Extensions version of http://maxs.io/typeclass-encoding-in-swift/ instead of explicit type class instance

gistfile1.swift

protocol Num {
    typealias N
    class func zero() -> N
    func succ() -> N
    func add(other: N) -> N
    func multiply(other: N) -> N
}

extension Int8: Num {
    typealias N = Int8
    static func zero() -> N { return 0 }
    func succ() -> N { return self + 1 }
    func add(other: N) -> N { return self + other }
    func multiply(other: N) -> N { return self * other }
}

extension Int32: Num {
    typealias N = Int32
    static func zero() -> N { return 0 }
    func succ() -> N { return self + 1 }
    func add(other: N) -> N { return self + other }
    func multiply(other: N) -> N { return self * other }
}

func sumVec<A: Num where A.N == A>(xs: Array<A>) -> A {
    return xs.reduce(A.zero()) { $0.add($1) }
}

let xs8: Array<Int8> = [0, 1, 4, 8]
let xs32: Array<Int32> = [0, 1, 4, 8]

let q = sumVec(xs8)  // 13
let r = sumVec(xs32) // 13