> On 17. Oct 2017, at 09:48, Jonathan Hull via swift-evolution
> <swift-evolution@swift.org> wrote:
>
>
>> On Oct 17, 2017, at 12:04 AM, Gwendal Roué via swift-evolution
>> <swift-evolution@swift.org> wrote:
>>
>>>> Modeling is, by definition, imperfect. The question is, what imperfect
>>>> model is most useful _to Swift_. The idea is that conforming Set and
>>>> Dictionary to Collection is on balance more useful than not doing so; that
>>>> having two protocols, Sequence and Collection, is on balance more useful
>>>> than having one or three, and that the set of semantic guarantees of
>>>> Collection are on balance more useful than other possible sets of semantic
>>>> guarantees.
>>>
>>> That is your idea which is disputed and underlined with arguments whereas
>>> you keep repeating that Set behaves as dictated by its conformance without
>>> giving use cases why this should be useful.
>>
>> Hello,
>>
>> You can't *control* the ordering of a set or a dictionary, but you can still
>> *rely* on it.
>>
>> For example, to find a key in a dictionary that is associated a given value,
>> you can rely on the fact that a dictionary's order is guaranteed to be
>> stable, and that on top of that its indexes can address the dictionary
>> itself, but also its keys and values sequences. The code below has no bug;
>>
>> let dict = ["a": "foo", "b": "bar", "c": "needle"]
>>
>> // Find a key associated with "needle"
>> if let index = dict.values.index(of: "needle") {
>> let key = dict.keys[index]
>> print(key) // prints "c"
>> }
>
> You are using the index from one collection to index into another collection.
> Isn’t that something the documentation explicitly tells us not to do? Or is
> there a special (documented) guarantee that these collections will always
> sync indices?
The documentation of Dictionary.values states that
"When iterated over, values appear in this collection in the same order as they
occur in the dictionary’s key-value pairs."
and similarly for Dictionary.keys. So that should be safe, as I understand it.
Martin
>
> I guess you could do:
>
> if let key = dict.first(where {$1 == “needle}).0 {
> print(key)
> }
>
> …although note that which element is “first” if there are multiple keys with
> needle as the value may shift around on you as you mess with the dictionary
> (and also on separate runs even if you don’t mess with the dictionary).
>
>> It's more difficult to find a use case for set's ordering and indexes. But
>> since you ask, here is an example. The goal is to find any element which is
>> not equal to another value, in any collection:
>>
>> extension Collection where Element: Equatable {
>> /// Returns any element which is not equal to the given element
>> func anyElement(notEqualTo v: Element) -> Element? {
>> if let i = index(of: v) {
>> if let alt = index(i, offsetBy: 1, limitedBy: endIndex), alt !=
>> endIndex {
>> return self[alt]
>> }
>> if i == startIndex {
>> return nil
>> }
>> return first
>> }
>> return first
>> }
>> }
>>
>> Set([1, 2, 3]).anyElement(notEqualTo: 1) // 2 or 3
>> Set([1, 2]).anyElement(notEqualTo: 1) // 2
>> Set([1]).anyElement(notEqualTo: 1) // nil
>> Set([2]).anyElement(notEqualTo: 1) // 2
>> Set([]).anyElement(notEqualTo: 1) // nil
>>
>> That *can* be useful, isn't it?
>
> Yes, although this could easily be provided using iteration without indexing.
>
>
> Note that my current favorite solution is to simply provide an additional
> guarantee on set/dictionary that the iteration order will always be the same
> for the same contents (regardless of history), but the order would still be
> otherwise arbitrary. That is a non-source-breaking change (renaming IS
> source breaking). It won’t fix everything, but it will fix some of the
> biggest gotchas. As a bonus, elementsEqual will 'just work'™ for
> sets/dictionaries.
>
> Thanks,
> Jon
>
>
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