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

Xiaodi Wu xiaodi.wu at gmail.com
Fri Oct 13 22:56:57 CDT 2017

On Fri, Oct 13, 2017 at 4:52 PM, Christopher Whidden <
christopher.whidden at gmail.com> wrote:

> Using the term “lexicographically” implies to me that the Element type
> must conform to Comparable, as would be required for a total order.  The
> Sequence method you mention, lexicographicallyPrecedes(_:), does have this
> constraint, whereas the method in question for elementsEqual(_:) /
> lexicographicallyEquals(_:) only has the constraint that the Element is
> Equatable.  As an example, an array of simple enums has no default
> lexicographical ordering but is still able to use this method because enums
> (without associated values) are Equatable by default:
> *enum* Foo {
>     *case* bar
>     *case* baz
> }
> *let* f1 = [Foo.bar, Foo.baz]
> *let* f2 = [Foo.baz, Foo.bar]
> f1.elementsEqual(f2) //false
> f1.elementsEqual(f2.reversed()) //true
> I also share Jonathan’s concerns that some programmers may misinterpret
> [lexicographically][Equals] to mean [sorted in lexicographical
> order][compare sequence equality], which is not what the method in question
> does.
> Xiaodi, I think you are right that Sequence.sequentiallyEquals is to close
> to "==" to use, but I think we have to find something better here.
> I’ll recommend that we use the name *Sequence.iterativelyEquals(_:)* since
> this describes the body of the method concisely.  A rough abbreviation of
> this algorithm is:
> 1. Iterate over elements in two sequences
> a. Compare elements for equality
> “iterativelyEquals" concisely describes this.

That's an intriguing alternative. The key quality to be communicated,
practically speaking, is that if two instances `x` and `y` compare `true`
with `elementsEqual`, then `for i in x { ... }` and `for i in y { ... }`
should be substitutable (if the elements of `x` and `y` are not consumed on
the first iteration), because the semantics of `Equatable` demand that
"equivalence means substitutability." This would be captured by something
like the name you suggest, or to be even more explicit:

x.substitutesForThePurposesOfIteratedAccess(for: y)

This is intentionally unwieldy to attract others to try to do better :) But
I think it captures the meaning we are going for here.

> On Oct 12, 2017, at 6:24 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.
> <https://gist.github.com/xwu/1f0ef4e18a7f321f22ca65a2f56772f6#motivation>
> Motivation
> As outlined by Ole Begemann
> <https://twitter.com/olebegemann/status/916291785185529857>, use of
> Sequence.elementsEqual(_:) can lead to surprising results if the
> sequences compared are unordered:
> var set1: Set<Int> = Set(1...5)var set2: Set<Int> = Set((1...5).reversed())
> set1 == set2 // trueset1.elementsEqual(set2) // false
> This result does reflect the *intended and documented* behavior of the
> elementsEqual(_:) method, which performs a *lexicographical* elementwise
> comparison. That is, the method first compares set1.first to set2.first,
> then (if the two elements compare equal) compares the next element stored
> internally in set1 to the next element stored internally in set2, and so
> on.
> In almost all circumstances where a set is compared to another set, or a
> dictionary is compared to another dictionary, users should use == instead
> of elementsEqual(_:).
> <https://gist.github.com/xwu/1f0ef4e18a7f321f22ca65a2f56772f6#proposed-solution>Proposed
> solution
> The proposed solution is the result of an iterative process of reasoning,
> presented here:
> The first and most obvious solution is to remove the elementsEqual(_:)
> method altogether in favor of ==. This prevents its misuse. However,
> because elementsEqual(_:) is a generic method on Sequence, we can use it
> to compare an instance of UnsafeBufferPointer<Int> to an instance of [Int].
> This is a useful and non-redundant feature which would be eliminated if the
> method is removed altogether.
> A second solution <https://github.com/apple/swift/pull/12318> is to
> create overloads that forbid the use of the elementsEqual(_:) method
> specifically in non-generic code. This would prevent misuse in non-generic
> code; however, it would also forbid legitimate mixed-type comparisons in
> non-generic code while failing to prevent misuse in generic code. The
> solution also creates a difference in the behavior of generic and
> non-generic code calling the same method, which is potentially confusing,
> without solving the problem completely.
> A third solution is to dramatically overhaul the protocol hierarchy for
> Swift sequences and collections so that unordered collections no longer
> have members such as first and elementsEqual(_:). However, this would be
> a colossal and source-breaking undertaking, and it is unlikely to be
> satisfactory in addressing all the axes of differences among sequence and
> collection types:
>    - Finite versus infinite
>    - Single-pass versus multi-pass
>    - Ordered versus unordered
>    - Lazy versus eager
>    - Forward/bidirectional/random-access
> A fourth solution is proposed here. It is predicated on the following
> observation:
> *Another method similar to elementsEqual(_:) already exists on Sequence
> named lexicographicallyPrecedes(_:). Like first, elementsEqual(_:),
> drop(while:), and others, it relies on the internal order of elements in a
> manner that is not completely suitable for an unordered collection.
> However, like first and unlike elementsEqual(_:), this fact is called out
> in the name of the method; unsurprisingly, like first and unlike
> elementsEqual(_:), there is no evidence that lexicographicallyPrecedes(_:)
> has been a pitfall for users.*
> This observation suggests that a major reason for confusion over
> elementsEqual(_:) stems from its name. So, *it is proposed that
> elementsEqual(_:) should be renamed to lexicographicallyEquals(_:)*. The
> function will remain somewhat of a poor fit for unordered collections, but
> no more so than many other methods that cannot trivially be removed from
> the API of unordered collections (as discussed above). The key is that,
> with such a renaming, the behavior of this method will no longer be
> confusing.
> <https://gist.github.com/xwu/1f0ef4e18a7f321f22ca65a2f56772f6#detailed-design>Detailed
> design
> extension Sequence where Element : Equatable {
>   @available(*, deprecated, message: "Use '==' if possible to compare two sequences of the same type, or use 'lexicographicallyEquals' to compare two ordered sequences.")
>   public func elementsEqual<Other : Sequence>(
>     _ other: Other
>   ) -> Bool where Other.Element == Element {
>     return lexicographicallyEquals(other)
>   }
>   public func lexicographicallyEquals<Other : Sequence>(
>     _ other: Other
>   ) -> Bool where Other.Element == Element {
>     // The body of this method is unchanged.    var iter1 = self.makeIterator()
>     var iter2 = other.makeIterator()
>     while true {
>       switch (iter1.next(), iter2.next()) {
>       case let (e1?, e2?):
>         if e1 != e2 { return false }
>       case (_?, nil), (nil, _?):
>         return false
>       case (nil, nil):
>         return true
>       }
>     }
>   }
> }
> A parallel change will be made with respect to elementsEqual(_:by:); that
> is, it will be deprecated in favor of lexicographicallyEquals(_:by:).
> <https://gist.github.com/xwu/1f0ef4e18a7f321f22ca65a2f56772f6#source-compatibility>Source
> compatibility
> Existing code that uses elementsEqual will gain a deprecation warning.
> <https://gist.github.com/xwu/1f0ef4e18a7f321f22ca65a2f56772f6#effect-on-abi-stability>Effect
> on ABI stability
> None.
> <https://gist.github.com/xwu/1f0ef4e18a7f321f22ca65a2f56772f6#effect-on-api-resilience>Effect
> on API resilience
> This proposal adds new methods to the public API of Sequence and
> conforming types.
> <https://gist.github.com/xwu/1f0ef4e18a7f321f22ca65a2f56772f6#alternatives-considered>Alternatives
> considered
> It is to be noted that lexicographicallyPrecedes(_:by:) and
> elementsEqual(_:by:) are essentially the same method, since both perform
> a lexicographical comparison using a custom predicate. However, there is
> not a good unifying name. (lexicographicallyCompares(to:by:) reads
> poorly.) Moreover, the predicate supplied is intended to have very
> different semantics, and maintaining two distinct methods may be a superior
> fit with the typical user's mental model of the intended behavior and may
> also be clearer to readers of the code. Therefore, this proposal does not
> seek to unify the two methods; instead, elementsEqual(_:by:) will be
> renamed lexicographicallyEquals(_:by:) as detailed above.
> _______________________________________________
> swift-evolution mailing list
> swift-evolution at swift.org
> https://lists.swift.org/mailman/listinfo/swift-evolution
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