[swift-evolution] [Idea] Repurpose Void

[Антон Жилин]

SE-0066 disallows Void to be used on left side of function types.

Some people, including me, argue that Void should be removed altogether,
because:
1) () is more consistent with curried functions: (Int) -> () -> Double
2) () follows functional programming traditions

Now, why don't we repurpose Void to follow functional programming
traditions as well?
Its definition will look like:
enum Void { }

Basically, Void is a type which cannot have values.
With it, we can eliminate at least two Swift special cases:

1. Noreturn functions

func exit(code: Int = 0) -> Void

>From this signature, it's obvious that `exit` cannot return normally

2. Rethrows

func call<T, U>(block: () throws T -> U) throws T -> U

Non-throwing functions are functions throwing Void.
So if T=Void, we get the non-throwing version of `call`.

- Anton
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