sum.swift

protocol Zero {
    static var zero: Self { get }
}

protocol Sumable {
    static func +(lhs: Self, rhs: Self) -> Self
}

extension Int: Sumable {}

extension Double: Sumable {}

extension Int: Zero {
    static let zero = 0
}

extension Double: Zero {
    static let zero = 0.0
}

extension Array where Element: Sumable & Zero {
    func sum() -> Element {
        return self.reduce(Element.zero, +)
    }
}

[1, 2, 3].sum() // 6

[1.1, 2.2, 3.3].sum() // 6.6