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

Jonathan Hull jhull at gbis.com
Sat Oct 14 22:55:21 CDT 2017

> On Oct 14, 2017, at 7:55 PM, Xiaodi Wu via swift-evolution <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. 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.

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