What is type '()' in swift? [duplicate]

Question

I'm receiving an event that passes a parameter of type ().

Is it an empty tuple..?

I'm referring to this type of expression:

let x = ()

Answer 1

() is both a type (and Void is a type alias for the empty tuple type) and the only value of that type. So

let x = ()

defines x as a constant property of type () (aka Void) with the value ().

Equivalent statements would be

let x:() = ()
let x:Void = ()

but not

let x = Void // error: expected member name or constructor call after type name

because Void is a type but not a value.

When defining function types with no parameters and/or no return value there seems to be a consensus to use () for an empty parameter list, and Void for no return value. This can for example be seen in UIKit completion handlers, or for execution blocks submitted to GCD dispatch queues, e.g.

func sync(execute block: () -> Void)

Answer 2

Looking at the docs for the Swift grammar:

All tuple types contain two or more types, except for Void which is a type alias for the empty tuple type, ().

tuple-type → (­)­ | (­ tuple-type-element­ , ­tuple-type-element-list­ )­
tuple-type-element-list → tuple-type-element­ | tuple-type-element ­, ­tuple-type-element-list­
tuple-type-element → element-name­ type-annotation­ | type­
element-name → identifier­

Therefore yes, it is a tuple of zero types / elements.