How to copy a struct and modify one of its properties at the same time?

Note, that while you use placeholder values for constants a and b you are not able to construct instance of A with any other values but this placeholders. Write initializer instead. You may write custom method that change any value in struct also:

struct A {
    let a: Int
    let b: Int

    init(a: Int = 2, b: Int = 3) {
        self.a = a
        self.b = b
    }

    func changeValues(a: Int? = nil, b: Int? = nil) -> A {
        return A(a: a ?? self.a, b: b ?? self.b)
    }
}

let state = A()
let state2 = state.changeValues(b: 4)