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

Jonathan Hull jhull at gbis.com
Mon Oct 16 18:10:48 CDT 2017

> On Oct 16, 2017, at 1:05 PM, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
> On Mon, Oct 16, 2017 at 10:49 Jonathan Hull <jhull at gbis.com <mailto:jhull at gbis.com>> wrote:
>> On Oct 16, 2017, at 7:20 AM, Xiaodi Wu <xiaodi.wu at gmail.com <mailto:xiaodi.wu at gmail.com>> wrote:
>> 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 that iteration order is undefined and could easily change due to changes in the private/internal structure of sets/dictionaries.  Algorithms that rely on that “publicly observable behavior” (i.e. leaking of internals) will suddenly break.
> And an algorithm in which such “sudden breakage” would occur is…?

Here are a few off the top of my head:

func hasPrefix(Sequence)->Bool
func hasSuffix(Sequence)->Bool
func containsSubsequence(Sequence)->Bool

What do these methods mean with regards to Set’s “publicly observable behavior”?

> You keep claiming that this bug is a feature because it is the current behavior… but that is tautological reasoning.
> Thanks,
> Jon

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