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

Thorsten Seitz tseitz42 at icloud.com
Tue Oct 17 01:03:50 CDT 2017

> Am 17.10.2017 um 01:43 schrieb Xiaodi Wu via swift-evolution <swift-evolution at swift.org>:
>> 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 argument.

Why do you not answe the question, what these methods *mean* for a Set?
Still waiting for a use case.


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