Swift array subscript extension "Range<T> is not convertible to Int"

Question

I'm defining an extension on Array to override Slice creation:

struct S<T> {
    private var array: [T] = []
    private var first = 0
    private var len = 0

    init(_ array: [T], _ range: Range<Int>? = nil) {
        self.array = array
        if let range = range {
            self.first = range.startIndex
            self.len = range.endIndex
        } else {
            self.first = 0
            self.len = array.count
        }
    }
}

extension Array {
    subscript(subRange: Range<Int>) -> S<T> {
        return S<T>(self, subRange)
    }
}

let a = [4, 3, 2, 1, 0, -1][2..<4 as Range<Int>]

However, I'm getting an error on defining a: "Range is not convertible to Int" (without the cast the error is "HalfOpenInterval ..."). What am I doing wrong?

Answer 1

Because Array already have subslice functionality:

typealias SubSlice = Slice<T>
subscript (subRange: Range<Int>) -> Slice<T>

So, in order to your implementation work, you have to explicitly specify the return type:

let a = [4, 3, 2, 1, 0, -1][2..<4] as S<Int>