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

Tue Oct 17 16:51:42 CDT 2017

> On Oct 17, 2017, at 2:08 PM, Kevin Nattinger <swift at nattinger.net> wrote:
> 
>> 
>> On Oct 17, 2017, at 11:47 AM, Michael Ilseman <milseman at apple.com <mailto:milseman at apple.com>> wrote:
>> 
>> 
>> 
>>> On Oct 17, 2017, at 10:15 AM, Kevin Nattinger via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>> 
>>>> Because, in my analysis, the problem is that the method is incorrectly named. The problem affects all types that conform to Sequence and not just Set and Dictionary; elementsEqual is a distinct function from ==, and it must either continue to be distinct or cease to exist, but its name does nothing to clarify any distinction.
>>> 
>>> In my analysis, the problem is the method's implementation. As I see it, the only use for `elementsEqual` is as a replacement for `==` when two objects are different types (or not known to be the same)—equal elements, and IF the sequences have an order, in the same order. Could you provide an example where `elementsEqual` randomly returning either true or false depending on internal state alone is a legitimate and desirable result?
>>> 
>> 
>> It doesn’t randomly return true or false, it consistently returns true or false for the *same* pair of Sequences. What *same* means, of course, is complicated and exists at two levels (as we have two ways of talking about *same*). 
> 
> I didn't mean literally random, but the result for any two equal sets is unpredictable and depends on implementation details.
> 

Exactly, and that’s why having this method is important. If you wish to pass a Set to a generic operation that cares about the order (e.g. generic over Sequence and not e.g. SetAlgebra), it is only substitutable with another Sequence if it happens to have the same ordering of the same elements. All orderings for Sets are arbitrary, at some level.

>> 
>> I apologize for not reading every email in depth in this thread (they are coming in faster than I can parse them), but let me try to present motivation for this and hopefully provide more shared understanding.
>> 
>> We have two forms of equality we’re talking about: equality of Sequence and equality of the elements of Sequences in their respective ordering. `==` covers the former, and I’ll use the existing (harmful) name of `elementsEqual` for the latter.
>> 
>> `==` conveys substitutability of the two Sequences. This does not necessarily entail anything about their elements, how those elements are ordered, etc., it just means two Sequences are substitutable. `elementsEqual` means that the two Sequences produce substitutable elements. These are different concepts and both are independently useful.
>> 
>> Cases:
>> 
>> 1. Two Sequences are substitutable and produce substitutable elements when iterated. `==` and `elementsEqual` both return true. 
>> 
>> Example: Two arrays with the same elements in the same order.
>> 
>> 
>> 2. Two Sequences are substitutable, but do not produce substitutable elements when iterated. `==` returns true, while `elementsEqual` returns false.
>> 
>> Example: Two Sets that contain the same elements but in a different order.
>> 
>> Contrived Example: Two Lorem Ipsum generators are the same generator (referentially equal, substitutable for the purposes of my library), but they sample the user’s current battery level (global state) each time they produce text to decide how fancy to make the faux Latin. They’re substitutable, but don’t generate the same sequence.
> 
> Evidently I disagree with your definition of "substitutable." How can you say one thing can be substituted for another when doing so gives a different result?
> 

They are substitutable for the purposes of certain operations. The key question is “what” gives a different result.

Sets are primarily about membership, and equal sets are substitutable (hand wavy) for Set-like purposes. But, two equal Sets are not substitutable for iteration purposes unless they produce the same elements in the same order. This latter requirement is far less important than the former for Sets, but can still come up in generic contexts.

Two Lorem Ipsum generators could be equal in that they are substitutable for (hand wavy) faux-Latin generation purposes, even if the sequence of generated characters happens to differ. But, if I cared about the sequences of generated characters being the same, then they are only substitutable if they produce the same characters in the same order. For example, this has ramifications on how I might test my generators.

I agree that this case is a little unintuitive, and I’m still nailing it down in my mind.

>> 
>> 
>> 3. Two Sequences are not substitutable, but produce substitutable elements when iterated. `==` returns false, while `elementsEqual` returns true.
>> 
>> Example: Consider two sequences that have differing identity. `==` operates on an identity level, `elementsEqual` operates at an element level.
>> 
>> Contrived Example: InfiniteMonkeys and Shakespeare both produce the same sonnet, but they’re not substitutable for my library’s purposes. 
> 
> The way I see it, this is exactly the reason for this function—determining whether two objects give the same sequence without regards to their types.
> 

What if they are the same type? What if they are the same type but that type has identity? These are two different and (potentially) orthogonal axes.

>> 
>> 
>> 4. Two Sequences are not substitutable and don’t produce substitutable elements when iterated. `==` and `elementsEqual` both return false.
>> 
>> Example: `[1,2,3]` compared to `[4,5,6]`
>> 
>> 
>> It is true that situations #2 and #3 are a little harder to grok, but they are what illustrate the subtle difference at hand. I think situation #2 is the most confusing, and has been the primary focus of this thread as Set exists and exhibits it.
> 
> Indeed. My opinion is that #2 is not just confusing, but an entirely invalid result/state. 
> 

It isn’t *entirely* invalid if even contrived examples exist. It might be harmful for some types such as Set, and an imperfect remedy might be to warn when used on concrete Set types. Certainly not calling it `elementsEqual` but instead something that implies there’s ordering impact is an improvement, however incremental.

>> 
>> 
>> Now, onto naming. `elementsEqual` is a very poor choice of name for the concept of equality of elements in their respective orderings, as it doesn’t highlight the “in their respective orderings” part. `lexicographicallyEqual` highlights the ordering much better, as “abc” is not lexicographically equal to “cba” despite having equal elements. I think it is clearly an improvement over the status quo. I like something a little more explicit (e.g. `elementsOrderedEqual`), personally, but I don’t care that strongly. I’m just glad to see `elementsEqual` getting some clarification.
>> 
>> 
>> 
>> 
>>>> 
>>>> Thanks,
>>>> Jon
>>>> _______________________________________________
>>>> swift-evolution mailing list
>>>> swift-evolution at swift.org <mailto:swift-evolution at swift.org>
>>>> https://lists.swift.org/mailman/listinfo/swift-evolution <https://lists.swift.org/mailman/listinfo/swift-evolution>
>>> _______________________________________________
>>> swift-evolution mailing list
>>> swift-evolution at swift.org <mailto:swift-evolution at swift.org>
>>> https://lists.swift.org/mailman/listinfo/swift-evolution <https://lists.swift.org/mailman/listinfo/swift-evolution>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.swift.org/pipermail/swift-evolution/attachments/20171017/aba5ee14/attachment.html>