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Rename flatten() to joined()

Introduction

Swift currently defines two similar functions, joined(separator:) and flatten(). joined(separator:) has a specialized implementation for Strings, and flatten() has implementations for various kinds of collections.

extension Sequence where Iterator.Element : Sequence {
  public func joined<Separator: Sequence>(separator: Separator) -> JoinedSequence<Self>
  public func flatten() -> FlattenSequence<Self>
}

extension Collection where Element : Collection {  // and similar variants
  public func flatten() -> FlattenCollection<Self>
}

This proposal renames flatten() to joined() (with no separator argument). It also adds a default separator of "" to the String-specific version of joined(separator:).

https://github.com/apple/swift/blob/f72a82327b172e1a2979e46cb7a579e3cc2f3bd6/stdlib/public/core/Join.swift https://github.com/apple/swift/blob/c6e828f761fc30f7ce444431de7da52814f96595/stdlib/public/core/String.swift#L769 https://github.com/apple/swift/blob/f72a82327b172e1a2979e46cb7a579e3cc2f3bd6/stdlib/public/core/Flatten.swift.gyb

Swift-evolution threads:

Motivation

To a user, there should be no distinction between flatten()ing a sequence or collection, and "joining" it with no separator.

Hence, the following expressions should be valid:

[[1,2],[3]].joined()                // [1,2,3]  -- currently named flatten()
[[1,2],[3]].joined(separator: [])   // [1,2,3]
[[1,2],[3]].joined(separator: [0])  // [1,2,0,3]

["ab","d"].joined()                // "abd"  -- currently no nullary function to do this
["ab","d"].joined(separator: "")   // "abd"
["ab","d"].joined(separator: "_")  // "ab_d"

Proposed solution

Rename flatten() to joined() with no argument. For now, it's acceptable to keep the code currently in Join.swift and that in Flatten.swift.gyb separate — in the future it might make sense to unify the algorithms.

The String-specific version of joined(separator:) is independent of the Sequence protocol extension (since String is not a Sequence), but the functionality is still useful. For consistency, a default value of "" should be added to the separator parameter:

extension Sequence where Iterator.Element == String {
  func joined(separator: String = "") -> String {
    ...
  }
}

(Or, if the standard library team deems it a better solution, joined() could be a separate method that simply calls joined(separator: "").)

Impact on existing code

Users of flatten() will need to migrate to joined(); this is straightforward with an availability attribute. Application behavior should not change.

Alternatives considered

An alternative is to leave flatten() and joined(separator:) as separate APIs. The distinction, however, seems unnecessary, and unifying them is a minor win for API clarity.