Can we require StringProtocol.UTF8View be a BidirectionalCollection?

Karl · February 21, 2021, 2:25pm

StringProtocol.UTF8View only requires Collection conformance, even though StringProtocol itself, as well as the UTF16View and UnicodeScalarView, are BidirectionalCollections. The actual UTF8Views for String and Substring are of course bidirectional. It's actually commented out from the protocol definition, which makes me think it may have been an oversight not to un-comment it?

github.com

apple/swift/blob/main/stdlib/public/core/StringProtocol.swift#L27


      
          public protocol StringProtocol
            : BidirectionalCollection,
            TextOutputStream, TextOutputStreamable,
            LosslessStringConvertible, ExpressibleByStringInterpolation,
            Hashable, Comparable
            where Iterator.Element == Character,
                  Index == String.Index,
                  SubSequence: StringProtocol,
                  StringInterpolation == DefaultStringInterpolation
          {
            associatedtype UTF8View: /*Bidirectional*/Collection
            where UTF8View.Element == UInt8, // Unicode.UTF8.CodeUnit
                  UTF8View.Index == Index
          
          
  associatedtype UTF16View: BidirectionalCollection
            where UTF16View.Element == UInt16, // Unicode.UTF16.CodeUnit
                  UTF16View.Index == Index
          
          
  associatedtype UnicodeScalarView: BidirectionalCollection
            where UnicodeScalarView.Element == Unicode.Scalar,
                  UnicodeScalarView.Index == Index

This is just a really weird asymmetry which forces me to write a bunch of additional constraints. Given that StringProtocol is kind of "sealed", would it be possible to fix this? Or would it break ABI?