[swift-evolution] [Proposal] Change Void meaning

Tue Jun 13 12:40:11 CDT 2017

Note that “inout Void” is a distinct type from “Void”; it is not possible
to specify a default value for an inout Void parameter even explicitly
(“error: cannot pass immutable value of type ()...”), so naturally it
cannot be done implicitly either.
On Tue, Jun 13, 2017 at 12:29 Jens Persson <jens at bitcycle.com> wrote:

> 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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>>
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