"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)"
}

This is obviously very silly when defined for Arrays, but who knows maybe it's useful in some domain-specific language for someone some day.

A bit of extra effort was needed for nested subscripting because »« is considered as an operator of its own by swiftc.

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