[swift-evolution] [Pitch] String revision proposal #1

Xiaodi Wu xiaodi.wu at gmail.com
Wed Mar 29 20:59:32 CDT 2017

This looks great. The restored conformances to *Collection will be huge.

Is this to be the first of several or the only major part of the manifesto
to be delivered in Swift 4?

Nits on naming: are we calling it Substring or SubString (à la
SubSequence)? and shouldn't it be UnicodeParsedResult rather than

On Wed, Mar 29, 2017 at 19:32 Ben Cohen via swift-evolution <
swift-evolution at swift.org> wrote:

Hi Swift Evolution,

Below is a pitch for the first part of the String revision. This covers a
number of changes that would allow the basic internals to be overhauled.

Online version here:

String Revision: Collection Conformance, C Interop, Transcoding

   - Proposal: SE-0161
   - Authors: Ben Cohen <https://github.com/airspeedswift>, Dave Abrahams
   - Review Manager: TBD
   - Status: *Awaiting review*


This proposal is to implement a subset of the changes from the Swift 4
String Manifesto


   - Make String conform to BidirectionalCollection
   - Make String conform to RangeReplaceableCollection
   - Create a Substring type for String.SubSequence
   - Create a Unicode protocol to allow for generic operations over both
   - Consolidate on a concise set of C interop methods.
   - Revise the transcoding infrastructure.

Other existing aspects of String remain unchanged for the purposes of this

This proposal follows up on a number of recommendations found in the

Collection conformance was dropped from String in Swift 2. After
reevaluation, the feeling is that the minor semantic discrepancies (mainly
with RangeReplaceableCollection) are outweighed by the significant benefits
of restoring these conformances. For more detail on the reasoning, see here

While it is not a collection, the Swift 3 string does have slicing
operations. String is currently serving as its own subsequence, allowing
substrings to share storage with their “owner”. This can lead to memory
leaks when small substrings of larger strings are stored long-term (see here
more detail on this problem). Introducing a separate type of Substring to
serve as String.Subsequence is recommended to resolve this issue, in a
similar fashion to ArraySlice.

As noted in the manifesto, support for interoperation with nul-terminated C
strings in Swift 3 is scattered and incoherent, with 6 ways to transform a
C string into a String and four ways to do the inverse. These APIs should
be replaced with a simpler set of methods on String.
Proposed solution

A new type, Substring, will be introduced. Similar to ArraySlice it will be
documented as only for short- to medium-term storage:

Long-term storage of Substring instances is discouraged. A substring holds
a reference to the entire storage of a larger string, not just to the
portion it presents, even after the original string’s lifetime ends.
Long-term storage of a substring may therefore prolong the lifetime of
elements that are no longer otherwise accessible, which can appear to be
memory leakage.

Aside from minor differences, such as having a SubSequence of Self and a
larger size to describe the range of the subsequence, Substring will be
near-identical from a user perspective.

In order to be able to write extensions accross both String and Substring,
a new Unicode protocol to which the two types will conform will be
introduced. For the purposes of this proposal, Unicode will be defined as a
protocol to be used whenver you would previously extend String. It should
be possible to substitute extension Unicode { ... } in Swift 4
wherever extension
String { ... } was written in Swift 3, with one exception: any passing of
self into an API that takes a concrete String will need to be rewritten as
String(self). If Self is a String then this should effectively optimize to
a no-op, whereas if Self is a Substring then this will force a copy,
helping to avoid the “memory leak” problems described above.

The exact nature of the protocol – such as which methods should be protocol
requirements vs which can be implemented as protocol extensions, are
considered implementation details and so not covered in this proposal.

Unicode will conform to BidirectionalCollection.
RangeReplaceableCollection conformance
will be added directly onto the String and Substring types, as it is
possible future Unicode-conforming types might not be range-replaceable
(e.g. an immutable type that wraps a const char *).

The C string interop methods will be updated to those described here
a single withCString operation and two init(cString:) constructors, one for
UTF8 and one for arbitrary encodings. The primary change is to remove
“non-repairing” variants of construction from nul-terminated C strings. In
both of the construction APIs, any invalid encoding sequence detected will
have its longest valid prefix replaced by U+FFFD, the Unicode replacement
character, per the Unicode specification. This covers the common case. The
replacement is done physically in the underlying storage and the validity
of the result is recorded in the String’s encoding such that future
accesses need not be slowed down by possible error repair separately.
Construction that is aborted when encoding errors are detected can be
accomplished using APIs on the encoding.

The current transcoding support will be updated to improve usability and
performance. The primary changes will be:

   - to allow transcoding directly from one encoding to another without
   having to triangulate through an intermediate scalar value
   - to add the ability to transcode an input collection in reverse,
   allowing the different views on String to be made bi-directional
   - to have decoding take a collection rather than an iterator, and return
   an index of its progress into the source, allowing that method to be static

The standard library currently lacks a Latin1 codec, so a enum Latin1:
UnicodeEncoding type will be added.
Detailed design

The following additions will be made to the standard library:

protocol Unicode: BidirectionalCollection {
  // Implementation detail as described above
extension String: Unicode, RangeReplaceableCollection {
  typealias SubSequence = Substring
struct Substring: Unicode, RangeReplaceableCollection {
  typealias SubSequence = Substring
  // near-identical API surface area to String

The subscript operations on String will be amended to return Substring:

struct String {
  subscript(bounds: Range<String.Index>) -> Substring { get }
  subscript(bounds: ClosedRange<String.Index>) -> Substring { get }

Note that properties or methods that due to their nature create new
String storage
(such as lowercased()) will *not* change.

C string interop will be consolidated on the following methods:

extension String {
  /// Constructs a `String` having the same contents as `nulTerminatedUTF8`.
  /// - Parameter nulTerminatedUTF8: a sequence of contiguous UTF-8 encoded
  ///   bytes ending just before the first zero byte (NUL character).
  init(cString nulTerminatedUTF8: UnsafePointer<CChar>)

  /// Constructs a `String` having the same contents as
  /// - Parameter nulTerminatedCodeUnits: a sequence of contiguous code units in
  ///   the given `encoding`, ending just before the first zero code unit.
  /// - Parameter encoding: describes the encoding in which the code units
  ///   should be interpreted.
  init<Encoding: UnicodeEncoding>(
    cString nulTerminatedCodeUnits: UnsafePointer<Encoding.CodeUnit>,
    encoding: Encoding)

  /// Invokes the given closure on the contents of the string, represented as a
  /// pointer to a null-terminated sequence of UTF-8 code units.
  func withCString<Result>(
    _ body: (UnsafePointer<CChar>) throws -> Result) rethrows -> Result

Additionally, the current ability to pass a Swift String into C methods
that take a C string will remain as-is.

A new protocol, UnicodeEncoding, will be added to replace the current
UnicodeCodec protocol:

public enum UnicodeParseResult<T, Index> {/// Indicates valid input
was recognized.////// `resumptionPoint` is the end of the parsed
regioncase valid(T, resumptionPoint: Index)  // FIXME: should these be
reordered?/// Indicates invalid input was recognized.//////
`resumptionPoint` is the next position at which to continue parsing
after/// the invalid input is repaired.case error(resumptionPoint:
/// Indicates that there was no more input to consume.case emptyInput

  /// If any input was consumed, the point from which to continue parsing.
  var resumptionPoint: Index? {
    switch self {
    case .valid(_,let r): return r
    case .error(let r): return r
    case .emptyInput: return nil
/// An encoding for text with UnicodeScalar as a common currency
typepublic protocol UnicodeEncoding {
  /// The maximum number of code units in an encoded unicode scalar value
  static var maxLengthOfEncodedScalar: Int { get }

  /// A type that can represent a single UnicodeScalar as it is encoded in this
  /// encoding.
  associatedtype EncodedScalar : EncodedScalarProtocol

  /// Produces a scalar of this encoding if possible; returns `nil` otherwise.
  static func encode<Scalar: EncodedScalarProtocol>(
    _:Scalar) -> Self.EncodedScalar?

  /// Parse a single unicode scalar forward from `input`.
  /// - Parameter knownCount: a number of code units known to exist in `input`.
  ///   **Note:** passing a known compile-time constant is strongly advised,
  ///   even if it's zero.
  static func parseScalarForward<C: Collection>(
    _ input: C, knownCount: Int /* = 0, via extension */
  ) -> ParseResult<EncodedScalar, C.Index>
  where C.Iterator.Element == EncodedScalar.Iterator.Element

  /// Parse a single unicode scalar in reverse from `input`.
  /// - Parameter knownCount: a number of code units known to exist in `input`.
  ///   **Note:** passing a known compile-time constant is strongly advised,
  ///   even if it's zero.
  static func parseScalarReverse<C: BidirectionalCollection>(
    _ input: C, knownCount: Int /* = 0 , via extension */
  ) -> ParseResult<EncodedScalar, C.Index>
  where C.Iterator.Element == EncodedScalar.Iterator.Element
/// Parsing multiple unicode scalar valuesextension UnicodeEncoding {
  public static func parseForward<C: Collection>(
    _ input: C,
    repairingIllFormedSequences makeRepairs: Bool = true,
    into output: (EncodedScalar) throws->Void
  ) rethrows -> (remainder: C.SubSequence, errorCount: Int)

  public static func parseReverse<C: BidirectionalCollection>(
    _ input: C,
    repairingIllFormedSequences makeRepairs: Bool = true,
    into output: (EncodedScalar) throws->Void
  ) rethrows -> (remainder: C.SubSequence, errorCount: Int)
  where C.SubSequence : BidirectionalCollection,
        C.SubSequence.SubSequence == C.SubSequence,
        C.SubSequence.Iterator.Element == EncodedScalar.Iterator.Element

UnicodeCodec will be updated to refine UnicodeEncoding, and all existing
codecs will conform to it.

Note, depending on whether this change lands before or after some of the
generics features, generic where clauses may need to be added temporarily.
Source compatibility

Adding collection conformance to String should not materially impact source
stability as it is purely additive: Swift 3’s String interface currently
fulfills all of the requirements for a bidirectional range replaceable

Altering String’s slicing operations to return a different type is source
breaking. The following mitigating steps are proposed:


   Add a deprecated subscript operator that will run in Swift 3
   compatibility mode and which will return a String not a Substring.

   Add deprecated versions of all current slicing methods to similarly
   return a String.


extension String {
  @available(swift, obsoleted: 4)
  subscript(bounds: Range<Index>) -> String {
    return String(characters[bounds])

  @available(swift, obsoleted: 4)
  subscript(bounds: ClosedRange<Index>) -> String {
    return String(characters[bounds])

In a review of 77 popular Swift projects found on GitHub, these changes
resolved any build issues in the 12 projects that assumed an explicit
String type
returned from slicing operations.

Due to the change in internal implementation, this means that these
operations will be *O(n)* rather than *O(1)*. This is not expected to be a
major concern, based on experiences from a similar change made to Java, but
projects will be able to work around performance issues without upgrading
to Swift 4 by explicitly typing slices as Substring, which will call the
Swift 4 variant, and which will be available but not invoked by default in
Swift 3 mode.

The C string interoperability methods outside the ones described in the
detailed design will remain in Swift 3 mode, be deprecated in Swift 4 mode,
and be removed in a subsequent release. UnicodeCodec will be similarly
Effect on ABI stability

As a fundamental currency type for Swift, it is essential that the String type
(and its associated subsequence) is in a good long-term state before being
locked down when Swift declares ABI stability. Shrinking the size of String to
be 64 bits is an important part of this.
Effect on API resilience

Decisions about the API resilience of the String type are still to be
determined, but are not adversely affected by this proposal.
Alternatives considered

For a more in-depth discussion of some of the trade-offs in string design,
see the manifesto and associated evolution thread

This proposal does not yet introduce an implicit conversion from Substring
 to String. The decision on whether to add this will be deferred pending
feedback on the initial implementation. The intention is to make a preview
toolchain available for feedback, including on whether this implicit
conversion is necessary, prior to the release of Swift 4.
Several of the types related to String, such as the encodings, would
ideally reside inside a namespace rather than live at the top level of the
standard library. The best namespace for this is probably Unicode, but this
is also the name of the protocol. At some point if we gain the ability to
nest enums and types inside protocols, they should be moved there. Putting
them inside String or some other enum namespace is probably not worthwhile
in the mean-time.
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