Swift 4: 'subscript' is unavailable error

Question

I'm using the MapBox navigation framework that hasn't quite been updated to Swift 4. I have one 'subscript' error that I can't quite get around. Here's the code. I'd really appreciate any help. Thank you.

private func extractNextChunk(_ encodedString: inout String.UnicodeScalarView) throws -> String {
    var currentIndex = encodedString.startIndex

    while currentIndex != encodedString.endIndex {
        let currentCharacterValue = Int32(encodedString[currentIndex].value)
        if isSeparator(currentCharacterValue) {
            let extractedScalars = encodedString[encodedString.startIndex...currentIndex]
            encodedString = encodedString[encodedString.index(after: currentIndex)..<encodedString.endIndex]

            return String(extractedScalars)
        }

        currentIndex = encodedString.index(after: currentIndex)
    }

    throw PolylineError.chunkExtractingError
}

Answer 1

The error message is misleading. The real problem is that subscripting a String.UnicodeScalarView with a range returns a String.UnicodeScalarView.SubSequence, so you cannot assign that back to encodedString.

One solution would be to create a String.UnicodeScalarView from the subsequence:

encodedString = String.UnicodeScalarView(encodedString[encodedString.index(after: currentIndex)...])

Alternatively (and perhaps simpler) go the other way around and remove the initial part of encodedString instead:

encodedString.removeSubrange(...currentIndex)

In either case, you can take use "one-sided ranges", compare SE-0172