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One-sided Ranges

Introduction

This proposal introduces the concept of a "one-sided" range, created via prefix/postfix versions of the existing range operators.

It also introduces a new protocol, RangeExpression, to simplify the creation of methods that take different kinds of ranges.

Motivation

It is common, given an index into a collection, to want a slice up to or from that index versus the start/end.

For example (assuming String is once more a Collection):

let s = "Hello, World!"
let i = s.index(of: ",")!
let greeting = s[s.startIndex..<i]

When performing lots of slicing like this, the verbosity of repeating s.startIndex is tiresome to write and harmful to readability.

Swift 3’s solution to this is a family of methods:

let greeting = s.prefix(upTo: i)
let withComma = s.prefix(through: i)
let location = s.suffix(from: i)

The two very different-looking ways to perform a similar task is jarring. And as methods, the result cannot be used as an l-value.

A variant of the one-sided slicing syntax found in Python (i.e. s[i:]) is proposed to resolve this.

Proposed solution

Introduce a one-sided range syntax, where the "missing" side is inferred to be the start/end:

// half-open right-handed range
let greeting = s[..<i]
// closed right-handed range
let withComma = s[...i]
// left-handed range (no need for half-open variant)
let location = s[i...]

Additionally, when the index is a countable type, i... should form a Sequence that counts up from i indefinitely. This is useful in forming variants of Sequence.enumerated() when you either want them non-zero-based i.e. zip(1..., greeting), or want to flip the order i.e. zip(greeting, 0...).

This syntax would supercede the existing prefix and suffix operations that take indices, which will be deprecated in a later release. Note that the versions that take distances are not covered by this proposal, and would remain.

This will require the introduction of new range types (e.g. PartialRangeThrough). There are already multiple range types (e.g. ClosedRange, CountableHalfOpenRange), which require overloads to allow them to be used wherever a Range can be.

To unify these different range types, a new protocol, RangeExpression will be created and all ranges conformed to it. Existing overloads taking concrete types other than Range can then be replaced with a single generic method that takes a RangeExpression, converts it to a Range, and then forward the method on.

A generic version of ~= will also be implemented for all range expressions:

switch i {
case 9001...: print("It’s over NINE THOUSAAAAAAAND")
default: print("There's no way that can be right!")
}

The existing concrete overloads that take ranges other than Range will be deprecated in favor of generic ones that take a RangeExpression.

Detailed design

Add the following to the standard library:

(a fuller work-in-progress implementation can be found here: https://github.com/apple/swift/pull/8710)

NOTE: The following is subject to change depending on pending compiler features. Methods may actually be on underscored protocols, and then moved once recursive protocols are implemented. Types may be collapsed using conditional conformance. This should not matter from a usage perspective – users are not expected to use these types directly or override any of the behaviors in their own types. Any final implementation will follow the below in spirit if not in practice.

public protocol RangeExpression {
    associatedtype Bound: Comparable

    /// 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.
    func relative<C: _Indexable>(to collection: C) -> Range<Bound> where C.Index == Bound

    func contains(_ element: Bound) -> Bool
}

extension RangeExpression {
  public static func ~= (pattern: Self, value: Bound) -> Bool
}

prefix operator ..<
public struct PartialRangeUpTo<T: Comparable>: RangeExpression {
  public init(_ upperBound: T) { self.upperBound = upperBound }
  public let upperBound: T
}
extension Comparable {
  public static prefix func ..<(x: Self) -> PartialRangeUpTo<Self>
}

prefix operator ...
public struct PartialRangeThrough<T: Comparable>: RangeExpression {
  public init(_ upperBound: T)
  public let upperBound: T
}
extension Comparable {
  public static prefix func ...(x: Self) -> PartialRangeThrough<Self>
}

postfix operator ...
public struct PartialRangeFrom<T: Comparable>: RangeExpression {
  public init(_ lowerBound: T)
  public let lowerBound: T
}
extension Comparable {
  public static postfix func ...(x: Self) -> PartialRangeFrom<Self>
}

// The below relies on Conditional Conformance. Pending that feature,
// this may require an additional CountablePartialRangeFrom type temporarily.
extension PartialRangeFrom: Sequence 
  where Index: _Strideable, Index.Stride : SignedInteger


extension Collection {
  public subscript<R: RangeExpression>(r: R) -> SubSequence
   where R.Bound == Index { get }
}
extension MutableCollection {
  public subscript<R: RangeExpression>(r: R) -> SubSequence
   where R.Bound == Index { get set }
}
  
extension RangeReplaceableColleciton {
  public mutating func replaceSubrange<C: Collection, R: RangeExpression>(
    _ subrange: ${Range}<Index>, with newElements: C
  ) where C.Iterator.Element == Iterator.Element, R.Bound == Index

  public mutating func removeSubrange<R: RangeExpression>(
    _ subrange: ${Range}<Index>
  ) where R.Bound == Index
}

Additionally, these new ranges will implement appropriate protocols such as CustomStringConvertible.

It is important to note that these new methods and range types are extensions only. They are not protocol requirements, as they should not need to be customized for specific collections. They exist only as shorthand to expand out to the full slicing operation.

Where PartialRangeFrom is a Sequence, it is left up to the type of Index to control the behavior when the type is incremented past its bounds. In the case of an Int, the iterator will trap when iterating past Int.max. Other types, such as a BigInt that could be incremented indefinitely, would behave differently.

The prefix and suffix methods that take an index are currently protocol requirements, but should not be. This proposal will fix that as a side-effect.

Source compatibility

The new operators/types are purely additive so have no source compatibility consequences. Replacing the overloads taking concrete ranges other than Range with a single generic version is source compatible. prefix and suffix will be deprecated in Swift 4 and later removed.

Effect on ABI stability

The prefix/suffix methods being deprecated should be eliminated before declaring ABI stability.

Effect on API resilience

The new operators/types are purely additive so have no resilience consequences.

Alternatives considered

i... is favored over i..< because the latter is ugly. We have to pick one, two would be redundant and likely to cause confusion over which is the "right" one. Either would be reasonable on pedantic correctness grounds – (i as Int)... includes Int.max consistent with ..., whereas a[i...] is interpreted as a[i..<a.endIndex] consistent with i..<.

It might be nice to consider extend this domain-specific language inside the subscript in other ways. For example, to be able to incorporate the index distance versions of prefix, or add distance offsets to the indices used within the subscript. This proposal explicitly avoids proposals in this area. Such ideas would be considerably more complex to implement, and would make a good project for investigation by an interested community member, but would not fit within the timeline for Swift 4.