Mutable struct via block in swift

mutable.swift

protocol Updateable {
    func updated(block: (inout Self) -> ()) -> Self
}

extension Updateable {
    func updated(block: (inout Self) -> ()) -> Self  {
        var item = self
        block(&item)
        return item
    }
}

extension Activity: Updateable { }

struct Activity {
    var a: String
    var b: String
}

let p = Activity(a: "a", b:"b")
let c = p.updated {
    $0.a = "12"
    $0.b = "13"
}
print("\(c)")

Is it - instead of the lens? Cool.