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 <[email protected]> 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 < > [email protected]> wrote: > >> >> On Jun 13, 2017, at 4:41 AM, Xiaodi Wu <[email protected]> wrote: >> >> On Tue, Jun 13, 2017 at 3:06 AM, John McCall <[email protected]> wrote: >> >>> On Jun 13, 2017, at 3:30 AM, Jérémie Girault <[email protected]> >>> 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 ([email protected]) wrote: >>> >>> On Mon, Jun 12, 2017 at 5:38 PM, Xiaodi Wu <[email protected]> wrote: >>> >>>> On Mon, Jun 12, 2017 at 5:25 PM, Jérémie Girault < >>>> [email protected]> wrote: >>>> >>>>> >>>>> >>>>> — >>>>> very short reply expected - vsre.info >>>>> Jérémie Girault >>>>> >>>>> On 12 juin 2017 at 23:56:37, Xiaodi Wu ([email protected]) wrote: >>>>> >>>>> On Mon, Jun 12, 2017 at 4:47 PM, Jérémie Girault < >>>>> [email protected]> 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 ([email protected]) wrote: >>>>>> >>>>>> >>>>>> On Jun 12, 2017, at 4:48 AM, Jérémie Girault via swift-evolution < >>>>>> [email protected]> 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 >> [email protected] >> https://lists.swift.org/mailman/listinfo/swift-evolution >> >>
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