[swift-evolution] [Proposal] Change Void meaning

[Xiaodi Wu]

On Mon, Jun 12, 2017 at 2:32 PM, Jérémie Girault <jeremie.girault at gmail.com>
wrote:

> @xiaodi
> I disagree on many points, for example what is the type of x when we type
> `let x = *Void` ?
>

That would not be a legal statement. Exploding a tuple is an operation that
only makes sense inside an argument list. Likewise `let x = &Void` will not
compile.


> This is the essence of the problem and this proposition wants to solve
> this.
>
> The regression is due to both reason combined : typealias Void = () AND
> SE-0110
>
> My proposition is to change the meaning of Void from () to “something
> else” that is type-compatible with SE-0110 (and splatting in the future).
>

I'm not sure I understand your motivation. Void is just a typealias. If
tomorrow Void meant something else, all functions must still return (), and
there is still no implicit tuple splatting.


> If you want an example of the changes needed to migrate to swift4, just
> look at the 42 files of handling parenthesis PR of RxSwift needed for
> swift4 upgrade : https://github.com/ReactiveX/RxSwift/pull/1282/files
>

Indeed, that's the result of SE-0110; these parentheses are needed because
there is no implicit tuple splatting. They would be required even if `Void`
did not exist in the language at all.


>
> —
> very short reply expected - vsre.info
> Jérémie Girault
>
> On 12 juin 2017 at 21:18:06, Xiaodi Wu (xiaodi.wu at gmail.com) wrote:
>
> On Mon, Jun 12, 2017 at 2:05 PM, David Hart <david at hartbit.com> wrote:
>
>>
>> On 12 Jun 2017, at 19:25, Xiaodi Wu via swift-evolution <
>> swift-evolution at swift.org> wrote:
>>
>> Unfortunately, I think this proposal appears to be mistaken as to this
>> key premise: Void was never (IIUC) meant to model the absence of arguments;
>> it is a type with one possible value.
>>
>> If I recall, a number of conversations have been raised about Void being
>> a typealias of (), and the definitive response has been that this falls
>> into the ship-has-sailed category of out-of-scope changes.
>>
>> More generally, the recent spate of complaints about regressions to a
>> particular coding style have to do with loss of implicit tuple splatting,
>> the cure for which is a proper implementation of tuple splatting, not
>> poking holes into settled parts of the type system.
>>
>>
>> But you can’t deny that SE-0110 has also caused regressions in the use of
>> Void as generic argument because Void is modelled as the empty tuple.
>>
>
> I'm not sure I understand this statement. Void is a synonym for the empty
> tuple, and that hasn't ever changed, so it can't be the root cause of any
> regressions.
>
>
>> And tuple splatting will not fix those regressions.
>>
>
> How come? If `*` is the splat operator, then it would be legal to call a
> function `foo` that takes no arguments with `foo(*Void)`; if implicit tuple
> splatting returns in fully implemented form, then it would be legal to call
> it once again with `foo(Void)`.
>
> And contrary to what some people might think, this is not an “edge-case”.
>> Most useful monads modelled with generics have good reasons to use Void:
>>
>> *The Result<T> monad:* Result<Void> represents the result of an
>> operation with no return value
>> *The Promise<T> monad:* Promise<Void> represents the result of an
>> asynchronous operation with no return value
>> *The Observable<T> monad (in functional reactive programming):*
>> Observable<Void> represents a stream of events with no values
>>
>> I use all three monads in my code and I’ve had to modify a lot of code
>> when migrating to Swift 4 beta1 because of Void.
>>
>
> Can you give examples of the modifications needed during migration? From
> here, I can only see that the reason any code needs modification is the
> complete removal of implicit tuple splatting. Nothing has changed about
> Void being a synonym for the empty tuple; even if you rename Void,
> functions will still return () by some other name, and unless there is
> tuple splatting in some form, the migration you performed is inevitable.
>
> On Mon, Jun 12, 2017 at 12:15 John McCall via swift-evolution <
>> swift-evolution at swift.org> wrote:
>>
>>>
>>> On Jun 12, 2017, at 4:48 AM, Jérémie Girault via swift-evolution <
>>> swift-evolution at swift.org> wrote:
>>>
>>> Hi here,
>>>
>>> As I tested swift4 in xcode9b1 I noticed a lot of regressions about
>>> tuples usage.
>>>
>>> After documenting myself about the changes which happened, I thought
>>> that they could be improved. Instead of fighting these propositions (which
>>> make sense), I wanted create a few proposal which would improve these
>>> recent changes with a few simple rules.
>>>
>>> My propositions are based on the recent decisions and in the
>>> continuation of SE-0110. The first one is about Void.
>>> Void is historically defined as the type of the empty tuple. The reason
>>> of this is that arguments were initially considered as tuple.
>>>
>>>
>>> The dominant consideration here was always return types, not
>>> parameters.  I'm not sure there was ever much point in writing Void in a
>>> parameter list, but whatever reasons there were surely vanished with
>>> SE-0066.
>>>
>>> Note that 'void' in C was originally exclusively a return type.  ANSI
>>> gave it a new purpose it with void*, but the meaning is totally unrelated.
>>>
>>> John.
>>> _______________________________________________
>>> swift-evolution mailing list
>>> swift-evolution at swift.org
>>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>>
>> _______________________________________________
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>> swift-evolution at swift.org
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>>
>>
>>
>
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