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0178-character-unicode-view.md

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Add unicodeScalars property to Character

Introduction

This proposal adds a unicodeScalars view to Character, similar to that on String.

Motivation

The Character element type of String is currently a black box that provides little functionality besides comparison, literal construction, and to be used as an argument to String.init.

Many operations on String could be neatly/readably implemented as operations on each character in the string, if Character exposed its scalars more directly. Many useful things can be determined by examining the scalars in a grapheme (for example is this an ASCII character?).

For example, today you can write this:

let s = "one two three"
s.split(separator: " ")

But you cannot write this:

let ws = CharacterSet.whitespacesAndNewlines
s.split { $0.unicodeScalars.contains(where: ws.contains) }

Proposed solution

Add a unicodeScalars property to Character, presenting a lazy view of the scalars in the character, along similar lines to the one on String.

Unlike the view on String, this will not be a mutable view – it will be read-only. The preferred method for creating and manipulating non-literal Character values will be through String. While there may be some good use cases to manipulating a Character directly, these are outweighed by the complexity of ensuring the invariant that it contain exactly one grapheme.

Detailed design

Add the following nested type to Character:

extension Character {
  public struct UnicodeScalarView : BidirectionalCollection {
    public struct Index
    public var startIndex: Index { get }
    public var endIndex: Index { get }
    public func index(after i: Index) -> Index
    public func index(before i: Index) -> Index
    public subscript(i: Index) -> UnicodeScalar
  }
  public var unicodeScalars: UnicodeScalarView { get }
}

Additionally, this type will conform to appropriate convenience protocols such as CustomStringConvertible.

All initializers will be declared internal, as unlike the String equivalent, this type will only ever be vended by Character.

Source compatibility

Purely additive, so no impact.

Effect on ABI stability

Purely additive, so no impact.

Effect on API resilience

Purely additive, so no impact.

Alternatives considered

Adding other views, such as utf8 or utf16, was considered but not deemed useful enough compared to using these operations on String instead.

In the future, this feature could be used to implement convenience methods such as isASCII on Character. This could be done additively, given this building block, and is outside the scope of this initial proposal.