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

[Xiaodi Wu]

On Fri, Oct 13, 2017 at 12:12 PM, Kevin Nattinger <swift at nattinger.net>
wrote:

>
> On Oct 13, 2017, at 10:01 AM, Michael Ilseman <milseman at apple.com> wrote:
>
>
>
> On Oct 12, 2017, at 9:57 PM, Kevin Nattinger via swift-evolution <
> swift-evolution at swift.org> wrote:
>
> –∞
>
> 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.
>
>
> FWIW, in the context of String, "lexicographical ordering” does not imply
> human-written-language-alphabetical order at all, as there’s no universal
> alphabetical ordering for human language. I.e., such a concrete notion for
> Strings does not exist, not even theoretically. “Lexicographical” derives
> its meaning from the mathematical usage[1] which uses that term as well as
> “alphabet” without being restricted to human-written-language, in which it
> means some total ordering over a finite set.
>
> [1] https://en.wikipedia.org/wiki/Lexicographical_order
>
>
> I see, apologies for the mistake.
> Regardless of the specific type of ordering, lexicographicallyEquals says
> to me that the objects should be sorted into lexicographical order and
> compared. Precisely the opposite of the proposal.
>

That is an interesting interpretation. I suppose anyone has the right to
interpret anything in any way they please, but this seems a uniquely
idiosyncratic interpretation. The total ordering is over the set of all
sequences, and the comparison is between two sequences--note that the
function name uses the singular ("equals") and makes no reference to the
lexicography or ordering of elements. Likewise, dictionaries don't sort
"hello" and "world" by comparing "ehllo" and "dlorw".


>
>
> 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.
> [...]
>
> _______________________________________________
> swift-evolution mailing list
> swift-evolution at swift.org
> https://lists.swift.org/mailman/listinfo/swift-evolution
>
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.swift.org/pipermail/swift-evolution/attachments/20171013/48fb5924/attachment.html>