[swift-evolution] [Proposal] Change Void meaning

Mon Jun 12 15:17:30 CDT 2017

I think this proposal would be complicating rather than simplifying the
type system, it would be adding a special rule.
And it is not a step towards resolving the many parentheses-related
inconsistencies that still remain.

Here is an example of one such remaining inconsistency, it's still in
(latest dev snapshot) Swift 4, so this is the behavior of both Swift 3(.2)
and 4:
func foo() -> Void {}
func foo(_: Void) -> Void {} // This is OK
func bar(fn: () -> Void) {}
func bar(fn: (_: Void) -> Void) {} // ERR: Invalid redecl.

I think the least surprising behavior here would be no error, rather than
two.
The current behavior with one error is very surprising.

On Mon, Jun 12, 2017 at 10:09 PM, Xiaodi Wu via swift-evolution <
swift-evolution at swift.org> wrote:

> On Mon, Jun 12, 2017 at 3:04 PM, Jérémie Girault <
> jeremie.girault at gmail.com> wrote:
>
>> Exactly, that means that your implementation of the tuple splatting
>> operator is out of the type system.
>> Can you expose it’s signature ?
>>
>> If you want the operator to be “compiler-magic” it’s possible.
>>
>
> Yes, it requires compiler support.
>
>
>> This proposal is an alternate solution.
>>
>> My point is that updating Void according to this proposal would
>>  - preserve the type system in a better way
>>  - have better source compatibility (in time for swift 4 release, were we
>> probably won’t see tuple splatting)
>>  - also keep the elegant original syntax of swift instead of stacking
>> parenthesis
>>
>> The impact for code writers would be minimized on time for swift 4 release
>>
>> As for return values: this proposition does not intend to change the how
>> return value of Void functions works.
>>
>> —
>> very short reply expected - vsre.info
>> Jérémie Girault
>>
>> On 12 juin 2017 at 21:45:08, Xiaodi Wu (xiaodi.wu at gmail.com) wrote:
>>
>> 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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>>>>
>>>>
>>>>
>>>
>>
>>
>
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