Convertible.swift

protocol Convertible {
    func to<T>() -> T;
//    var <T>to: T { get }     // multiple instances of generic var is not allowed
}

extension String {
    func to() -> String {
        return self
    }
    func to() -> Int {
        return (self as NSString).integerValue
    }
    func to() -> Double {
        return (self as NSString).doubleValue
    }
}

let s = "5.1"
let d: Double = s.to()          // 5.1
let i: Int = s.to()             // 5