@swift-5.3.3
func modify<T>(_ t: inout T, _ f: (inout T) -> Void) { f(&t) } struct S { var a: Int = 3 var b: Int = 1 } var s = S() modify(&s) { (s) in modify(&s.a) { (a) in modify(&s.b) { (b) in a = a + b b = a - b } } } print(s)