On Fri, Oct 13, 2017 at 4:52 PM, Christopher Whidden < christopher.whid...@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@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@swift.org > https://lists.swift.org/mailman/listinfo/swift-evolution > > >
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