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Substring performance affordances


This proposal modifies a small number of methods in the standard library that are commonly used with the Substring type:

  • Modify the init on floating point and integer types, to construct them from StringProtocol rather than String.
  • Change join to be an extension where Element: StringProtocol
  • Have Substring.filter to return a String


Swift 4 introduced Substring as the slice type for String. Previously, String had been its own slice type, but this leads to issues where string buffers can be unexpectedly retained. This approach was adopted instead of the alternative of having the slicing operation make a copy. A copying slicing operation would have negative performance consequences, and would also conflict with the requirement that Collection be sliceable in constant time. In cases where an API requires a String, the user must construct a new String from a Substring. This can be thought of as a "deferral" of the copy that was avoided at the time of the slice.

There are a few places in the standard library where it is notably inefficient to force a copy of a substring in order to use it with a string: joining substrings, and converting substrings to integers. In particular, these operations are likely to be used inside a loop over a number of substrings extracted from a string – for example, splitting a string into substrings, then rejoining them.

Additionally, per SE-163, operations on Substring that produce a fresh string (such as .uppercase) should return a String. This changes Substring.filter to do so.

Proposed solution

Add the following to the standard library:

extension FixedWidthInteger {
  public init?<S : StringProtocol>(_ text: S, radix: Int = 10)

extension Float/Double/Float80 {
  public init?<S : StringProtocol>(_ text: S, radix: Int = 10)

extension Sequence where Element: StringProtocol {
  public func joined(separator: String = "") -> String

extension Substring {
  public func filter(_ isIncluded: (Element) throws -> Bool) rethrows -> String

These additions are deliberately narrow in scope. They are not intended to solve a general problem of being able to interchange substrings for strings (or more generally slices for collections) generically in different APIs. See the alternatives considered section for more on this.

Source compatibility

No impact, these are generalizing an existing API to a protocol (in case of numeric conversion/joining) or modify a type newly introduced in Swift 4 (in case of filter).

Effect on ABI stability

The switch from conrete to generic types needs to be made before ABI stability.

Alternatives considered

The goal of this proposal is to generalize existing methods that are specific to string processing. Further affordances, such as implicit or explicit conversions of String to Substring, might solve this problem more generally but are considered out of scope for this proposal.

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