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Add StaticString.UnicodeScalarView

Introduction

There is no way to create a substring of a StaticString that is still typed as StaticString. There should be.

Swift Evolution Discussion Thread, Review

Motivation

It is occasionally useful to be able to produce a substring of a StaticString that can be passed to APIs expecting a StaticString. For example, extracting the filename from __FILE__. But there is no way to do this today, as StaticString does not provide any means by which to create a new instance beyond the trivial nullary init() initializer (which creates an empty string).

Proposed solution

We add a new type StaticString.UnicodeScalarView that conforms to CollectionType and a new property unicodeScalars on StaticString. We also add 2 initializers to StaticString

init(_ unicodeScalars: UnicodeScalarView)
init(_ unicodeScalars: Slice<UnicodeScalarView>)

Together, this allows the user to manipulate the unicode scalar view to produce the desired slice, and then to create a StaticString from the results. This has the added benefit of providing a convenient way to work with StaticStrings as a sequence of UnicodeScalars instead of as a UTF8 buffer.

Detailed design

The API looks like this:

extension StaticString {
  /// The value of `self` as a collection of [Unicode scalar values](http://www.unicode.org/glossary/#unicode_scalar_value).
  public var unicodeScalars: UnicodeScalarView { get }

  /// Construct the `StaticString` corresponding to the given
  /// `UnicodeScalarView`.
  public init(_: UnicodeScalarView)

  /// Construct the `StaticString` corresponding to the given
  /// `UnicodeScalarView` slice.
  public init(_: Slice<UnicodeScalarView>)

  /// A collection of [Unicode scalar values](http://www.unicode.org/glossary/#unicode_scalar_value) that
  /// encode a `StaticString`.
  public struct UnicodeScalarView : CollectionType {

    init(_: StaticString)

    /// A position in a `StaticString.UnicodeScalarView`.
    public struct Index : BidirectionalIndexType, Comparable {
      /// Returns the next consecutive value after `self`.
      ///
      /// - Requires: The next value is representable.
      @warn_unused_result
      public func successor() -> Index

      /// Returns the previous consecutive value before `self`.
      ///
      /// - Requires: The previous value is representable.
      @warn_unused_result
      public func predecessor() -> Index
    }

    /// The position of the first `UnicodeScalar` if the `StaticString` is
    /// non-empty; identical to `endIndex` otherwise.
    public var startIndex: Index { get }

    /// The "past the end" position.
    ///
    /// `endIndex` is not a valid argument to `subscript`, and is always
    /// reachable from `startIndex` by zero or more applications of
    /// `successor()`.
    public var endIndex: Index { get }

    /// Returns `true` iff `self` is empty.
    public var isEmpty: Bool { get }

    public subscript(position: Index) -> UnicodeScalar { get }
  }
}

Impact on existing code

None.

Alternatives considered

We could add a subscript(bounds: Range<Index>) to StaticString directly, but there's no good way to define Index (for the same reasons String doesn't conform to CollectionType).

We could expose an unsafe initializer from a pointer, so the user can manipulate utf8Start to produce the desired pointer, but this would be very unsafe and allow users to try and trick code taking StaticString into accepting a dynamic string instead.