[swift-evolution] [Discussion] Zero-element tuple glitch

[Xiaodi Wu]

Why can't () as an argument list be regarded as the same as (()) just like
no return type is regarded as the same as `-> ()`?
On Sun, Jul 3, 2016 at 17:41 Austin Zheng via swift-evolution <
swift-evolution at swift.org> wrote:

> Void is just a typealias for (). () is the only type in Swift whose type
> and only instance are spelled the same way, as far as I can tell. This
> isn't true for Void, since Void is only a type, not the value.
>
> As for the zero-argument/one-argument problem...how often does realistic
> code need to pass in a zero-argument function to a function type that takes
> one argument? If the answer is "often", we should consider what it would
> take to define an implicit conversion. Otherwise, maybe we can get away
> with a standard library adapter that can be used to wrap a () -> T as a U
> -> T (where U = () or something).
>
> Austin
>
>
> > On Jul 3, 2016, at 3:35 PM, Vladimir.S via swift-evolution <
> swift-evolution at swift.org> wrote:
> >
> > On 04.07.2016 0:47, Anton Zhilin via swift-evolution wrote:
> >> Vladimir.S via swift-evolution <swift-evolution at ...> writes:
> >>
> >>> On 03.07.2016 23:53, Anton Zhilin via swift-evolution wrote:
> >>>> let x = (Int, Int)(1, 2)  //=> (1, 2)
> >>>> let y = (Int)(1)          //=> 1
> >>>> let z = ()()              // error :(
> >>>>
> >>>> Am I requesting too much from type checker?
> >>>
> >>> As I understand, because () is Void, you can't have anything additional
> >> for
> >>> it. It is just Void, like you write 'let z = Void Void'. I.e. only 'let
> >> z =
> >>> ()' allowed.
> >>
> >> In the example, that's how I expect that to parse:
> >>
> >> let z = ().init()  // just demo, does not compile
> >>
> >> I mean, first () is type, and second () is its initializer.
> >>
> >> By the way, this is one thing that Void can do, but () cannot.
> >> Are there any others?
> >>
> >
> > I believe in this case () is playing a role of instance of type Void,
> i.e. empty tuple. I don't think you can expect ().init()
> >
> > Also:
> > func f(_ x: ())->() {}
> > f(())
> > //f(Void) // argument passed to call that takes no arguments
> >
> > And just some fun:
> >
> > let x1 : Void
> > //let x2 : Void = Void // cannot convert value of type 'Void.Type' (aka
> '().Type') to specified type 'Void' (aka '()')
> > let x3 : ()
> > let x4 : () = ()
> > let x5 : Void = ()
> > //let x4 : () = Void  // cannot convert value of type 'Void.Type' (aka
> '().Type') to specified type '()'
> >
> > print(().dynamicType) // ()
> > print(().self) // ()
> > // print("Void.dynamicType = ", Void.dynamicType) // '.dynamicType' is
> not allowed after a type name
> > print(Void.self) // ()
> > print(().dynamicType == Void.self) // true
> >
> >
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