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

[Xiaodi Wu]

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.


> -Thorsten
>
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