But yes, I agree with the overall point that (while a very interesting idea (IMO)) there are definitely a _lot_ of cases to consider before it’s clearly viable as a solution. On Tue, Jun 13, 2017 at 12:43 Jens Persson <j...@bitcycle.com> wrote:
> Ah, right! > > On Tue, Jun 13, 2017 at 7:40 PM, Xiaodi Wu <xiaodi...@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 <j...@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@swift.org> wrote: >>> >>>> >>>> On Jun 13, 2017, at 4:41 AM, Xiaodi Wu <xiaodi...@gmail.com> wrote: >>>> >>>> On Tue, Jun 13, 2017 at 3:06 AM, John McCall <rjmcc...@apple.com> >>>> wrote: >>>> >>>>> On Jun 13, 2017, at 3:30 AM, Jérémie Girault < >>>>> jeremie.gira...@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...@gmail.com) wrote: >>>>> >>>>> On Mon, Jun 12, 2017 at 5:38 PM, Xiaodi Wu <xiaodi...@gmail.com> >>>>> wrote: >>>>> >>>>>> On Mon, Jun 12, 2017 at 5:25 PM, Jérémie Girault < >>>>>> jeremie.gira...@gmail.com> wrote: >>>>>> >>>>>>> >>>>>>> >>>>>>> — >>>>>>> very short reply expected - vsre.info >>>>>>> Jérémie Girault >>>>>>> >>>>>>> On 12 juin 2017 at 23:56:37, Xiaodi Wu (xiaodi...@gmail.com) wrote: >>>>>>> >>>>>>> On Mon, Jun 12, 2017 at 4:47 PM, Jérémie Girault < >>>>>>> jeremie.gira...@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 (rjmcc...@apple.com) >>>>>>>> wrote: >>>>>>>> >>>>>>>> >>>>>>>> On Jun 12, 2017, at 4:48 AM, Jérémie Girault via swift-evolution < >>>>>>>> swift-evolution@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. >>>>>>>> >>>>>>>> >>>>> >>>> >>>> >>>> _______________________________________________ >>>> swift-evolution mailing list >>>> swift-evolution@swift.org >>>> https://lists.swift.org/mailman/listinfo/swift-evolution >>>> >>>> >
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