> Am 17.10.2017 um 03:20 schrieb Xiaodi Wu via swift-evolution > <swift-evolution@swift.org>: > >> On Mon, Oct 16, 2017 at 8:03 PM, David Sweeris <daveswee...@mac.com> wrote: >> >>> On Oct 16, 2017, at 1:07 PM, Xiaodi Wu <xiaodi...@gmail.com> wrote: >>> >>> >>>> On Mon, Oct 16, 2017 at 13:15 David Sweeris <daveswee...@mac.com> wrote: >>>> >>>>> On Oct 16, 2017, at 09:21, Michael Ilseman <milse...@apple.com> wrote: >>>>> >>>>> >>>>> >>>>>> On Oct 16, 2017, at 8:46 AM, David Sweeris via swift-evolution >>>>>> <swift-evolution@swift.org> wrote: >>>>>> >>>>>> >>>>>> On Oct 16, 2017, at 07:20, Xiaodi Wu via swift-evolution >>>>>> <swift-evolution@swift.org> wrote: >>>>>> >>>>>>> >>>>>>>> On Mon, Oct 16, 2017 at 05:48 Jonathan Hull <jh...@gbis.com> wrote: >>>>>>>> >>>>>>>>> On Oct 15, 2017, at 9:58 PM, Xiaodi Wu <xiaodi...@gmail.com> wrote: >>>>>>>>> >>>>>>>>>> On Sun, Oct 15, 2017 at 8:51 PM, Jonathan Hull <jh...@gbis.com> >>>>>>>>>> wrote: >>>>>>>>>> >>>>>>>>>>>> On Oct 14, 2017, at 10:48 PM, Xiaodi Wu <xiaodi...@gmail.com> >>>>>>>>>>>> wrote: >>>>>>>>>>>>>>> That ordering can be arbitrary, but it shouldn’t leak internal >>>>>>>>>>>>>>> representation such that the method used to create identical >>>>>>>>>>>>>>> things affects the outcome of generic methods because of >>>>>>>>>>>>>>> differences in internal representation. >>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>> It would be better to say that the iteration order is >>>>>>>>>>>>>>>>> well-defined. That will almost always mean documented, and >>>>>>>>>>>>>>>>> usually predictable though obviously e.g. RNGs and iterating >>>>>>>>>>>>>>>>> in random order will not be predictable by design. >>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>>> That's actually more semantically constrained than what >>>>>>>>>>>>>>>>>>> Swift calls a `Collection` (which requires conforming types >>>>>>>>>>>>>>>>>>> to be multi-pass and(?) finite). By contrast, Swift's >>>>>>>>>>>>>>>>>>> `SpongeBob` protocol explicitly permits conforming >>>>>>>>>>>>>>>>>>> single-pass, infinite, and/or unordered types. >>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>> I think you’re talking about Sequence here, I’ve lost track >>>>>>>>>>>>>>>>>> of your nonsense by now. Yes, the current Swift protocol >>>>>>>>>>>>>>>>>> named Sequence allows unordered types. You seem to keep >>>>>>>>>>>>>>>>>> asserting that but not actually addressing my argument, >>>>>>>>>>>>>>>>>> which is that allowing Sequences to be unordered with the >>>>>>>>>>>>>>>>>> current API is undesired and actively harmful, and should >>>>>>>>>>>>>>>>>> therefore be changed. >>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>> What is harmful about it? >>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>> After thinking about it, I think the harmful bit is that >>>>>>>>>>>>>>>> unordered sequences are leaking internal representation (In >>>>>>>>>>>>>>>> your example, this is causing people to be surprised when two >>>>>>>>>>>>>>>> sets with identical elements are generating different >>>>>>>>>>>>>>>> sequences/orderings based on how they were created). You are >>>>>>>>>>>>>>>> correct when you say that this problem is even true for for-in. >>>>>>>>>>>>>>> >>>>>>>>>>>>>>> I would not say it is a problem. Rather, by definition, >>>>>>>>>>>>>>> iteration involves retrieving one element after another; if >>>>>>>>>>>>>>> you're allowed to do that with Set, then the elements of a Set >>>>>>>>>>>>>>> are observably ordered in some way. Since it's not an >>>>>>>>>>>>>>> OrderedSet--i.e., order doesn't matter--then the only sensible >>>>>>>>>>>>>>> conclusion is that the order of elements obtained in a for...in >>>>>>>>>>>>>>> loop must be arbitrary. If you think this is harmful, then you >>>>>>>>>>>>>>> must believe that one should be prohibited from iterating over >>>>>>>>>>>>>>> an instance of Set. Otherwise, Set is inescapably a Sequence by >>>>>>>>>>>>>>> the Swift definition of Sequence. All extension methods on >>>>>>>>>>>>>>> Sequence like drop(while:) are really just conveniences for >>>>>>>>>>>>>>> common things that you can do with iterated access; to my mind, >>>>>>>>>>>>>>> they're essentially just alternative ways of spelling various >>>>>>>>>>>>>>> for...in loops. >>>>>>>>>>>>>> >>>>>>>>>>>>>> I think an argument could be made that you shouldn’t be able to >>>>>>>>>>>>>> iterate over a set without first defining an ordering on it >>>>>>>>>>>>>> (even if that ordering is somewhat arbitrary). Maybe we have >>>>>>>>>>>>>> something like a “Sequenc(e)able” protocol which defines things >>>>>>>>>>>>>> which can be turned into a sequence when combined with some sort >>>>>>>>>>>>>> of ordering. One possible ordering could be the internal >>>>>>>>>>>>>> representation (At least in that case we are calling it out >>>>>>>>>>>>>> specifically). If I had to say >>>>>>>>>>>>>> “setA.arbitraryOrder.elementsEqual(setB.arbitraryOrder)” I would >>>>>>>>>>>>>> definitely be less surprised when it returns false even though >>>>>>>>>>>>>> setA == setB. >>>>>>>>>>>>> >>>>>>>>>>>>> Well, that's a totally different direction, then; you're arguing >>>>>>>>>>>>> that `Set` and `Dictionary` should not conform to `Sequence` >>>>>>>>>>>>> altogether. That's fine (it's also a direction that some of us >>>>>>>>>>>>> explored off-list a while ago), but at this point in Swift's >>>>>>>>>>>>> evolution, realistically, it's not within the realm of possible >>>>>>>>>>>>> changes. >>>>>>>>>>>> >>>>>>>>>>>> I am actually suggesting something slightly different. Basically, >>>>>>>>>>>> Set and Dictionary’s conformance to Collection would have a >>>>>>>>>>>> different implementation. They would conform to another protocol >>>>>>>>>>>> declaring that they are unordered. That protocol would fill in >>>>>>>>>>>> part of the conformance to sequence/collection using a default >>>>>>>>>>>> ordering, which is mostly arbitrary, but guaranteed to produce the >>>>>>>>>>>> same ordering for the same list of elements (even across >>>>>>>>>>>> collection types). This would be safer, but a tiny bit slower >>>>>>>>>>>> than what we have now (We could also potentially develop a way for >>>>>>>>>>>> collections like set to amortize the cost). For those who need to >>>>>>>>>>>> recover speed, the new protocol would also define a property which >>>>>>>>>>>> quickly returns a sequence/iterator using the internal ordering (I >>>>>>>>>>>> arbitrarily called it .arbitraryOrder). >>>>>>>>>>>> >>>>>>>>>>>> I believe it would not be source breaking. >>>>>>>>>>> >>>>>>>>>>> That is indeed something slightly different. >>>>>>>>>>> >>>>>>>>>>> In an ideal world--and my initial understanding of what you were >>>>>>>>>>> suggesting--Set and Dictionary would each have a member like >>>>>>>>>>> `collection`, which would expose the underlying data as a >>>>>>>>>>> `SetCollection` or `DictionaryCollection` that in turn would >>>>>>>>>>> conform to `Collection`; meanwhile, Set and Dictionary themselves >>>>>>>>>>> would not offer methods such as `prefix`, or indexing by subscript, >>>>>>>>>>> which are not compatible with being unordered. For those who want a >>>>>>>>>>> particular ordering, there'd be something like `collection(ordered >>>>>>>>>>> areInIncreasingOrder: (T, T) -> Bool) -> >>>>>>>>>>> {Set|Dictionary}Collection`. >>>>>>>>>>> >>>>>>>>>>> What you suggest here instead would be minimally source-breaking. >>>>>>>>>>> However, I'm unsure of where these guarantees provide benefit to >>>>>>>>>>> justify the performance cost. Certainly not for `first` or >>>>>>>>>>> `dropFirst(_:)`, which still yields an arbitrary result which >>>>>>>>>>> doesn't make sense for something _unordered_. We *could* have an >>>>>>>>>>> underscored customization point named something like >>>>>>>>>>> `_customOrderingPass` that is only invoked from `elementsEqual` or >>>>>>>>>>> other such methods to pre-rearrange the internal ordering of >>>>>>>>>>> unordered collections in some deterministic way before comparison. >>>>>>>>>>> Is that what you have in mind? >>>>>>>>>> >>>>>>>>>> >>>>>>>>>> Something like that. Whatever we do, there will be a tradeoff >>>>>>>>>> between speed, correctness, and ergonomics. >>>>>>>>>> >>>>>>>>>> My suggestion trades speed for correctness, and provides a way to >>>>>>>>>> recover speed through additional typing (which is slightly less >>>>>>>>>> ergonomic). >>>>>>>>> >>>>>>>>> You haven't convinced me that this is at all improved in >>>>>>>>> "correctness." It trades one arbitrary iteration order for another on >>>>>>>>> a type that tries to model an unordered collection. >>>>>>>>> >>>>>>>>>> We could do something like you suggest. I don’t think the method >>>>>>>>>> would need to be underscored… the ordering pass could just be a >>>>>>>>>> method on the protocol which defines it as unordered. Then we could >>>>>>>>>> provide a special conformance for things where order really matters >>>>>>>>>> based on adherence to that protocol. That might be an acceptable >>>>>>>>>> tradeoff. It would give us speed at the cost of having the correct >>>>>>>>>> implementation being less ergonomic and more error prone (you have >>>>>>>>>> to remember to check that it is unordered and call the ordering >>>>>>>>>> method when it mattered). >>>>>>>>>> >>>>>>>>>> I’d still be a bit worried that people would make incorrect generic >>>>>>>>>> algorithms based on expecting an order from unordered things, but at >>>>>>>>>> least it would be possible for them check and handle it correctly. >>>>>>>>>> I think I could get behind that tradeoff/compromise, given where we >>>>>>>>>> are in the swift process and Swift's obsession with speed (though I >>>>>>>>>> still slightly prefer the safer default). At least the standard >>>>>>>>>> library would handle all the things correctly, and that is what will >>>>>>>>>> affect the majority of programmers. >>>>>>>>> >>>>>>>>> What is an example of such an "incorrect" generic algorithm that >>>>>>>>> would be made correct by such a scheme? >>>>>>>> >>>>>>>> To start with, the one you gave as an example at the beginning of this >>>>>>>> discussion: Two sets with identical elements which have different >>>>>>>> internal storage and thus give different orderings as sequences. You >>>>>>>> yourself have argued that the confusion around this is enough of a >>>>>>>> problem that we need to make a source-breaking change (renaming it) to >>>>>>>> warn people that the results of the ‘elementsEqual’ algorithm are >>>>>>>> undefined for sets and dictionaries. >>>>>>> >>>>>>> No, I am arguing that the confusion about ‘elementsEqual’ is foremost a >>>>>>> problem with its name; the result of this operation is not at all >>>>>>> undefined for two sets but actually clearly defined: it returns true if >>>>>>> two sets have the same elements in the same iteration order, which is a >>>>>>> publicly observable behavior of sets (likewise dictionaries). >>>>>> >>>>>> How is the iteration order of an unordered set or dictionary “publicly >>>>>> observable”? If either is implemented such that it can asynchronously >>>>>> optimize its storage (maybe by rebalancing a tree or merging two >>>>>> non-contiguous array segments or something), its iteration order could >>>>>> change without changing what values it contains. Seems like consecutive >>>>>> calls to “elementsEquals” (or whatever we’re calling it) should return >>>>>> the same answer, if we don’t add, remove, or mutate elements. >>>>> >>>>> Sets are values. If you add, remove, or mutate any elements you have a >>>>> different Set and thus a potentially different ordering of elements. >>>> >>>> From the “value semantics” PoV, yes. But from the “unordered collection of >>>> values” PoV, Sets/Dictionaries, being unordered, are semantically free to >>>> rearrange the in-memory ordering of their elements without user >>>> intervention. >>> >>> No, they are not semantically free to do so. The semantics of Collection >>> forbid it, because the iteration order must be multi-pass. As long as the >>> value is unchanged, the iteration order is unchanged. That is a documented, >>> public guarantee of the API. >> >> If the semantics of unordered collections (with a lowercase c) and of >> `Collection` (with an uppercase C) differ on such a basic level, then why >> are we trying to force them together? I mean, I understand that >> source-compatibility is important, but so is correctly modeling that which >> we claim to model. > > Modeling is, by definition, imperfect. The question is, what imperfect model > is most useful _to Swift_. The idea is that conforming Set and Dictionary to > Collection is on balance more useful than not doing so; that having two > protocols, Sequence and Collection, is on balance more useful than having one > or three, and that the set of semantic guarantees of Collection are on > balance more useful than other possible sets of semantic guarantees.
That is your idea which is disputed and underlined with arguments whereas you keep repeating that Set behaves as dictated by its conformance without giving use cases why this should be useful. -Thorsten > _______________________________________________ > swift-evolution mailing list > swift-evolution@swift.org > https://lists.swift.org/mailman/listinfo/swift-evolution
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