[swift-evolution] [Proposal] Change Void meaning

[Jens Persson]

The std lib swap could perhaps be an interesting example to consider:
public func swap<T>(_ a: inout T, _ b: inout T)

What would happen with that?
Will inout arguments be an exception to the rule of Void getting a default
value, and if so, what would the effects of that be?
Or would it somehow be allowed to call swap()?
Or is there a third alternative?
/Jens


On Tue, Jun 13, 2017 at 7:15 PM, John McCall via swift-evolution <
swift-evolution at swift.org> wrote:

>
> On Jun 13, 2017, at 4:41 AM, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
>
> On Tue, Jun 13, 2017 at 3:06 AM, John McCall <rjmccall at apple.com> wrote:
>
>> On Jun 13, 2017, at 3:30 AM, Jérémie Girault <jeremie.girault at gmail.com>
>> wrote:
>>
>> Exactly,
>> The reflexion behind it is:
>>
>> - Let's understand that 0110 and other tuple SE are important for the
>> compiler, we do not want them to rollback
>> - However we have number of regressions for generics / functional
>> programmers
>> - Let’s solve this step by step like a typical problem
>>
>> - Step 0 is adressing this Void tuple of size zero :
>> - Zero is in many problems of CS an edge case, so let’s handle this case
>> first
>> - The compiler knows what Void is, and its only value (or non-value)
>> - It was handled historically by the compiler because of implicit side
>> effects
>> - Let’s handle it explicitely with rules in current context
>> - one effect of the proposal is source compatibility
>> - but the goal is to build atop and strengthen 0110, 0066 and other
>> tuple-related SE
>>
>>
>> There are four difficulties I see with this proposal.
>>
>> The first is that it is a first step that quite clearly does not lead to
>> anything.  It resolves a difficulty with exactly one case of function
>> composition, but we would need completely different solutions to handle any
>> of the other compositional regressions of SE-0110.
>>
>> The second is that it's a huge source of complexity for the type system.
>> The type checker would not be able to do even rudimentary type matching,
>> e.g. when checking a call, without having first resolved all of the
>> argument and parameter types to prove that they are not Void.  This would
>> probably render it impossible to type-check many programs without some
>> ad-hoc rule of inferring that certain types are not Void.  It would
>> certainly make type-checking vastly more expensive.
>>
>> The third is that it is not possible to prevent values of Void from
>> existing, because (unlike Never, which cannot be constructed) they are
>> always created by returning from a Void-returning function, and a generic
>> function can do anything it likes with that value — turn it into an Any,
>> store it in an Array, whatever.  The proposal seems to only consider using
>> the value as a parameter.
>>
>
> Hang on, though. If Jérémie is interested only in addressing the issue of
> Void as a parameter and his idea can be adequately carried out by inferring
> a default value of Void for every parameter of type Void, this should be a
> fairly self-contained change, should it not? And would the impact on the
> cost of type checking really be vastly greater in that case?
>
>
> If the proposal was phrased in terms of defaults, e.g. "trailing
> parameters do not require a matching argument if they have Void type", then
> yes, that would be implementable because it still admits a "local"
> reduction on call constraints, one which does not need to immediately
> reason about the actual types of arguments.  It is not clear that this rule
> allows function compositions of the sort that Jérémie is looking for,
> though.
>
> Anyway, that is not the proposal; the proposal is that parameters — in any
> position — are simply removed from the parameter sequence if they have Void
> type.  In order to allow composition (i.e. f(g(x)), where g: X -> Void),
> you then need a matching rule that arguments are dropped from the argument
> sequence (for purposes of type-checking) if they have Void type.  Either of
> these rules is sufficient to turn the reduction of function-type matches
> into an extremely messy combinatoric matching problem where e.g. (τ0, Int)
> can be passed to a function taking (Int, τ1) if we can decide that τ0 == τ1
> == Void.
>
> This idea is now rather intriguing to me because it extends beyond just
> addressing one symptom of SE-0110. Swift allows us to omit the spelling out
> of return types that are Void, it allows warning-free discarding of return
> values that are Void, etc. This could add a nice consistency and
> rationalize some of the weirdness of passing a value that is stipulated by
> the parameter type.
>
> Finally, it would allow a lot of inadvertent errors with the use of
>> generic functions, because any argument of unconstrained type could be
>> accidentally specialized with Void.  For example, if you forgot to pass an
>> argument to this function, it would simply infer T=Void:
>>   func append<T>(value: T)
>> It seems more likely that this would lead to unexpected, frustrating bugs
>> than that this would actually be desired by the programmer.  You really
>> just want this to kick in in more generic situations.
>>
>
> Hmm, at first glance, that seemed like it could be bad. But if, say, a
> particular collection can store an element of type Void, is it so
> undesirable to allow `append()` to append Void?
>
>
> append on a Collection is not an unconstrained generic; its parameter type
> is determined by the type of the collection.  Perhaps this was a
> poorly-chosen example.
>
> John.
>
>
>
>> John.
>>
>>
>> —
>> very short reply expected - vsre.info
>> Jérémie Girault
>>
>> On 13 juin 2017 at 00:44:52, Xiaodi Wu (xiaodi.wu at gmail.com) wrote:
>>
>> On Mon, Jun 12, 2017 at 5:38 PM, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
>>
>>> On Mon, Jun 12, 2017 at 5:25 PM, Jérémie Girault <jeremie.girault at gmail
>>> .com> wrote:
>>>
>>>>
>>>>
>>>> —
>>>> very short reply expected - vsre.info
>>>> Jérémie Girault
>>>>
>>>> On 12 juin 2017 at 23:56:37, Xiaodi Wu (xiaodi.wu at gmail.com) wrote:
>>>>
>>>> On Mon, Jun 12, 2017 at 4:47 PM, Jérémie Girault <jeremie.girault at gmail
>>>> .com> wrote:
>>>>
>>>>> - Void as arguments is pretty common when using generics, that’s a
>>>>> core point of this proposal. An maybe that’s why we misunderstood ourselves
>>>>> (around 0110 / 0066). This proposal addresses arguments.
>>>>> - maybe it should be revised around this ? Simple example :
>>>>>
>>>>> `typealias Callback<T> = (T) -> Void` -> `Callback<Void>` will give
>>>>> `(Void) => Void`.
>>>>>
>>>>> It was acceptable before swift4 but no more. However nobody cares
>>>>> about this `Void` argument and actually we know it’s value. So why let the
>>>>> developer type it ?
>>>>>
>>>>
>>>> Ah, I see. The purpose of SE-0029...SE-0110 was to make it possible to
>>>> distinguish an argument list `(Void)` from an argument list `()`. This does
>>>> cause some verbosity where previously users relied on implicit tuple
>>>> splatting. Ideally, we would bring back some syntactic sugar to make this
>>>> more ergonomic. But, whether or not the spelling is made more
>>>> user-friendly, the point here is that _everybody_ should care about this
>>>> `Void` argument.
>>>>
>>>> It is still be typechecked and appropriate errors should be reported to
>>>> the user so _nobody_ will ignore it.
>>>>
>>>> But with the proposal the code will be striped out of Void arguments at
>>>> compile-time. I think it's a win for the developer on a lot of grounds. The
>>>> fact that this proposal integrates with the type-system is also important.
>>>>
>>>> If you are not comfortable about Void being stripped, we can also
>>>> discuss alternatives: someone was suggesting me that it would be possible
>>>> to replace :
>>>>
>>>> ```
>>>>
>>>> func foo<T, U, V>(t: T, u: U) -> V {
>>>>
>>>>   // do something with t and u
>>>>
>>>>   // return some V
>>>>
>>>> }
>>>>
>>>> ```
>>>>
>>>> with
>>>>
>>>> ```
>>>>
>>>> func foo<Void, Int, String>(u: Int) -> String { let t = ()
>>>>
>>>>   // do something with t and u
>>>>
>>>>   // return some V
>>>>
>>>> }
>>>>
>>>> ```
>>>>
>>>> or
>>>>
>>>> ```
>>>>
>>>> func foo<Void, Int, String>(t: Void = (), u: Int) -> String {
>>>>
>>>>   // do something with t and u
>>>>
>>>>   // return some V
>>>>
>>>> }
>>>>
>>>> ```
>>>>
>>>> I don’t know what you would consider more effective or elegant (at an
>>>> implementation level) but it’s the same result for the developper.
>>>>
>>>
>> Ah, but I think I catch your drift with the last example. Is this a more
>> general point that the compiler should treat every parameter of type Void
>> as having an implied default value of Void? That would be an interesting
>> idea.
>>
>> What is the goal of such changes? Is it to allow you to write `foo()`
>>> instead of `foo(())` for a function `foo` of type `(T) -> Void`?
>>>
>>> If so, then I think what you're seeking to do is reverse SE-0066 (as
>>> Vladimir points out), which explicits details how it's is an intentional
>>> change to require such a spelling. I think you're starting from the premise
>>> that this is unintended or undesirable, when in fact it is deliberate and
>>> approved.
>>>
>>> It is also, unless I'm mistaken, not the issue that was raised initially
>>> with respect to SE-0110, which had to do with the extra boilerplate of
>>> destructuring a tuple inside a closure, something that was not so obvious
>>> before implementation.
>>>
>>> My point here is that `Void` should be “striped” by “reducing” argument
>>>>> list signatures.
>>>>>
>>>>> —
>>>>> very short reply expected - vsre.info
>>>>> Jérémie Girault
>>>>>
>>>>> On 12 juin 2017 at 19:15:18, John McCall (rjmccall at apple.com) 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.
>>>>>
>>>>>
>>
>
>
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