> On Oct 16, 2017, at 1:08 PM, Xiaodi Wu via swift-evolution > <swift-evolution@swift.org> wrote: > > On Mon, Oct 16, 2017 at 13:15 David Sweeris <daveswee...@mac.com > <mailto:daveswee...@mac.com>> wrote: > > On Oct 16, 2017, at 09:21, Michael Ilseman <milse...@apple.com > <mailto:milse...@apple.com>> wrote: >> >> 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.
Even if a Set value has a fixed order, a copy of that value may have a *different* order. How many generic algorithm implementations are going to be confused by that? -- Greg Parker gpar...@apple.com <mailto:gpar...@apple.com> Runtime Wrangler
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