[swift-users] See documentation comment for discussion

[Brent Royal-Gordon]

> I've previously used myString[0...1] to get the first character of a string.

I'm not sure how, because this has never worked in Swift. Strings use opaque indices which specifically prevent you from doing this, so you have never been able to subscript a string with integers or ranges of integers. It's done this way because indexing into a Unicode string intrinsically requires walking through the data structure and counting characters one at a time; the lack of integer indices is meant to force you to explicitly write `index(after:)` or `index(_:offsetBy:)` (formerly `successor()` or `advancedBy(_:)`) in your code to make this cost obvious.

> What documentation is this referring to? There's no right-click definition available, nothing in Navigator, and Google fails to turn up anything.

The standard library includes definitions of these subscripts which are meant to direct you to documentation explaining why they're not supported, but these definitions don't show up in the generated interfaces. Presumably Xcode is removing all APIs marked "unavailable". The doc comment it's trying to direct you to is here <https://github.com/apple/swift/blob/b8401e1fde52d95e5a8ce7b043a3c5a3bcf72181/stdlib/public/core/UnavailableStringAPIs.swift.gyb#L15>:

  /// Subscripting strings with integers is not available.
  ///
  /// The concept of "the `i`th character in a string" has
  /// different interpretations in different libraries and system
  /// components.  The correct interpretation should be selected
  /// according to the use case and the APIs involved, so `String`
  /// cannot be subscripted with an integer.
  ///
  /// Swift provides several different ways to access the character
  /// data stored inside strings.
  ///
  /// - `String.utf8` is a collection of UTF-8 code units in the
  ///   string. Use this API when converting the string to UTF-8.
  ///   Most POSIX APIs process strings in terms of UTF-8 code units.
  ///
  /// - `String.utf16` is a collection of UTF-16 code units in
  ///   string.  Most Cocoa and Cocoa touch APIs process strings in
  ///   terms of UTF-16 code units.  For example, instances of
  ///   `NSRange` used with `NSAttributedString` and
  ///   `NSRegularExpression` store substring offsets and lengths in
  ///   terms of UTF-16 code units.
  ///
  /// - `String.unicodeScalars` is a collection of Unicode scalars.
  ///   Use this API when you are performing low-level manipulation
  ///   of character data.
  ///
  /// - `String.characters` is a collection of extended grapheme
  ///   clusters, which are an approximation of user-perceived
  ///   characters.
  ///
  /// Note that when processing strings that contain human-readable
  /// text, character-by-character processing should be avoided to
  /// the largest extent possible.  Use high-level locale-sensitive
  /// Unicode algorithms instead, for example,
  /// `String.localizedStandardCompare()`,
  /// `String.localizedLowercaseString`,
  /// `String.localizedStandardRangeOfString()` etc.


-- 
Brent Royal-Gordon
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