: Int? = 0

Is in reality

var x: Optional<Int> = 0 

Here x can take either an Integer value or a nil value representing absence of an Integer value.

All nils are not the same.

nil is equal to itself. However, nils of two differnt type are not comparable.

let ix: Int? = nil
let fx: Float? = nil
fx == ix // This will raise a type error, since by default Equatable '==' is not defined to compare between different types 

Size of nil value

A nil value has same size as the size reserved for other values of the same type.

var x: Int? = nil
MemoryLayout.size(OfValue: x)

Int = 0