Ah, right! On Tue, Jun 13, 2017 at 7:40 PM, Xiaodi Wu <[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]> 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 <jeremie.girault@ >>>>> gmail.com> 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 <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 ([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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