StateCallAsFunction.swift

enum _Value<Initial, Changed> {
    case initial(Initial)
    case changed(Changed)
}

typealias StateRepresentable<T> = _Value<T, T>
typealias ChangeHandleable<T> = _Value<Void, T>

extension _Value where Initial == Void {
    static func initial() -> _Value<Void, Changed> {
        .initial(())
    }

    func callAsFunction() -> Changed? {
        switch self {
        case .initial:
            return nil
        case let .changed(value):
            return value
        }
    }
}

extension _Value where Initial == Changed {
    func callAsFunction() -> Changed {
        switch self {
        case let .initial(value),
             let .changed(value):
            return value
        }
    }
}

var value1 = ChangeHandleable<Int>.initial()
value1 = .changed(1)
print(String(describing: value1()))

var value2: StateRepresentable<Int>? = .initial(0)
value2 = nil
value2 = .changed(1)
print(String(describing: value2?()))