generics.swift

protocol Summable { func +(lhs: Self, rhs: Self) -> Self }
extension Int: Summable {}
extension Double: Summable {}
extension String: Summable {}

func sum<T: Summable>(lhs:T, _ rhs:T) -> T {
    return lhs + rhs
}

sum("Hello ", "World") // "Hello World"
sum(1, 4) // 5