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Rationalizing Sequence end-operation names

Introduction

Sequence and Collection offer many special operations which access or manipulate its first or last elements, but they are plagued by inconsistent naming which can make it difficult to find inverses or remember what the standard library offers. We propose that we standardize these names so they follow consistent, predictable patterns.

Swift-evolution thread: [Draft] Rationalizing Sequence end-operation names

Scope

This proposal does not aim to add or remove any functionality; it merely renames and redesigns existing operations. Adding new operations is out of scope for this proposal unless it's incidental to the new designs.

Nonetheless, we do want the new designs to support adding more operations in the future. The names we've chosen are informed by some of the speculative APIs discussed in "Future directions", although we think they are perfectly sensible names even if nothing else changes.

Motivation

The Sequence and Collection protocols offer a wide variety of APIs which are defined to operate on, or from, one end of the sequence:

Get Index Exclude Remove (1) Pop (1) Equate (2)
Fixed Size
First 1 C.first - S.dropFirst() C.removeFirst() C.popFirst() -
Last 1 C.last - S.dropLast() C.removeLast() C.popLast() -
First (n: Int) S.prefix(3) - S.dropFirst(3) C.removeFirst(3) - S.starts(with: [x,y,z])
  ...with closure S.prefix(while: isPrime) - S.drop(while: isPrime) - - S.starts(with: [x,y,z], by: ==)
Last (n: Int) S.suffix(3) - S.dropLast(3) C.removeLast(3) - -
  ...with closure - - - - - -
Searching From End
First matching element - C.index(of: x) - - - -
  ...with closure S.first(where: isPrime) C.index(where: isPrime) - - - -
Last matching element - - - - - -
  ...with closure - - - - - -
Based on Index
startIndex ..< (i: Index) C.prefix(upTo: i) - - - - -
startIndex ... (i: Index) C.prefix(through: i) - - - - -
(i: Index) ..< endIndex C.suffix(from: i) - - - - -

We have included several blank rows for operands which fit the APIs' patterns, even if they don't happen to have any operations currently.

Type abbreviations:

  • S = Sequence
  • C = Collection (or a sub-protocol like BidirectionalCollection)

Notes:

  1. remove and pop both mutate the array to delete the indicated element(s), but remove assumes as a precondition that the indicated elements exist, while pop checks whether or not they exist.

  2. String and NSString have bespoke versions of first n and last n Equate operations, in the form of their hasPrefix and hasSuffix methods.

Leaving aside the question of whether any gaps in these tables ought to be filled, we see a number of issues with existing terminology.

Inconsistent use of prefix and suffix

Some APIs which operate on a variable number of elements anchored at one end or the other use the terms prefix or suffix:

  • Sequence.prefix(_:) and Sequence.suffix(_:)
  • Sequence.prefix(while:)
  • String.hasPrefix(_:) and String.hasSuffix(_:)

Others, however, use first or last:

  • Sequence.dropFirst(_:) and Sequence.dropLast(_:)
  • Sequence.removeFirst(_:) and Sequence.removeLast(_:)

Still others use neither:

  • Sequence.starts(with:)
  • Sequence.drop(while:)

These methods are all closely related, but because of this inconsistent terminology, they fail to form predictable method families.

first has multiple meanings

The word first can mean three different things in these APIs:

  1. Just the very first element of the sequence.

  2. A subsequence of elements anchored at the beginning of the sequence, as mentioned in the last point.

  3. The first element encountered in the sequence which matches a given criterion when walking from the beginning of the sequence towards the end.

It would be nice to have more clarity here—particularly around #2, which implies different return value behavior.

drop is misleading and scary

In a Swift context, we believe the drop methods are actively confusing:

  • drop does not have the -ing or -ed suffix normally used for a nonmutating method.

  • drop has strong associations with destructive operations; it's the term used, for instance, for deleting whole tables in SQL. Even dropping would probably sound more like a mutating operation than alternatives.

  • As previously mentioned, the use of dropFirst and dropLast for single-drop operations and multiple-drop operations breaks up method families.

drop, dropFirst, and dropLast are terms of art, so we allow them a certain amount of leeway. However, we believe the drop functions go well beyond what we should permit. They are relatively uncommon operations, associated primarily with functional languages rather than mainstream object-oriented or imperative languages, and their violation of the normal Swift naming guidelines is especially misleading.

The term-of-art exception is not a suicide pact; it is meant to aid understanding by importing common terminology, not bind us to follow every decision made by any language that came before us. In this case, we think we should ignore precedent and forge our own path.

Unstated direction of operation

Several APIs could theoretically be implemented by working from either end of the sequence, and would return different results depending on the direction, but do not indicate the direction in their names:

  • Sequence.drop(while:)
  • Collection.index(of:)

Adding a direction to these APIs would make their behavior clearer and permit us to offer opposite-end equivalents in the future. (Unmerged swift-evolution pull request 329 would add lastIndex methods.)

Operations taking an index are really slicing

prefix(upTo:), prefix(through:), and suffix(from:) at first appear to belong to the same family as the other prefix and suffix methods, but deeper examination reveals otherwise. They are the only operations which take indices, and they don't cleanly extend to the other operations which belong to these families. (For instance, it would not make sense to add a dropPrefix(upTo:) method; it would be equivalent to suffix(from:).)

Also, on Int-indexed collections like Array, prefix(_:) and prefix(upTo:) are identical, but there is little relationship between suffix(_:) and suffix(from:), which is confusing.

suffix(from:) is a particularly severe source of confusion. The other suffix APIs all have parameters relative to the end of the collection, but suffix(from:)'s index is still relative to the beginning of the array. This is obvious if you think deeply about the meaning of an index, but we don't really want to force our users to stare at a strange API until they have an epiphany.

We believe these operations have much more in common with slicing a collection using a range, and that reimagining them as slicing APIs will be more fruitful. Thus, the bottom of the table above should probably be split into a separate one and combined with a table of subrange APIs:

Type Get Remove Replace
Based on Index, Arbitrary
(i: Index) ..< (j: Index) Range<Index> C[i ..< j] C.removeSubrange(i ..< j) C.replaceSubrange(i ..< j, with: [x,y])
  ...Countable CountableRange<Index> C[i ..< j] C.removeSubrange(i ..< j) C.replaceSubrange(i ..< j, with: [x,y])
(i: Index) ... (j: Index) ClosedRange<Index> C[i ... j] C.removeSubrange(i ... j) C.replaceSubrange(i ... j, with: [x,y])
  ...Countable CountableClosedRange<Index> C[i ... j] C.removeSubrange(i ... j) C.replaceSubrange(i ... j, with: [x,y])
Based on Index, From End
startIndex ..< (i: Index) upTo: Index C.prefix(upTo: i) - -
(i: Index) ..< endIndex from: Index C.suffix(from: i) - -
startIndex ... (i: Index) through: Index C.prefix(through: i) - -

Why does it matter?

Many of these APIs are only occasionally necessary, so it's important that they be easy to find when needed and easy to understand when read. If you know that prefix(10) will get the first ten elements but don't know what its inverse is, you will probably not guess that it's dropFirst(10). The confusing, conflicting names in these APIs are a barrier to users adopting them where appropriate.

Proposed solution

We sever the index-taking APIs from the others, forming two separate families, which we will call the "Sequence-end operations" and the "index-based operations". We then consider and redesign them along separate lines.

Sequence-end operations

Each of these APIs should be renamed to use a directional word based on its row in the table:

Operand Directional word
Fixed Size
First 1 first
Last 1 last
First (n: Int) prefix
  ...with closure prefix
Last (n: Int) suffix
  ...with closure suffix
Searching From End
First matching element first
  ...with closure first
Last matching element last
  ...with closure last

To accomplish this, starts(with:) should be renamed to hasPrefix(_:), and other APIs should have directional words replaced or added as appropriate.

Additionally, the word drop in the "Exclude" APIs should be replaced with removing. These operations omit the same elements which the remove operations delete, so even though the types are not always the same (removing returns SubSequence, not Self), we think they are similar enough to deserve to be treated as nonmutating forms.

These changes yield (altered names bold):

Get Index Exclude Remove (1) Pop (1) Equate (2)
Fixed Size
First 1 C.first - S.removingFirst() C.removeFirst() C.popFirst() -
Last 1 C.last - S.removingLast() C.removeLast() C.popLast() -
First (n: Int) S.prefix(3) - S.removingPrefix(3) C.removePrefix(3) - S.hasPrefix([x,y,z])
  ...with closure S.prefix(while: isPrime) - S.removingPrefix(while: isPrime) - - S.hasPrefix([x,y,z], by: ==)
Last (n: Int) S.suffix(3) - S.removingSuffix(3) C.removeSuffix(3) - -
  ...with closure - - - - - -
Searching From End
First matching element - C.firstIndex(of: x) - - - -
  ...with closure S.first(where: isPrime) C.firstIndex(where: isPrime) - - - -
Last matching element - - - - - -
  ...with closure - - - - - -

Index-based operations

Because these APIs look up elements based on their indices, we believe these operations should be exposed as subscripts, and ideally should look like other slicing operations:

let head = people[..<i]
let tail = people[i..<]
let rearrangedPeople = tail + head

We will accomplish this by introducing two new types, IncompleteRange and IncompleteClosedRange. These are similar to Range and ClosedRange, except that the bounds are optional.

To construct them, we will introduce both prefix and suffix operators taking a non-optional bound, and infix operators taking optional bounds. (We offer both because c[..<i] is more convenient than c[nil ..< i], but doesn't allow you to dynamically choose between supplying and omitting a bound.) These will follow the existing convention: ..< will construct the half-open IncompleteRange, while ... will construct IncompleteClosedRange.

Rather than continuing to proliferate overloads of slicing subscripts, we will also introduce a new RangeExpression protocol which allows any range-like type to convert itself into a plain Range<Index> appropriate to the collection in question. Thus, there should only be two range subscripts: one taking Range<Index>, and one taking everything else.

We will also modify the existing removeSubrange(_:) and replaceSubrange(_:with:) calls to take RangeExpression instances, thereby merging many existing variants into one while simultaneously extending them to support IncompleteRange and IncompleteClosedRange. Though technically additive, we believe this is an easy win.

Thus, the table above becomes:

Type Get Remove Replace
Based on Index, Arbitrary
(i: Index) ..< (j: Index) Range<Index> C[i ..< j] C.removeSubrange(i ..< j) C.replaceSubrange(i ..< j, with: [x,y])
  ...Countable CountableRange<Index> C[i ..< j] C.removeSubrange(i ..< j) C.replaceSubrange(i ..< j, with: [x,y])
(i: Index) ... (j: Index) ClosedRange<Index> C[i ... j] C.removeSubrange(i ... j) C.replaceSubrange(i ... j, with: [x,y])
  ...Countable CountableClosedRange<Index> C[i ... j] C.removeSubrange(i ... j) C.replaceSubrange(i ... j, with: [x,y])
Based on Index, From End
startIndex ..< (i: Index) IncompleteRange<Index> C[..<i] C.removeSubrange(..<i) C.replaceSubrange(..<i, with: [x,y])
(i: Index) ..< endIndex IncompleteRange<Index> C[i..<] C.removeSubrange(i..<) C.replaceSubrange(i..<, with: [x,y])
startIndex ... (i: Index) IncompleteClosedRange<Index> C[...i] C.removeSubrange(...i) C.replaceSubrange(...i, with: [x,y])

However, it should be implemented with merely:

Type Get Remove Replace
(i: Index) ..< (j: Index) Range<Index> C[i ..< j] C.removeSubrange(i ..< j) C.replaceSubrange(i ..< j, with: [x,y])
Everything else RangeExpression where Bound == Index C[i ... j] C.removeSubrange(i ... j) C.replaceSubrange(i ... j, with: [x,y])

Detailed design

Sequence-end operations

The following methods should be renamed as follows wherever they appear in the standard library. These are simple textual substitutions; we propose no changes whatsoever to types, parameter interpretations, or other semantics.

Old method New method
dropFirst() -> SubSequence removingFirst() -> SubSequence
dropLast() -> SubSequence removingLast() -> SubSequence
dropFirst(_ n: Int) -> SubSequence removingPrefix(_ n: Int) -> SubSequence
drop(@noescape while predicate: (Iterator.Element) throws -> Bool) rethrows -> SubSequence removingPrefix(@noescape while predicate: (Iterator.Element) throws -> Bool) rethrows -> SubSequence
dropLast(_ n: Int) -> SubSequence removingSuffix(_ n: Int) -> SubSequence
removeFirst(_ n: Int) removePrefix(_ n: Int)
removeLast(_ n: Int) removeSuffix(_ n: Int)
starts<PossiblePrefix: Sequence>(with possiblePrefix: PossiblePrefix) -> Bool where ... hasPrefix<PossiblePrefix: Sequence>(_ possiblePrefix: PossiblePrefix) -> Bool where ...
starts<PossiblePrefix : Sequence>(with possiblePrefix: PossiblePrefix, by areEquivalent: @noescape (Iterator.Element, Iterator.Element) throws -> Bool) rethrows -> Bool where ... hasPrefix<PossiblePrefix : Sequence>(_ possiblePrefix: PossiblePrefix, by areEquivalent: @noescape (Iterator.Element, Iterator.Element) throws -> Bool) rethrows -> Bool where ...
index(of element: Iterator.Element) -> Index? firstIndex(of element: Iterator.Element) -> Index?
index(where predicate: @noescape (Iterator.Element) throws -> Bool) rethrows -> Index? firstIndex(where predicate: @noescape (Iterator.Element) throws -> Bool) rethrows -> Index?

Index-based operations

We will first present an idealized version of the design which cannot be fully implemented in Swift 3 due to generics bugs and limitations. Then we will present the minor changes necessary to implement it. The two should be source-compatible unless users conform their own types to RangeExpression.

Idealized design

The RangeExpression protocol is defined like so:

/// A type which can be used to slice a collection. A `RangeExpression` can 
/// convert itself to a `Range<Bound>` of indices within a given collection; 
/// the collection can then slice itself with that `Range`.
public protocol RangeExpression {
  /// Returns `self` expressed as a range of indices within `collection`.
  /// 
  /// -Parameter collection: The collection `self` should be 
  ///                        relative to.
  /// 
  /// -Returns: A `Range<Bound>` suitable for slicing `collection`. 
  ///           The return value is *not* guaranteed to be inside 
  ///           its bounds. Callers should apply the same preconditions 
  ///           to the return value as they would to a range provided 
  ///           directly by the user.
  public func relative<C: Collection>(to collection: C) -> Range<Bound> where C.Index == Bound {
}

The following existing types will be conformed to RangeExpressible:

  • Range
  • CountableRange
  • ClosedRange
  • CountableClosedRange

The Range conformance is not strictly necessary, but allows APIs which do not need to be overridden to implement only a RangeExpression-based variant. Type inference favors concrete members over generic ones, so it should prefer to use parameters explicitly typed as Range<Index> over parameters of a generic type constrained to RangeExpression where Bound == Index.

The Indexable and MutableIndexable subscripts which take range types other than Range itself will be removed. So will the RangeReplaceableCollection subscripts which take range types other than Range. Instead, they will be replaced with single generic versions taking a RangeExpression where Bound == Index, using relative(to:) to convert them to Ranges and then calling through to the plain Range variants:

extension Collection {
  /// Accesses a contiguous subrange of the collection's elements.
  ///
  /// The accessed slice uses the same indices for the same elements as the
  /// original collection. Always use the slice's `startIndex` property
  /// instead of assuming that its indices start at a particular value.
  ///
  /// This example demonstrates getting a slice of an array of strings, finding
  /// the index of one of the strings in the slice, and then using that index
  /// in the original array.
  ///
  ///     let streets = ["Adams", "Bryant", "Channing", "Douglas", "Evarts"]
  ///     let streetsSlice = streets[2 ..< streets.endIndex]
  ///     print(streetsSlice)
  ///     // Prints "["Channing", "Douglas", "Evarts"]"
  ///
  ///     let index = streetsSlice.index(of: "Evarts")    // 4
  ///     print(streets[index!])
  ///     // Prints "Evarts"
  ///
  /// - Parameter bounds: A range of the collection's indices. The bounds of
  ///   the range must be valid indices of the 
  public subscript<R>(bounds: R) -> SubSequence where R: RangeExpression, R.Bound == Index {
    get {
      return self[bounds.relative(to: self)]
    }
  }
}
extension MutableCollection {
  /// Accesses a contiguous subrange of the collection's elements.
  ///
  /// The accessed slice uses the same indices for the same elements as the
  /// original collection. Always use the slice's `startIndex` property
  /// instead of assuming that its indices start at a particular value.
  ///
  /// This example demonstrates getting a slice of an array of strings, finding
  /// the index of one of the strings in the slice, and then using that index
  /// in the original array.
  ///
  ///     let streets = ["Adams", "Bryant", "Channing", "Douglas", "Evarts"]
  ///     let streetsSlice = streets[2 ..< streets.endIndex]
  ///     print(streetsSlice)
  ///     // Prints "["Channing", "Douglas", "Evarts"]"
  ///
  ///     let index = streetsSlice.index(of: "Evarts")    // 4
  ///     streets[index!] = "Eustace"
  ///     print(streets[index!])
  ///     // Prints "Eustace"
  ///
  /// - Parameter bounds: A range of the collection's indices. The bounds of
  ///   the range must be valid indices of the collection.
  public subscript<R>(bounds: R) -> SubSequence where R: RangeExpression, R.Bound == Index {
    get {
      return self[bounds.relative(to: self)]
    }
    set {
      self[bounds.relative(to: self)] = newValue
    }
  }
}
extension RangeReplaceableCollection {
  /// Replaces the specified subrange of elements with the given collection.
  ///
  /// This method has the effect of removing the specified range of elements
  /// from the collection and inserting the new elements at the same location.
  /// The number of new elements need not match the number of elements being
  /// removed.
  ///
  /// In this example, three elements in the middle of an array of integers are
  /// replaced by the five elements of a `Repeated<Int>` instance.
  ///
  ///      var nums = [10, 20, 30, 40, 50]
  ///      nums.replaceSubrange(1...3, with: repeatElement(1, count: 5))
  ///      print(nums)
  ///      // Prints "[10, 1, 1, 1, 1, 1, 50]"
  ///
  /// If you pass a zero-length range as the `subrange` parameter, this method
  /// inserts the elements of `newElements` at `subrange.startIndex`. Calling
  /// the `insert(contentsOf:at:)` method instead is preferred.
  ///
  /// Likewise, if you pass a zero-length collection as the `newElements`
  /// parameter, this method removes the elements in the given subrange
  /// without replacement. Calling the `removeSubrange(_:)` method instead is
  /// preferred.
  ///
  /// Calling this method may invalidate any existing indices for use with this
  /// collection.
  ///
  /// - Parameters:
  ///   - subrange: The subrange of the collection to replace. The bounds of
  ///     the range must be valid indices of the collection.
  ///   - newElements: The new elements to add to the collection.
  ///
  /// - Complexity: O(*m*), where *m* is the combined length of the collection
  ///   and `newElements`. If the call to `replaceSubrange` simply appends the
  ///   contents of `newElements` to the collection, the complexity is O(*n*),
  ///   where *n* is the length of `newElements`.
  public mutating func replaceSubrange<R, C>(
    _ subrange: R,
    with newElements: C
  ) where R : RangeExpression, R.Bound == Index, C : Collection, C.Iterator.Element == Iterator.Element {
    replaceSubrange(subrange.relative(to: self), with: newElements)
  }

  /// Removes the elements in the specified subrange from the collection.
  ///
  /// All the elements following the specified position are moved to close the
  /// gap. This example removes two elements from the middle of an array of
  /// measurements.
  ///
  ///     var measurements = [1.2, 1.5, 2.9, 1.2, 1.5]
  ///     measurements.removeSubrange(1..<3)
  ///     print(measurements)
  ///     // Prints "[1.2, 1.5]"
  ///
  /// Calling this method may invalidate any existing indices for use with this
  /// collection.
  ///
  /// - Parameter bounds: The range of the collection to be removed. The
  ///   bounds of the range must be valid indices of the collection.
  ///
  /// - Complexity: O(*n*), where *n* is the length of the collection.
  public mutating func removeSubrange<R>(_ bounds: R) where R : RangeExpression, R.Bound == Index {
    removeSubrange(bounds.relative(to: self))
  }
}

The Collection- and RangeReplaceableCollection-mimicking APIs on String will be similarly replaced with RangeExpression-based versions, except that here the Range versions will be removed too, so all calls will be through RangeExpression.

The IncompleteRange and IncompleteClosedRange types will be very similar. They are designed to be useful generally, not merely with RangeExpressible. Below is the interface for IncompleteRange; IncompleteClosedRange would be analogous.

prefix operator ..< {}
postfix operator ..< {}

/// A `Range` which may not have all of its bounds specified.
/// The `IncompleteRange` can be completed by providing a 
/// `Range` or `CountableRange` from which it can retrieve 
/// default upper and lower bounds.
public struct IncompleteRange<Bound : Comparable> {
  /// The lowest value within the range. 
  /// If `nil`, completing the range will adopt the default value's 
  /// `lowerBound`.
  public let lowerBound: Bound?
  /// The value just above the highest value within the range.
  /// If `nil`, completing the range will adopt the default value's 
  /// `upperBound`.
  public let upperBound: Bound?
}

extension IncompleteRange {
  /// Returns a `Range` with the same `upperBound` 
  /// and `lowerBound` as the current instance. `nil` bounds are 
  /// filled in from `defaultBounds`.
  /// 
  /// This method does not check whether `lowerBound` and `upperBound` 
  /// lie within `defaultBounds`. 
  public func completed(by defaultBounds: Range<Bound>) -> Range<Bound>
  
  /// Returns a `Range` with the same `upperBound` 
  /// and `lowerBound` as the current instance. `nil` bounds are 
  /// filled in from `defaultBounds`.
  /// 
  /// This method does not check whether `lowerBound` and `upperBound` 
  /// lie within `defaultBounds`. 
  /// Nor does it check whether the resulting `lowerBound` is below 
  /// its `upperBound`.
  public func completed(byUnchecked defaultBounds: Range<Bound>) -> Range<Bound>
}

extension IncompleteRange where
    Bound : Strideable,
    Bound.Stride : SignedInteger {
  /// Returns a `CountableRange` with the same `upperBound` 
  /// and `lowerBound` as the current instance. `nil` bounds are 
  /// filled in from `defaultBounds`.
  /// 
  /// This method does not check whether `lowerBound` and `upperBound` 
  /// lie within `defaultBounds`. 
  public func completed(by defaultBounds: CountableRange<Bound>) -> CountableRange<Bound>
  
  /// Returns a `CountableRange` with the same `upperBound` 
  /// and `lowerBound` as the current instance. `nil` bounds are 
  /// filled in from `defaultBounds`.
  /// 
  /// This method does not check whether `lowerBound` and `upperBound` 
  /// lie within `defaultBounds`. 
  /// Nor does it check whether the resulting `lowerBound` is below 
  /// its `upperBound`.
  public func completed(byUnchecked defaultBounds: CountableRange<Bound>) -> CountableRange<Bound>
}

extension IncompleteRange: RangeExpression { ... }

/// Constructs an `IncompleteRange` with the provided upper 
/// bound and an unknown lower bound.
public prefix func ..< <Bound: Comparable>(upperBound: Bound) -> IncompleteRange<Bound>

/// Constructs an `IncompleteRange` with the provided lower 
/// bound and an unknown upper bound.
public postfix func ..< <Bound: Comparable>(lowerBound: Bound) -> IncompleteRange<Bound>

/// Constructs an `IncompleteRange` with the provided upper 
/// and lower bounds. Either or both may be `nil`, in which case the 
/// bound will be provided when the `IncompleteRange` is 
/// completed.
public func ..< <Bound: Comparable>(lowerBound: Bound?, upperBound: Bound?) -> IncompleteRange<Bound>

Finally, since they are now redundant, prefix(upTo:), prefix(through:), and suffix(from:) will be removed.

Actual design

The actual design varies from the ideal one in four ways:

  1. The CountableRange variants of completed(by:) require a slightly different set of constraints to match a workaround on those types.

  2. Swift 3 does not support generic subscripts, so we must instead generate subscripts for each known RangeExpression type.

  3. Because of the complex way Collection's protocols are layered, it is necessary to attach subscripts to Indexable and MutableIndexable, and constrain relative(to:)'s parameter to Indexable, instead of Collection and MutableCollection.

  4. Swift currently has trouble with the constraints on RangeExpression.relative(to:) if it is a protocol requirement. Thus, it is instead provided as an extension method. A method with simpler generic constraints is instead used as the requirement.

These will not affect source compatibility except when a user conforms their own types to RangeExpression, so we suggest we place warnings in the documentation, effectively pre-deprecating its required method.

The actual design of RangeExpression is thus as follows:

/// A type which can be used to slice a collection. A `RangeExpression` can 
/// convert itself to a `Range<Bound>` of indices within a given collection; 
/// the collection can then slice itself with that `Range`.
/// 
/// -Warning: The requirements of `RangeExpression` are likely to change 
///           in a future version of Swift. If you conform your own 
///           types to `RangeExpression`, be prepared to migrate them.
public protocol RangeExpression {
  /// The type of the bounds of the `Range` produced by this 
  /// type when used as a `RangeExpression`.
  associatedtype Bound : Comparable
  
  /// Returns `self` expressed as a `Range<Bound>` suitable for 
  /// slicing a collection with the indicated properties.
  /// 
  /// -Parameter bounds: The range of indices in the collection.
  ///                    Equivalent to `startIndex ..< endIndex`
  ///                    in `Collection`.
  /// 
  /// -Parameter offset: A function which can be used to add to or 
  ///                    subtract from a bound. Equivalent to 
  ///                    `index(_:offsetBy:)` in `Collection`.
  /// 
  /// -Returns: A `Range<Bound>` suitable for slicing a collection. 
  ///           The return value is *not* guaranteed to be inside 
  ///           `bounds`. Callers should apply the same preconditions 
  ///           to the return value as they would to a range provided 
  ///           directly by the user.
  /// 
  /// -Warning: This method is likely to be replaced in a future version of Swift.
  ///           If you are calling this method, we recommend using the 
  ///           `relative(to:)` extension method instead. If you are implementing 
  ///           it, be prepared to migrate your code.
  /// 
  /// -Recommended: `relative(to:)`
  // 
  // WORKAROUND unfiled - We want to have this requirement, but it triggers a generics bug
  // func relative<C: Indexable>(to collection: C) -> Range<Bound> where C.Index == Bound
  func relative<BoundDistance: SignedInteger>(to bounds: Range<Bound>, offsettingBy offset: (Bound, BoundDistance) -> Bound) -> Range<Bound>
}

extension RangeExpression {
  /// Returns `self` expressed as a range of indices within `collection`.
  /// 
  /// -Parameter collection: The collection `self` should be 
  ///                        relative to.
  /// 
  /// -Returns: A `Range<Bound>` suitable for slicing `collection`. 
  ///           The return value is *not* guaranteed to be inside 
  ///           its bounds. Callers should apply the same preconditions 
  ///           to the return value as they would to a range provided 
  ///           directly by the user.
  /// 
  /// -RecommendedOver: `relative(to:offsettingBy:)`
  public func relative<C: Indexable>(to collection: C) -> Range<Bound> where C.Index == Bound {
    let bounds = Range(uncheckedBounds: (lower: collection.startIndex, upper: collection.endIndex))
    return relative(to: bounds, offsettingBy: collection.index(_:offsetBy:))
  }
}

A full working prototype of RangeExpression and the IncompleteRange types is available in this pull request.

Impact on existing code

Obviously, any code using these APIs under their old names or designs would have to be transitioned to the new names and designs.

The sequence-end operations would be by far the simplest to handle; these are simple renamings and could be handed by @available(renamed:) and migration support. The only complication is that some overloads have transitioned to a new base name, while others have stayed with the old one, but we suspect the migrator is up to this task.

The index-based operations are more difficult to migrate. The patterns would be roughly:

collection.prefix(upTo: i)    => collection[..<i]
collection.prefix(through: i) => collection[...i]
collection.suffix(from: i)    => collection[i..<]

A custom fix-it would be ideal, but is probably not necessary; an @available(message:) would do. Presumably this would have to be a special case in the migrator as well.

The other changes to the handling of subranges are source-compatible.

Alternatives considered

skipping instead of removing

If the type differences are seen as disqualifying removing as a replacement for drop, we suggest using skipping instead.

There are, of course, many possible alternatives to skipping; this is almost a perfect subject for bikeshedding. We've chosen skipping because:

  1. It is not an uncommon word, unlike (say) omitting. This means non-native English speakers and schoolchildren are more likely to recognize it.

  2. It is an -ing verb, unlike (say) without. This makes it fit common Swift naming patterns more closely.

  3. It does not imply danger, unlike (say) dropping, nor some sort of ongoing process, unlike (say) ignoring. This makes its behavior more obvious.

If you want to suggest an alternative on swift-evolution, please do not merely mention a synonym; rather, explain why it is an improvement on either these axes or other ones. (We would be particularly interested in names other than removing which draw an analogy to something else in Swift.)

collection[to/through/from:] instead of IncompleteRange

Rather than add new types and operators to replace prefix(upTo/through:) and suffix(from:), we could merely transform these methods into subscripts with parameter labels. This would be a simpler design, but these terms have proven imperfect in the stride calls, and labeled subscripts are rare (actually, we believe they're unprecedented in the standard library).

longestPrefix(where:) instead of prefix(while:)

The name prefix(while:) isn't perfect; it seems to imply more state than is really involved. (There is a stateful loop, but it's contained within the method.) A name like longestPrefix(where:) might read better and avoid this implication, but we think it's important that prefix(_:) and prefix(while:) be given parallel names, and longestPrefix(3) doesn't make much sense.

No RangeExpression protocol

The RangeExpression protocol could be severed from this proposal, but this seems like a good opportunity to refactor our subrange handling.

Other alternatives

  • Rather than using first and last for the "First matching" and "Last matching" categories, we could use a distinct term. These methods have different performance characteristics than the others, and modeling that might be helpful. However, it's difficult to find a good term—an earlier version of this proposal used earliest and latest, which don't read perfectly—and the level of confusion is pretty low.

  • We considered using first and last as the basis for both single-element and multiple-element operations (such that prefix(3) would become first(3), etc.), but:

    1. These seemed like distinct functionalities, particularly since their types are different.

    2. We're not comfortable with heavily overloading a property with a bunch of methods, and didn't want to make first and last into methods.

    3. Most APIs work fine, but hasFirst(_:) is atrocious, and we see no better alternative which includes the word first.

  • We considered moving first and last to Sequence and possibly making them methods, but our understanding is that the core team has considered and rejected this approach in the past.

  • We considered moving removingFirst and removingLast to Collection and making them properties, to match first and last, but this seemed like the sort of foolish consistency that Ralph Waldo Emerson warned of.

Future directions

Note: The rest of this proposal is highly speculative and there's probably no need to read further.

Other Sequence API cleanups

Seriously source-breaking

  • There is an ongoing discussion about which, if any, of map, flatMap, filter, and reduce ought to be renamed to more closely match Swift naming conventions. There is also discussion about relabeling many closure parameters.

    The "Future directions" section below suggests every(where:) as an alternative to filter which could be extended in ways compatible with this proposal.

Significantly source-breaking

  • The removeSubrange(_:) and replaceSubrange(_:with:) APIs are rather oddly named. They might be better renamed to, for instance, remove(in:) and replace(in:with:).

  • It is not clear how important removingFirst() and removingLast() actually are, given that they're exactly equivalent to removingPrefix(1) and removingSuffix(1), and their corresponding "get" members are on Collection instead of Sequence. They could be removed.

Slightly source-breaking

  • removeFirst/Last() and popFirst/Last() are very nearly redundant; their only difference is that the remove methods have a non-Optional return type and require the collection not be empty, while the pop methods have an Optional return type and return nil if it's empty.

    These operations could be merged, with the remove operations taking on the preconditions of the current pop operations; additionally, removePrefix(_:) and removeSuffix(_:) could drop their equivalent preconditions requiring that the elements being removed exist. These changes would simplify the standard library and make these methods more closely parallel the equivalent removing methods, which do not have similar preconditions.

    Performance-critical code which wants to avoid the checks necessary to remove these preconditions could switch to remove(at:) and removeSubrange(_:), which would continue to reject invalid indices.

Adding sequence and collection operations

This exercise in renaming suggests all sorts of other APIs we might add, and a few we might rename.

In general, we have not attempted to carefully scrutinize the usefulness of each of these APIs; instead, we have merely listed the ones which we can imagine some kind of use for. The main exception is the "Pop" operation; we can imagine several different, and rather incompatible, ways to extend it, and we're not going to take the time to sort out our thoughts merely to write a "Future directions" section.

Filling in the sequence-end API table

The gaps in the table suggest a number of APIs we could offer in the future. Here, we have filled in all options which are at least coherent:

Get Index Exclude Remove (1) Pop (1) Equate (2)
Fixed Size
First 1 C.first C.firstIndex S.removingFirst() C.removeFirst() C.popFirst() -
Last 1 C.last C.lastIndex S.removingLast() C.removeLast() C.popLast() -
First (n: Int) S.prefix(_:) C.prefixIndex(_:) S.removingPrefix(_:) C.removePrefix(_:) - S.hasPrefix(_:)
  ...with closure S.prefix(while:) C.prefixIndex(while:) S.removingPrefix(while:) C.removePrefix(while:) - S.hasPrefix(_:by:)
Last (n: Int) S.suffix(_:) C.suffixIndex(_:) S.removingSuffix(_:) C.removeSuffix(_:) - S.hasSuffix(_:)
  ...with closure S.suffix(while:) C.suffixIndex(while:) S.removingSuffix(while:) C.removeSuffix(while:) - S.hasSuffix(_:by:)
Searching From End
First matching element S.first(_:) C.firstIndex(of:) S.removingFirst(_:) C.removeFirst(_:) - -
  ...with closure S.first(where:) C.firstIndex(where:) S.removingFirst(where:) C.removeFirst(where:) - -
Last matching element S.last(_:) C.lastIndex(of:) S.removingLast(_:) C.removeLast(_:) - -
  ...with closure S.last(where:) C.lastIndex(where:) S.removingLast(where:) C.removeLast(where:) - -

To explain a few entries which might not be immediately obvious: firstIndex and lastIndex would be nil if the collection is empty, and lastIndex would be the index before endIndex. prefixIndex would return the last index of the prefix, and suffixIndex would return the first index of the suffix; alternatively, these could be named with Indices and return ranges. first(_:) and last(_:) would return the first and last element equal to the provided value; on a Set, they would be roughly equivalent to NSSet.member(_:).

The changes we consider most worthy include:

  • Adding corresponding last and suffix methods for all first and prefix methods.

  • Adding corresponding while: versions of all appropriate prefix/suffix APIs.

Ones that could be useful, but can usually be emulated with more work:

  • Adding remove/removing-by-content APIs.

  • Adding prefix/suffixIndex(while:).

Ones that are mere conveniences or may not have strong use cases:

  • first/lastIndex and prefix/suffixIndex(_:).

  • first/last(_:).

"All" and "Every" as operands

One could imagine adding rows to this table for "all" and "every matching". In addition to creating some useful new API, this would also suggest some interesting renaming for existing APIs:

  • allIndices would be a name for indices.

  • removeAll() is actually an existing name which happens to fit this pattern.

  • every(where:) would be a name for filter. Though some of us believe filter is a strong term of art, we do note that every(where:) does not cause confusion about the sense of its test, a major complaint about filter.

In the table below, bold indicates new functionality; italics indicates existing functionality renamed to fit this pattern.

Get Index Exclude Remove (1) Pop (1) Equate (2)
Fixed Size
First 1 C.first C.firstIndex S.removingFirst() C.removeFirst() C.popFirst() -
Last 1 C.last C.lastIndex S.removingLast() C.removeLast() C.popLast() -
First (n: Int) S.prefix(_:) C.prefixIndex(_:) S.removingPrefix(_:) C.removePrefix(_:) - S.hasPrefix(_:)
  ...with closure S.prefix(while:) C.prefixIndex(while:) S.removingPrefix(while:) C.removePrefix(while:) - S.hasPrefix(_:by:)
Last (n: Int) S.suffix(_:) C.suffixIndex(_:) S.removingSuffix(_:) C.removeSuffix(_:) - S.hasSuffix(_:)
  ...with closure S.suffix(while:) C.suffixIndex(while:) S.removingSuffix(while:) C.removeSuffix(while:) - S.hasSuffix(_:by:)
All - allIndices - C.removeAll() - -
Searching From End
First matching element S.first(_:) C.firstIndex(of:) S.removingFirst(_:) C.removeFirst(_:) - -
  ...with closure S.first(where:) C.firstIndex(where:) S.removingFirst(where:) C.removeFirst(where:) - -
Last matching element S.last(_:) C.lastIndex(of:) S.removingLast(_:) C.removeLast(_:) - -
  ...with closure S.last(where:) C.lastIndex(where:) S.removingLast(where:) C.removeLast(where:) - -
Every matching element S.every(_:) C.everyIndex(of:) S.removingEvery(_:) C.removeEvery(_:) - -
  ...with closure S.every(where:) C.everyIndex(where:) S.removingEvery(where:) C.removeEvery(where:) - -

An alternative to the every methods is to give them names based on all or any, but these tend to require breaks from the naming patterns of the matching first and last methods to remain grammatical.