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

[Jonathan Hull]

> On Oct 14, 2017, at 9:21 PM, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
> 
> On Sat, Oct 14, 2017 at 10:55 PM, Jonathan Hull <jhull at gbis.com <mailto:jhull at gbis.com>> wrote:
> 
>> On Oct 14, 2017, at 7:55 PM, Xiaodi Wu via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> 
>> > Ordered, yes, but it’s only admittedly poor wording that suggests multi-pass, and I don’t think anything there suggests finite.
>> 
>> If a Sequence is "guaranteed to iterate the same every time," then surely it must be multi-pass; what's the alternative?
> 
> Single-pass, but where two dictionaries/sets with the same elements would be guaranteed to output the same ordering.
> 
> I'm not sure I understand. A single-pass sequence is one where iteration can happen only once because it is destructive. By definition, then, it is not guaranteed to "iterate the same" a second time. Neither sets nor dictionaries are single-pass sequences. Kevin says that his definition of a "Sequence" is something "guaranteed to iterate the same every time," which requires them to be multi-pass, does it not?

But if I am comparing two single-pass things, the order can still be defined when they compare that one time.  Single-pass doesn’t mean that the order is undefined.  On the contrary, as you point out, it has a “first” thing, and then a thing after that, and so on.

Regardless, most of the objects we are talking about here are multi-pass collections (e.g. sets).


>  
> 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.

If that is unergonomic, we could define an arbitrary but consistent ordering over all Swift types that can be used to create a predictable sequence order for unordered types.  That is necessarily slower, but much safer… and people concerned with speed could use something like ‘arbitraryOrder’ above to regain full speed.

I am not arguing that that is necessarily the right approach, just that we need more thought/discussion around what is actually causing the confusion here: The fact that we are assuming an ordering on something where the ordering is undefined.

Thanks,
Jon









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