navigation_model.swift

enum NavStyle {
    case push
    case modal
    case back
}

struct Nav {
    let navs: [Nav]
    let style: NavStyle = .push

    init(navs: [Nav] = []) {
        self.navs = navs
    }

    func push(_ nav: Nav) -> Nav {
        return Nav(navs: navs + [nav])
    }

    func pop() -> Nav {
        var navs = self.navs
        navs.removeLast()
        return Nav(navs: navs)
    }
}

struct NavAction {
}

func navigate(from nav: Nav, by action: NavAction) -> Nav {
    return Nav()
}

func runNavigation(from current: Nav, to new: Nav) {
    // perform actual navigation
}

func sample() {
    let nav = Nav()
    let action = NavAction()
    let newNav = navigate(from: nav, by: action)

    runNavigation(from: Nav(), to: newNav)
}