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

Michael Ilseman milseman at apple.com
Tue Oct 17 13:47:33 CDT 2017

> On Oct 17, 2017, at 10:15 AM, Kevin Nattinger via swift-evolution <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 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.


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.

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. 

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.

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
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