[swift-evolution] [Proposal] Change Void meaning

Jérémie Girault jeremie.girault at gmail.com
Mon Jun 12 15:16:50 CDT 2017

I invite you to read the proposal rules again with a fresh mindset and
benevolence spirit.
It’s my first one and may not be very clear but the rules are

Especially try to forget that Void is a tuple or anything.
Void is Nothing in the programmer’s mind. An instance of Void shouldn’t
even exist.

The proposed meaning of Void is to be a “lack” of arguments. It means that
each “Void” argument reduces the arity of the function by one, at its exact
position. Especially look at the canonical signature definition, and the
proposed rules of reduction.

That proposal, if implemented, would effectively allow `Callback<Void>` to
be called without arguments in the context of swift4.

very short reply expected - vsre.info
Jérémie Girault

On 12 juin 2017 at 22:06:54, Xiaodi Wu (xiaodi.wu at gmail.com) wrote:

On Mon, Jun 12, 2017 at 2:56 PM, Jérémie Girault <jeremie.girault at gmail.com>

> @Xiaodi Wu
> Disagree, and we would need the original designer here to help us, but my
> understanding of the original meaning of tuples-as-arguments is that when I
> define:
> `func foo(_ arg0: Any, _ arg1: Any) {}`
> I can afterwards “apply” a tuple to a function named `foo` and therefore
> execute the function on this tuple.
> Calling a function syntax was equivalent to put a tuple next to a function
> name:
> `foo(42, “hello")` the left-hand is `foo`, the right-hand is `(42,
> “hello")` is the tuple.
> The same way if I have
> `func foo()`
> `foo()` means calling `foo` with argument `()` and there we have our
> original `Void`
> That meaning changed recently due to multiple SE implementations actually.

Tuples-as-arguments never shipped in any version of Swift, and the ability
to "apply" a tuple like that was removed by SE-0029--the original goal was
to implement this change in time for Swift 2.2.

> The parenthesis around the call don't have a tuple meaning anymore.
> Therefore it breaks a lot of code relying on this feature of the language,
> which was quite elegant, but could not handle advanced functions features.

Yes, this is an intentional and approved part of SE-0029. The drawbacks
were enumerated in that proposal and were deemed acceptable.

> @john: Void can happen easily in parameters due to generics, the easiest
> example that breaks with swift4 is
> `typealias Callback<T> = (T) -> Void`
> here
> `Callback<Void>` which is perfectly reasonable introduces Void in the
> argument list and forces the caller to introduces an empty partenthesis set
> when invoking:
> ```
> let foo: Callback<Void> = { }
> foo(())
> ```

`Callback<T> = (T) -> Void` refers to a callback that takes exactly one
argument. Since argument lists are not tuples, it is not possible to invoke
such a callback with zero arguments, or for that matter with two, three,
four, or five arguments.

That’s were the proposal shines in my opinion: adapting the meaning of Void
> to the current context of swift4 gives back the language the elegant syntax
> it had while preserving the type system

I'm not sure what you mean by this. No change in `Void` can cause a
function of type (T) -> Void to accept zero arguments: it requires one
argument of type T.

On 12 juin 2017 at 20:04:18, Xiaodi Wu (xiaodi.wu at gmail.com) wrote:
> On Mon, Jun 12, 2017 at 12:44 Jérémie Girault <jeremie.girault at gmail.com>
> wrote:
>> Void was the empty tuple because arguments were tuples.
> As John explained, that is _not_ correct. Void was not motivated by
> anything to do with argument lists.
> So no arguments meant empty tuple.
>> If we consider the empty tuple to be an argument, then the type for the
>> type of empty tuple should be `Unit`
> It has been suggested to rename Void to Unit. I do believe it’s on the
> commonly rejected ideas list (and if it’s not, it ought to be).
> Void, however, seem naturally fitted for the absence of argument.
>> Should `func foo(Void)` be different from `func foo()`?
> SE-0110 determined that the two should in fact be different.
> I don’t think so. But different from `func foo(Unit)` ? Yes !
> It sounds like your quarrel is with the name of the typealias. I don’t see
> how that solves any issues with the loss of tuple splatting. Functions will
> still return (), and you foo(()) is not foo().
> My point here is that we probably won’t have splatting for swift4.
>> But if we consider the type system as a guide, we can consider 3 simple
>> set of rules and restore almost 100% source compatibility while keeping the
>> improvement of SE-0110
>> - Rules for swift3 tuples-arguments of cardinality zero (Void) in swift 4
>> (this proposition)
>> - Rules for swift3 tuples-arguments of cardinality one in swift 4
>> (proposition to be done)
>> - Rules for swift3 tuples-arguments of cardinality > 1 in swift 4
>> (proposition to be done)
>>>> very short reply expected - vsre.info
>> Jérémie Girault
>> On 12 juin 2017 at 19:25:31, Xiaodi Wu (xiaodi.wu at gmail.com) 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.
>> 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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