> On Jun 8, 2016, at 10:28 AM, Антон Жилин via swift-evolution
> <[email protected]> wrote:
>
> I'll talk about how "bottom" mechanic works there. (I'm not a Haskell expert,
> but I'll at least try ;) )
>
> Firstly, there is a Void type in standard library, which is exactly what
> enum Never {} in Swift is.
> Pretty trivial: it is useful as a parameter of higher kinded types, e.g.
> Either.
Not really. Void is a "Unit" type - it's not an empty enum, it's an enum with
exactly one value. The set of values for the Void type contains exactly one
possible value.
>
> Then, there is bottom. Long story short, each type in Haskell includes an
> undefined (bottom, _|_) value. That corresponds to partially defined
> functions in set theory.
>
> Example 1: Integer -> Integer function can either return a Z number, or be
> undefined at that input (return bottom)
> Example 2: () type actually contains two elements: () and _|_
> Example 3: Void contains one element: _|_
>
> Bottom value results in an error on most occasions.
>
> What does returning Void mean? In set theory, set of Integer -> Void is an
> empty set. In type theory, Integer -> Void can contain a single element
> that returns bottom.
>
> Applying that to Swift, we express bottom value by not returning on a
> particular input (due to a crash or an infinite loop). We are going to
> express bottom type by Never (or aliases). The only way to return bottom type
> is as well going to be returning bottom value (in Swift meaning).
>
> There is also such a function in Haskell: absurd :: forall a. Void -> a
> Of course, it just returns bottom value of type a. The philosophy of this is
> "from false follows anything" or like, "if we are going to crash, then we can
> do anything".
> Based on this, we can implement conversions from Void to any type in Swift.
> It would look like:
>
> func implementContract() -> Int {
> return fatalError("unimplemented")
> }
>
> `return` is crucial here.
> This is what I suggest to change in the proposal.
>
> - Anton
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