[Draft] Rename Sequence.elementsEqual

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(_:slight_smile: 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(_:slight_smile: 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(_:slight_smile:
method altogether in favor of ==. This prevents its misuse. However,
because elementsEqual(_:slight_smile: 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(_:slight_smile: 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(_:slight_smile: 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(_:slight_smile:
has been a pitfall for users.*

This observation suggests that a major reason for confusion over
elementsEqual(_:slight_smile: stems from its name. So, *it is proposed that
elementsEqual(_:slight_smile: 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.

I’ve always hated the use of the word “lexicographically” in that method 1)
because lexicographically is hard to spell, and 2) because it’s weird to
say that an unordered collection has a lexicographical order. But this
change is probably for the best.

···

On Thu, 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(_:slight_smile: 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(_:slight_smile: 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(_:slight_smile:
method altogether in favor of ==. This prevents its misuse. However,
because elementsEqual(_:slight_smile: 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(_:slight_smile: 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(_:slight_smile: 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(_:slight_smile:
has been a pitfall for users.*

This observation suggests that a major reason for confusion over
elementsEqual(_:slight_smile: stems from its name. So, *it is proposed that
elementsEqual(_:slight_smile: 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

–∞

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@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.

[...]

1 Like

The name that feels natural to me would be `sequentiallyEquals`. I don't dispute that the term "lexicographical" is used correctly here, although at least for me personally, it's not a word that I encounter frequently enough to understand what this method would do without reading the documentation. Like Kevin, if I were to guess what the method did without checking, I would probably think that it compared lexicographically-sorted versions of the collections.

But the consistency with `lexicographicallyPrecedes` is a pretty strong argument, although `sequentiallyPrecedes` would also feel more natural to me there, and my suspicion about the mentioned lack of evidence that the method has been a pitfall for users is that it's not actually used often enough out in the wild to have heard much about it. That's just a guess though. This proposal is the first time I've learned of its existence.

In any case, my ideal version of this proposal would use `sequentiallyEquals` and also rename `lexicographicallyPrecedes` to `sequentiallyPrecedes`, but I still like the proposal overall as-is. Thanks for bringing it forward!

Jarod

···

On Oct 12, 2017, 16:24 -0700, Xiaodi Wu via swift-evolution <swift-evolution@swift.org>, wrote:

Rename Sequence.elementsEqual

• Proposal: SE-NNNN
• Authors: Xiaodi Wu
• Review Manager: TBD
• Status: Awaiting review

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.
Motivation
As outlined by Ole Begemann, use of Sequence.elementsEqual(_:slight_smile: 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 // true
set1.elementsEqual(set2) // false
This result does reflect the intended and documented behavior of the elementsEqual(_:slight_smile: 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(_:).
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(_:slight_smile: method altogether in favor of ==. This prevents its misuse. However, because elementsEqual(_:slight_smile: 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 is to create overloads that forbid the use of the elementsEqual(_:slight_smile: 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(_:slight_smile: 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(_:slight_smile: has been a pitfall for users.
This observation suggests that a major reason for confusion over elementsEqual(_:slight_smile: stems from its name. So, it is proposed that elementsEqual(_:slight_smile: 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.
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:).
Source compatibility
Existing code that uses elementsEqual will gain a deprecation warning.
Effect on ABI stability
None.
Effect on API resilience
This proposal adds new methods to the public API of Sequence and conforming types.
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

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(_:slight_smile: / lexicographicallyEquals(_:slight_smile: 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(_:slight_smile: 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.

···

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(_:slight_smile: 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 // true
set1.elementsEqual(set2) // false
This result does reflect the intended and documented behavior of the elementsEqual(_:slight_smile: 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(_:slight_smile: method altogether in favor of ==. This prevents its misuse. However, because elementsEqual(_:slight_smile: 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(_:slight_smile: 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(_:slight_smile: 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(_:slight_smile: has been a pitfall for users.

This observation suggests that a major reason for confusion over elementsEqual(_:slight_smile: stems from its name. So, it is proposed that elementsEqual(_:slight_smile: 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

To answer more precisely this request (which remains valid no matter the
protocol hierarchy). I propose

"matchesSequence" ( or simply "matches" or "match", whatever is more
coherent with the naming guidelines).

So
var a: [Int] = [1,2,3]
a.matchesSequence([1,2,3]) returns true.

I first thought that the verb "matching" was too heavily associated to
regular expressions, but i think that it's the correct equivalent for
something as general as a sequence.

···

On Fri, Oct 13, 2017 at 1:24 AM, 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(_:slight_smile: 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(_:slight_smile: 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(_:slight_smile:
method altogether in favor of ==. This prevents its misuse. However,
because elementsEqual(_:slight_smile: 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(_:slight_smile: 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(_:slight_smile: 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(_:slight_smile:
has been a pitfall for users.*

This observation suggests that a major reason for confusion over
elementsEqual(_:slight_smile: stems from its name. So, *it is proposed that
elementsEqual(_:slight_smile: 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

I agree that the proposed name is a poor choice. If we just focus on the naming part, there is precedent in other languages for the name “sequenceEqual”. I think that name makes it a bit clearer that the result is whether the sequences match pair wise rather than whether they have the same elements irrespective of order. I don’t think it entirely solves the problem, but I like it a lot better than the proposed name.

···

--
Adam Kemp

On Oct 12, 2017, at 9:57 PM, Kevin Nattinger via swift-evolution <swift-evolution@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.

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@swift.org> wrote:

Rename Sequence.elementsEqual

Proposal: SE-NNNN
Authors: Xiaodi Wu
Review Manager: TBD
Status: Awaiting review
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@swift.org
https://lists.swift.org/mailman/listinfo/swift-evolution

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.

It occurred to me, if we keep (and document) Sequence as unordered and move all the ordered methods to a new OrderedSequence, then we'd only be breaking truly broken code and we could even potentially offer a fix-it to change Sequence to OrderedSequence when we see the user using the ordered methods in places where we can see and change the generic definition.

I think I'd still prefer Iterable/Sequence as far as naming, but this may be a more acceptable change, and I'd rather have this than no change.

+1

Agree with the reasoning and that this is the best solution. It reduces the risk of confusing the function for what it’s not.

···

On 13 Oct 2017, at 01:37, Kelvin Ma via swift-evolution <swift-evolution@swift.org> wrote:

I’ve always hated the use of the word “lexicographically” in that method 1) because lexicographically is hard to spell, and 2) because it’s weird to say that an unordered collection has a lexicographical order. But this change is probably for the best.

On Thu, Oct 12, 2017 at 6:24 PM, Xiaodi Wu via swift-evolution <swift-evolution@swift.org> wrote:
Rename Sequence.elementsEqual

Proposal: SE-NNNN
Authors: Xiaodi Wu
Review Manager: TBD
Status: Awaiting review
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.

Motivation

As outlined by Ole Begemann, use of Sequence.elementsEqual(_:slight_smile: 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 // true
set1.elementsEqual(set2) // false
This result does reflect the intended and documented behavior of the elementsEqual(_:slight_smile: 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(_:).

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(_:slight_smile: method altogether in favor of ==. This prevents its misuse. However, because elementsEqual(_:slight_smile: 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 is to create overloads that forbid the use of the elementsEqual(_:slight_smile: 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(_:slight_smile: 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(_:slight_smile: has been a pitfall for users.

This observation suggests that a major reason for confusion over elementsEqual(_:slight_smile: stems from its name. So, it is proposed that elementsEqual(_:slight_smile: 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.

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:).

Source compatibility

Existing code that uses elementsEqual will gain a deprecation warning.

Effect on ABI stability

None.

Effect on API resilience

This proposal adds new methods to the public API of Sequence and conforming types.

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

_______________________________________________
swift-evolution mailing list
swift-evolution@swift.org
https://lists.swift.org/mailman/listinfo/swift-evolution

–∞

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

···

On Oct 12, 2017, at 9:57 PM, Kevin Nattinger via swift-evolution <swift-evolution@swift.org> wrote:

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@swift.org <mailto: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.

[...]

_______________________________________________
swift-evolution mailing list
swift-evolution@swift.org
https://lists.swift.org/mailman/listinfo/swift-evolution

It has been a while, but I believe a lexicographical ordering is basically a mapping from a set to the natural numbers (which should, in effect, provide a total ordering). The mapping itself can be arbitrary, but the same set of things should never map to two different sequences (which just happens to be the example given).

This tells me that “lexicographicallyPrecedes” is actually misnamed unless it refers to something which has a defined totally ordering.

If we are looking to rename elementsEqual (which I would expect to check if the same elements are present regardless of ordering), I would recommend elementOrderingEqual or something like that that references that the answer will be affected by the internal representation/ordering of the sequence.

Thanks,
Jon

···

On Oct 13, 2017, at 11:10 AM, Jarod Long via swift-evolution <swift-evolution@swift.org> wrote:

The name that feels natural to me would be `sequentiallyEquals`. I don't dispute that the term "lexicographical" is used correctly here, although at least for me personally, it's not a word that I encounter frequently enough to understand what this method would do without reading the documentation. Like Kevin, if I were to guess what the method did without checking, I would probably think that it compared lexicographically-sorted versions of the collections.

But the consistency with `lexicographicallyPrecedes` is a pretty strong argument, although `sequentiallyPrecedes` would also feel more natural to me there, and my suspicion about the mentioned lack of evidence that the method has been a pitfall for users is that it's not actually used often enough out in the wild to have heard much about it. That's just a guess though. This proposal is the first time I've learned of its existence.

In any case, my ideal version of this proposal would use `sequentiallyEquals` and also rename `lexicographicallyPrecedes` to `sequentiallyPrecedes`, but I still like the proposal overall as-is. Thanks for bringing it forward!

Jarod

On Oct 12, 2017, 16:24 -0700, 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(_:slight_smile: 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 // true
set1.elementsEqual(set2) // false
This result does reflect the intended and documented behavior of the elementsEqual(_:slight_smile: 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(_:slight_smile: method altogether in favor of ==. This prevents its misuse. However, because elementsEqual(_:slight_smile: 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(_:slight_smile: 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(_:slight_smile: 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(_:slight_smile: has been a pitfall for users.

This observation suggests that a major reason for confusion over elementsEqual(_:slight_smile: stems from its name. So, it is proposed that elementsEqual(_:slight_smile: 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

_______________________________________________
swift-evolution mailing list
swift-evolution@swift.org
https://lists.swift.org/mailman/listinfo/swift-evolution

+1 on both points. As for your solutions, i see 1/ as the best solution.
Breaking source code that rely on badly defined, or improper behavior isn't
"breaking". You don't break something that's already half broken.
As an app developer relying on swift on my day to day job and making a
living of it, i want to emphasis this: I really don't mind if a language
version change is making me look more carefully on some parts of my code
that i probably had overlooked.
Sure i may pester a bit when the code doesn't compile, but it sure is
better than discovering the weird behavior of a badly defined protocol
hierarchy in customer support.

···

On Fri, Oct 13, 2017 at 6:57 AM, Kevin Nattinger via swift-evolution < swift-evolution@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.

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@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@swift.org
https://lists.swift.org/mailman/listinfo/swift-evolution

It means to skip the first element the set would normally have given you. Which element this will be may be arbitrary, but this is still not useless:

    set.dropFirst().reduce(set.first!, …)

Even elementsEqual(_:slight_smile: does tell you something potentially valuable: Whether two instances will end up giving the same result when processed by an ordering-sensitive algorithm.

We should change the name to something like orderEquals(_:), and maybe change the lexicographicallyPrecedes(_:slight_smile: method to something analogous like orderPrecedes(_:), and then be done with it.

···

On Oct 12, 2017, at 9:57 PM, Kevin Nattinger via swift-evolution <swift-evolution@swift.org> wrote:

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?

--
Brent Royal-Gordon
Sent from my iPhone

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(_:slight_smile: /
lexicographicallyEquals(_:slight_smile: 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 :slight_smile: But
I think it captures the meaning we are going for here.

···

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

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(_:slight_smile: 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(_:slight_smile: 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(_:slight_smile:
method altogether in favor of ==. This prevents its misuse. However,
because elementsEqual(_:slight_smile: 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(_:slight_smile: 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(_:slight_smile: 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(_:slight_smile:
has been a pitfall for users.*

This observation suggests that a major reason for confusion over
elementsEqual(_:slight_smile: stems from its name. So, *it is proposed that
elementsEqual(_:slight_smile: 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

Totally agree with Kevin’s analysis. I always thought „Sequence“ a very misleading name for an Iterable.
I wouldn’t mind the breakage at all.

-Thorsten

···

Am 13.10.2017 um 06:57 schrieb Kevin Nattinger via swift-evolution <swift-evolution@swift.org>:

–∞

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@swift.org <mailto: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.

[...]

_______________________________________________
swift-evolution mailing list
swift-evolution@swift.org
https://lists.swift.org/mailman/listinfo/swift-evolution

I think that “match” is a word to avoid for this, as this doesn’t have anything to do with pattern matching, fuzzy matching, etc., while “equals” is precisely the concept we’re using.

What about the name “sequentiallyEquals”? Highlight the fact that we’re talking about sequential ordering, i.e. whatever order the sequence provides, as opposed to first doing some lexicographical ordering of elements themselves.

var a: Set<Int> = [3, 1, 2]
a.sequentiallyEquals([1,2,3]) // result depends on application of equality in a (potentially-arbitrary) sequential ordering

Whereas I could see the following being more confusing:

var a: Set<Int> = [3, 1, 2]
a.lexicographicallyEquals([1,2,3]) // result depends on application of equality, but what meaning does “lexicographically” convey?

It’s not immediately clear to someone new to the API that “lexicographically” speaks to the nature of the sequence’s (potentially-arbitrary) order, irrespective of element. It could give the false impression that it speaks to some nature of the elements themselves, in this case Ints, which have an obvious lexicographical ordering. I don’t know how frequent that misconception would be in practice, but it does cause me to do a double-take in this contrived example.

···

On Oct 14, 2017, at 1:04 PM, Benjamin G via swift-evolution <swift-evolution@swift.org> wrote:

To answer more precisely this request (which remains valid no matter the protocol hierarchy). I propose

"matchesSequence" ( or simply "matches" or "match", whatever is more coherent with the naming guidelines).

So
var a: [Int] = [1,2,3]
a.matchesSequence([1,2,3]) returns true.

I first thought that the verb "matching" was too heavily associated to regular expressions, but i think that it's the correct equivalent for something as general as a sequence.

On Fri, Oct 13, 2017 at 1:24 AM, Xiaodi Wu via swift-evolution <swift-evolution@swift.org <mailto: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(_:slight_smile: 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 // true
set1.elementsEqual(set2) // false
This result does reflect the intended and documented behavior of the elementsEqual(_:slight_smile: 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(_:slight_smile: method altogether in favor of ==. This prevents its misuse. However, because elementsEqual(_:slight_smile: 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(_:slight_smile: 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(_:slight_smile: 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(_:slight_smile: has been a pitfall for users.

This observation suggests that a major reason for confusion over elementsEqual(_:slight_smile: stems from its name. So, it is proposed that elementsEqual(_:slight_smile: 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.

_______________________________________________
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swift-evolution@swift.org <mailto:swift-evolution@swift.org>
https://lists.swift.org/mailman/listinfo/swift-evolution

_______________________________________________
swift-evolution mailing list
swift-evolution@swift.org
https://lists.swift.org/mailman/listinfo/swift-evolution

I think that “match” is a word to avoid for this, as this doesn’t have
anything to do with pattern matching, fuzzy matching, etc., while “equals”
is precisely the concept we’re using.

What about the name “sequentiallyEquals”? Highlight the fact that we’re
talking about sequential ordering, i.e. whatever order the sequence
provides, as opposed to first doing some lexicographical ordering of
elements themselves.

var a: Set<Int> = [3, 1, 2]
a.sequentiallyEquals([1,2,3]) // result depends on application of equality
in a (potentially-arbitrary) sequential ordering

Whereas I could see the following being more confusing:

var a: Set<Int> = [3, 1, 2]
a.lexicographicallyEquals([1,2,3]) // result depends on application of
equality, but what meaning does “lexicographically” convey?

It’s not immediately clear to someone new to the API that
“lexicographically” speaks to the nature of the sequence’s
(potentially-arbitrary) order, irrespective of element. It could give the
false impression that it speaks to some nature of the elements themselves,
in this case Ints, which have an obvious lexicographical ordering. I don’t
know how frequent that misconception would be in practice, but it does
cause me to do a double-take in this contrived example.

I'm entirely puzzled that apparently large numbers of people believe
lexicographical comparison, a term with a very specific mathematical
definition and no colloquial use, to mean what it does not. I'd like to
avoid inventing Swift-specific new terms for this particular concept which
is not at all unique to Swift. The other plausible terms I can see with
some other use might be "elementwise equals" or "entrywise equals" or
"coordinatewise equals."

···

On Sat, Oct 14, 2017 at 8:11 PM, Michael Ilseman <milseman@apple.com> wrote:

On Oct 14, 2017, at 1:04 PM, Benjamin G via swift-evolution < > swift-evolution@swift.org> wrote:

To answer more precisely this request (which remains valid no matter the
protocol hierarchy). I propose

"matchesSequence" ( or simply "matches" or "match", whatever is more
coherent with the naming guidelines).

So
var a: [Int] = [1,2,3]
a.matchesSequence([1,2,3]) returns true.

I first thought that the verb "matching" was too heavily associated to
regular expressions, but i think that it's the correct equivalent for
something as general as a sequence.

On Fri, Oct 13, 2017 at 1:24 AM, 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(_:slight_smile: 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(_:slight_smile: 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(_:slight_smile:
method altogether in favor of ==. This prevents its misuse. However,
because elementsEqual(_:slight_smile: 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(_:slight_smile: 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(_:slight_smile: 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(_:slight_smile:
has been a pitfall for users.*

This observation suggests that a major reason for confusion over
elementsEqual(_:slight_smile: stems from its name. So, *it is proposed that
elementsEqual(_:slight_smile: 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

_______________________________________________
swift-evolution mailing list
swift-evolution@swift.org
https://lists.swift.org/mailman/listinfo/swift-evolution

I think that “match” is a word to avoid for this, as this doesn’t have anything to do with pattern matching, fuzzy matching, etc., while “equals” is precisely the concept we’re using.

What about the name “sequentiallyEquals”? Highlight the fact that we’re talking about sequential ordering, i.e. whatever order the sequence provides, as opposed to first doing some lexicographical ordering of elements themselves.

var a: Set<Int> = [3, 1, 2]
a.sequentiallyEquals([1,2,3]) // result depends on application of equality in a (potentially-arbitrary) sequential ordering

I really like this name.

It does make me think about .first which I now feel it should be .sequentiallyFirst :slight_smile:

I ultimately think that these methods/properties do not make sense for unordered types so probably they should just be unavailable.

···

On Oct 14, 2017, at 6:11 PM, Michael Ilseman via swift-evolution <swift-evolution@swift.org> wrote:

Whereas I could see the following being more confusing:

var a: Set<Int> = [3, 1, 2]
a.lexicographicallyEquals([1,2,3]) // result depends on application of equality, but what meaning does “lexicographically” convey?

It’s not immediately clear to someone new to the API that “lexicographically” speaks to the nature of the sequence’s (potentially-arbitrary) order, irrespective of element. It could give the false impression that it speaks to some nature of the elements themselves, in this case Ints, which have an obvious lexicographical ordering. I don’t know how frequent that misconception would be in practice, but it does cause me to do a double-take in this contrived example.

On Oct 14, 2017, at 1:04 PM, Benjamin G via swift-evolution <swift-evolution@swift.org <mailto:swift-evolution@swift.org>> wrote:

To answer more precisely this request (which remains valid no matter the protocol hierarchy). I propose

"matchesSequence" ( or simply "matches" or "match", whatever is more coherent with the naming guidelines).

So
var a: [Int] = [1,2,3]
a.matchesSequence([1,2,3]) returns true.

I first thought that the verb "matching" was too heavily associated to regular expressions, but i think that it's the correct equivalent for something as general as a sequence.

On Fri, Oct 13, 2017 at 1:24 AM, Xiaodi Wu via swift-evolution <swift-evolution@swift.org <mailto: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(_:slight_smile: 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 // true
set1.elementsEqual(set2) // false
This result does reflect the intended and documented behavior of the elementsEqual(_:slight_smile: 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(_:slight_smile: method altogether in favor of ==. This prevents its misuse. However, because elementsEqual(_:slight_smile: 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(_:slight_smile: 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(_:slight_smile: 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(_:slight_smile: has been a pitfall for users.

This observation suggests that a major reason for confusion over elementsEqual(_:slight_smile: stems from its name. So, it is proposed that elementsEqual(_:slight_smile: 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 <mailto:swift-evolution@swift.org>
https://lists.swift.org/mailman/listinfo/swift-evolution

_______________________________________________
swift-evolution mailing list
swift-evolution@swift.org <mailto:swift-evolution@swift.org>
https://lists.swift.org/mailman/listinfo/swift-evolution

_______________________________________________
swift-evolution mailing list
swift-evolution@swift.org
https://lists.swift.org/mailman/listinfo/swift-evolution

–∞

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.

···

On Oct 13, 2017, at 10:01 AM, Michael Ilseman <milseman@apple.com> wrote:

On Oct 12, 2017, at 9:57 PM, Kevin Nattinger via swift-evolution <swift-evolution@swift.org <mailto:swift-evolution@swift.org>> wrote:

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@swift.org <mailto: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.

[...]

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swift-evolution@swift.org <mailto:swift-evolution@swift.org>
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“Lexicographical comparison” is a pretty standard term, and easy to Google.
We didn’t make it up for Swift :slight_smile:

Since Swift names this protocol Sequence, something named
“Sequence.sequenceEqual” cannot distinguish this method from ==.

···

On Fri, Oct 13, 2017 at 01:28 Adam Kemp <adam.kemp@apple.com> wrote:

I agree that the proposed name is a poor choice. If we just focus on the
naming part, there is precedent in other languages for the name
“sequenceEqual”. I think that name makes it a bit clearer that the result
is whether the sequences match pair wise rather than whether they have the
same elements irrespective of order. I don’t think it entirely solves the
problem, but I like it a lot better than the proposed name.

--
Adam Kemp

On Oct 12, 2017, at 9:57 PM, Kevin Nattinger via swift-evolution < > swift-evolution@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.

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@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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https://lists.swift.org/mailman/listinfo/swift-evolution