Monad protocol in swift

monad.swift

protocol Monad {
    typealias F
    typealias U
    class func bind<M : Monad where M.U == U>(Self, F -> M) -> M
    class func `return`(F) -> Self
}


extension Array : Monad {
    typealias F = T
    typealias U = Array<()>
    static func bind<M : Monad where M.U == U>(m : Array, _ f : F -> M) -> M {
        var ret = Array<M.F>()
        for a in m {
            ret += f(a) as M.F[]
        }
        return ret as M
    }
    static func `return`(f : F) -> Array {
        return [f]
    }
}

operator infix >>= { associativity left }
func >>=<MA : Monad, MB : Monad where MA.U == MB.U>(lhs : MA, rhs : MA.F -> MB) -> MB {
    return MA.bind(lhs, rhs)
}

println([3] >>= {[$0, $0 + 1]} >>= {[Double($0), Double(10 + $0)]})