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

Xiaodi Wu xiaodi.wu at gmail.com
Mon Oct 16 18:42:46 CDT 2017

On Mon, Oct 16, 2017 at 6:10 PM, Jonathan Hull <jhull at gbis.com> wrote:

> 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> wrote:
>> On Oct 16, 2017, at 7:20 AM, Xiaodi Wu <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”?

In what way do these algorithms break? They would continue to
determine--correctly--whether an instance of Set, when iterated, begins
with, ends with, or contains (respectively) a subsequence that matches the
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