On Sat, Oct 14, 2017 at 10:55 PM, Jonathan Hull <jh...@gbis.com> wrote:
> > On Oct 14, 2017, at 7:55 PM, Xiaodi Wu via swift-evolution < > swift-evolution@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? > 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.
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