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

Kevin Nattinger swift at nattinger.net
Thu Oct 12 23:57:19 CDT 2017


1. I strongly object to the proposed name. It doesn't make it more clear to me what the method does, and is misleading at best. Among other issues, "lexicographical" is defined as alphabet order, and (1) this method applies to objects that are not Strings, and (2) this method's behavior isn't any more well-defined for Strings, so that name is even more of a lie than the original.

2. This is really just a symptom of a bigger problem. The fact that two Sets can compare equal and yet return different results for that method (among too many others) is logically inconsistent and points to a much deeper issue with Set and Sequence. It is probably about 3 releases too late to get this straightened out properly, but I'll outline the real issue in case someone has an idea for fixing it.

The root of the problem is that Set conforms to Sequence, but Sequence doesn't require a well-defined order. Since Set doesn't have a well-defined order, a significant portion of its interface is unspecified. The methods are implemented because they have to be, but they doesn't have well-defined or necessarily consistent results.

A sequence is, by definition, ordered. That is reflected in the fact that over half the methods in the main Sequence definition* make no sense and are not well-defined unless there is a well-defined order to the sequence itself. What does it even mean to `dropFirst()` in a Set? The fact that two objects that compare equal can give different results for a 100% deterministic function is illogical, nonsensical, and dangerous.

* 7/12 by my count, ignoring `_*` funcs but including the `var`

The current contents of Sequence can be cleanly divided into two groups; those that return SubSequence imply a specific ordering, and the rest do not.

 I think those should be/should have been two separate protocols:

public protocol Iterable {
  associatedtype Iterator: IteratorProtocol
  func map<T>(...) -> [T] // Iterable where .Iterator.Element == T
  func filter(...) -> [Iterator.Element] // Iterable where .Iterator.Element == Self.Iterator.Element
  func forEach(...)
  func makeIterator() -> Iterator
  var underestimatedCount: Int { get }

public protocol Sequence: Iterable { // Maybe OrderedSequence just to make the well-defined-order requirement explicit
  associatedtype SubSequence
  func dropFirst(...)   -> SubSequence   // Sequence where .Iterator.Element == Self.Iterator.Element
  func dropLast(...)    -> SubSequence   //    " "
  func drop(while...)   -> SubSequence   //    " "
  func prefix(...)      -> SubSequence   //    " "
  func prefix(while...) -> SubSequence   //    " "
  func suffix(...)      -> SubSequence   //    " "
  func split(...where...)  -> [SubSequence] // Iterable where .Iterator.Element == (Sequence where .Iterator.Element == Self.Iterator.Element)

(The comments, of course, would be more sensible types once the ideas can actually be expressed in Swift)

Then unordered collections (Set and Dictionary) would just conform to Iterable and not Sequence, so ALL the methods on those classes would make logical sense and have well-defined behavior; no change would be needed for ordered collections.

Now, the practical matter. If this were Swift 1->2 or 2->3, I doubt there would be a significant issue with actually making this change. Unfortunately, we're well beyond that and making a change this deep is an enormous deal. So I see two ways forward.

1. We could go ahead and make this separation. Although it's a potentially large breaking change, I would argue that because the methods are ill-defined anyway, the breakage is justified and a net benefit.

2. We could try and think of a way to make the distinction between ordered and unordered "sequences" in a less-breaking manner. Unfortunately, I don't have a good suggestion for this, but if anyone has ideas, I'm all ears. Or eyes, as the case may be.

> On Oct 12, 2017, at 4:24 PM, Xiaodi Wu via swift-evolution <swift-evolution at swift.org> wrote:
> Rename Sequence.elementsEqual
> Proposal: SE-NNNN <https://gist.github.com/xwu/NNNN-rename-elements-equal.md>
> Authors: Xiaodi Wu <https://github.com/xwu>
> Review Manager: TBD
> Status: Awaiting review
>  <https://gist.github.com/xwu/1f0ef4e18a7f321f22ca65a2f56772f6#introduction>Introduction
> The current behavior of Sequence.elementsEqual is potentially confusing to users given its name. Having surveyed the alternative solutions to this problem, it is proposed that the method be renamed to Sequence.lexicographicallyEquals.
> [...]
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