data Route
  = Page Page
  | NotFound
data Page
  = Home
  | ProductList
  | ProductView ProductId
  | About
  | ContactUs
type ProductId = Int

derive instance Generic Route _
derive instance Generic Page _
productId :: RouteDuplex' ProductId
productId = int segment
routes :: RouteDuplex' Route
routes =
  default NotFound $
    sum
      { "Page": pages
      , "NotFound": "404" / noArgs
      }
pages :: RouteDuplex' Page
pages =
  root $ end $
    sum
      { "Home": noArgs
      , "ProductList": "products" / noArgs
      , "ProductView": "products" / productId
      , "About": "about" / noArgs
      , "ContactUs": "about" / noArgs
      }
-- | This is the route parser we need to pass to the driver.
-- | It can produce any route which allows the parser to return a value of `NotFound` instead of failing.
parseRoute :: forall  String -> Either RouteError Route
parseRoute = parse routes
-- | This is the route printer we need to pass to the driver.
-- | It can only print paths to valid pages, which means a path can't be produced for the `NotFound` route.
-- | With this approach routes can be seperated based on whether they should be a navigation target and have a URL.
-- | Note: assymetry is not required, and a symmetrical printer works as well.
printRoute :: Page -> String
printRoute = print pages

onNavigation :: Maybe Route -> Route -> Router.RouterM Route Page Router.Routing Router.Resolved Unit
onNavigation previousRoute requestedRoute =
  case requestedRoute of
    NotFound ->
      case previousRoute of
        Just (Page page) -> Router.do
          liftEffect showBrokenNavigationMessage
          Router.redirect page -- redirect back to the previous page and show a message
        _ ->
          Router.continue -- no previous page, so just show the "not found" page
    _ -> Router.do
      access <- liftAff fetchUserAccess
      if userHasAccess requestedRoute access then
        Router.continue -- they have access, so resolve with the requested page
      else
        Router.override NotFound -- no access, so pretend the page doesn't exist
onEvent :: Router.RoutingEvent Route -> Effect Unit
onEvent newEvent =
  case newEvent of
    Router.Routing previousRoute requestedRoute ->
      showNavigationSpinner
    Router.Resolved previousRoute newRoute ->
      hideNavigationSpinner
      setCurrentRoute newRoute

mkRouter :: Effect (Router.Router Route Page)
mkRouter = do
  driver <- PushState.mkInterface parseRoute printRoute
  router <- Router.mkInterface onNavigation onEvent driver
  pure router