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

[Thorsten Seitz]

> Am 17.10.2017 um 14:46 schrieb Xiaodi Wu <xiaodi.wu at gmail.com>:
> 
>> On Tue, Oct 17, 2017 at 01:03 Thorsten Seitz <tseitz42 at icloud.com> wrote:
>> 
>> 
>>> 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.
> 
> The method means exactly what I just said: the iteration order of one set matches the iteration order of another sequence. I’ve given you one use case and others have given more.

Sorry, the use case you gave was just a clever trick instead of using `contains` and Float.isNaN. No one else has provided a use case for `elementsEqual` yet.

-Thorsten


> 
>> 
>> -Thorsten
>> 
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