[swift-evolution] [Proposal] Change Void meaning

[Jens Persson]

Ah, right!

On Tue, Jun 13, 2017 at 7:40 PM, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:

> 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@
>>>>> 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@
>>>>>> 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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>>> swift-evolution mailing list
>>> swift-evolution at swift.org
>>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>>
>>>
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