I would like to note that overloading on inout is a code smell and something that should be removed in a future version of the language. inout is not a type, it’s a SIL type residency annotation.
~Robert Widmann > On Jun 13, 2017, at 10:40 AM, Xiaodi Wu via swift-evolution > <[email protected]> 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 <[email protected] > <mailto:[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] <mailto:[email protected]>> wrote: > >> On Jun 13, 2017, at 4:41 AM, Xiaodi Wu <[email protected] >> <mailto:[email protected]>> wrote: >> >> On Tue, Jun 13, 2017 at 3:06 AM, John McCall <[email protected] >> <mailto:[email protected]>> wrote: >>> On Jun 13, 2017, at 3:30 AM, Jérémie Girault <[email protected] >>> <mailto:[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 <http://vsre.info/> >>> Jérémie Girault >>> >>> On 13 juin 2017 at 00:44:52, Xiaodi Wu ([email protected] >>> <mailto:[email protected]>) wrote: >>> >>>> On Mon, Jun 12, 2017 at 5:38 PM, Xiaodi Wu <[email protected] >>>> <mailto:[email protected]>> wrote: >>>> On Mon, Jun 12, 2017 at 5:25 PM, Jérémie Girault >>>> <[email protected] <mailto:[email protected]>> wrote: >>>> >>>> >>>> — >>>> very short reply expected - vsre.info <http://vsre.info/> >>>> Jérémie Girault >>>> >>>> On 12 juin 2017 at 23:56:37, Xiaodi Wu ([email protected] >>>> <mailto:[email protected]>) wrote: >>>> >>>>> On Mon, Jun 12, 2017 at 4:47 PM, Jérémie Girault >>>>> <[email protected] <mailto:[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 <http://vsre.info/> >>>>> Jérémie Girault >>>>> >>>>> On 12 juin 2017 at 19:15:18, John McCall ([email protected] >>>>> <mailto:[email protected]>) wrote: >>>>> >>>>>> >>>>>>> On Jun 12, 2017, at 4:48 AM, Jérémie Girault via swift-evolution >>>>>>> <[email protected] <mailto:[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] <mailto:[email protected]> > https://lists.swift.org/mailman/listinfo/swift-evolution > <https://lists.swift.org/mailman/listinfo/swift-evolution> > > _______________________________________________ > swift-evolution mailing list > [email protected] > https://lists.swift.org/mailman/listinfo/swift-evolution
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