JSONValueConvertible.swift

protocol JSONValueConvertible {
    var jsonValue: JSONValue { get }
}

protocol JSONValue: JSONValueConvertible { }

extension JSONValue {
    var jsonValue: JSONValue {
        return self
    }
}

extension String: JSONValue { }

extension Int: JSONValue { }

extension Double: JSONValue { }

extension Bool: JSONValue { }

extension Array: JSONValue { }

extension Array where Element: JSONValueConvertible {
    var jsonValue: JSONValue {
        return self.map({ $0.jsonValue })
    }
}

struct Convertible: JSONValueConvertible {
    let string: String

    var jsonValue: JSONValue {
        return string
    }
}

let a = [Convertible(string: "bing")]
let b = a.jsonValue

print(a)
print(b)