"Throwing subscripts" in Swift 2

ThrowingSubscripts.swift

infix operator « { associativity left }
infix operator »« { associativity left }
postfix operator » {}

struct Index {
    let value: Int
}

enum Error : ErrorType {
    case OutOfBounds(index: Int, count: Int)
}

postfix func » (index: Int) -> Index {
    return Index(value: index)
}

func « <T>(array: [T], index: Int) throws -> T {
    if array.indices ~= index { return array[index] }
    throw Error.OutOfBounds(index: index, count: array.count)
}

func « <T>(array: [T], index: Index) throws -> T {
    if array.indices ~= index.value { return array[index.value] }
    throw Error.OutOfBounds(index: index.value, count: array.count)
}

func »« <T>(array: [T], index: Index) throws -> T {
    if array.indices ~= index.value { return array[index.value] }
    throw Error.OutOfBounds(index: index.value, count: array.count)
}

do {
    let xs = [[0],[10, 20, 30]]
    print(try xs«0»)    //=> "[0]"
    print(try xs«0»«0») //=> "0"
    print(try xs«1»«2») //=> "30"
    print(try xs«0»)    //=> "[0]"
    print(try xs«3»)    // throws OutOfBounds(3, 3)
    print(try xs«0»)
} catch let e {
    print(e) //=> "OutOfBounds(3, 3)"
}

Yes it is useful while we wait for language support for throwing subscripts!