[swift-evolution] [Draft] Rename Sequence.elementsEqual

[Jonathan Hull]

> On Oct 17, 2017, at 5:44 AM, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
> 
> 
> On Tue, Oct 17, 2017 at 00:56 Thorsten Seitz <tseitz42 at icloud.com <mailto:tseitz42 at icloud.com>> wrote:
> 
> 
> Am 17.10.2017 um 00:13 schrieb Xiaodi Wu <xiaodi.wu at gmail.com <mailto:xiaodi.wu at gmail.com>>:
> 
>> 
>> On Mon, Oct 16, 2017 at 14:21 Thorsten Seitz <tseitz42 at icloud.com <mailto:tseitz42 at icloud.com>> wrote:
>>> Am 16.10.2017 um 16:20 schrieb Xiaodi Wu via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>>:
>>> 
>>> 
>>> On Mon, Oct 16, 2017 at 05:48 Jonathan Hull <jhull at gbis.com <mailto:jhull at gbis.com>> wrote:
>>> 
>>>> On Oct 15, 2017, at 9:58 PM, Xiaodi Wu <xiaodi.wu at gmail.com <mailto:xiaodi.wu at gmail.com>> wrote:
>>>> 
>>>> On Sun, Oct 15, 2017 at 8:51 PM, Jonathan Hull <jhull at gbis.com <mailto:jhull at gbis.com>> wrote:
>>>> 
>>>>> On Oct 14, 2017, at 10:48 PM, Xiaodi Wu <xiaodi.wu at gmail.com <mailto:xiaodi.wu at 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).
>> 
>> But it is a behavior which has absolutely no meaning at all because the order does not depend on the elements of the set but on the history of how the set has been reached its current state.
>> So why should I ever use this method on a set? 
>> What is the use case?
>> 
>> One example: you can use it to check an instance of Set<Float> to determine if it has a NaN value. (The “obvious” way of doing it is not guaranteed to work since NaN != NaN.)
> 
> How would I do that? I'd rather expect to use a property isNaN on Float to do that.
> 
> set.elementsEqual(set)

I see why that would work (thanks to Set being a collection vs a sequence), but it still feels like a hack.  I definitely wouldn’t want to be maintaining code with that in it. Especially when compared to something like:

	set.contains(where: {$0.isNaN})


>>> I don’t see why a non-source-breaking change is suddenly off-limits.
>>> 
>>> But more than that, any generic algorithm which is assuming that the sequence is coming from an ordered source (i.e. many things using first/last).  Some uses of first are ok because the programmer actually means ‘any’, but anywhere where they actually mean first/last may be problematic.
>>> 
>>> Such as...?
>>> 
>>> Currently, there is no way to test for ordered-ness, so there is no way for even a careful programmer to mitigate this problem.  By adding a protocol which states that something is unordered, we can either branch on it, or create a separate version of an algorithm for things which conform.
>>> 
>>> It is clearly the case that Swift’s protocol hierarchy fits sets and collections imperfectly; however, it is in the nature of modeling that imperfections are present. The question is not whether it is possible to incur performance, API surface area, and other trade-offs to make the model more faithful, but rather whether this usefully solves any problem. What is the problem being mitigated? As I write above, Swift’s Set and Dictionary types meet the semantic requirements for Collection and moonlight as ordered collections. What is a generic algorithm on an ordered collection that is  “not OK” for Set and Dictionary? (“elementsEqual”, as I’ve said, is not such an example.)
>> 
>> On the contrary, `elementsEqual` is exactly such an example, because it makes no sense to use it on a Set.
>> 
>> let s1 = Set([1,2,3,4,5,6])
>> let s2 = Set([6,5,4,3,2,1])
>> 
>> Both sets have different iteration orders. Comparing those sets with some other collection using `elementsEqual` will give no meaningful result because the order - and therefore the result of `elementsEqual` - is in effect random.
>> 
>> No, it is not such an example; it’s misleadingly named but works correctly—that is, its behavior matches exactly the documented behavior, which relies on only the semantic guarantees of Sequence, which Set correctly fulfills.
> 
> Fulfills to the letter. Again, what can you do with it if the result is random??
> 
> The result is not random.

It is undefined though.  As you said earlier, by the guarantees we have been given, it may shift over different builds/runs of a program.  Thus in one run, it might return true and then false in another (without changing our code).  As Greg pointed out, it is also possible with the guarantees we are given, for set/dict to have different orderings with copies of themselves. (It will happen for sure when deep copying a dictionary with reference-type keys).

As I keep saying, relying on the behavior of a leaking internal implementation is a bad plan.


We should add an additional guarantee to set/dict that the order returned will be the same for the same contents regardless of history (but can be otherwise arbitrary).  That will fix the behavior for algorithms like elementsEqual (i.e. They will return the same result across builds/runs).  It will also implicitly provide as a result, the constraint you were arguing is needed across copies of a collection type.  I agree that that is an important guarantee.  Why not fix both issues with a single non-source-breaking change?

Why is the source-breaking change of renaming things better?

Thanks,
Jon





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