"SetPage" duplicated when dispatched through Cmd.ofMsg

When the router is connected to the Elmish state using Router.infer, routing commands issued through Cmd.ofMsg are duplicated.

Here is a minimal example (see github):

type Page =
    | [<EndPoint "/">] Home
    | [<EndPoint "/secondPage">] NewPage    
type Message =
    | SetPage of Page
    | DoAThing
let update message model =
   // This demonstrates which messages are duplicated  
    Console.WriteLine($"{message}")
    match message with
    | SetPage page ->
        {model with page = page}, Cmd.none
    | DoAThing ->
        model, Cmd.ofMsg (SetPage NewPage)
let router = Router.infer SetPage (fun model -> model.page)

When DoAThing is dispatched from Home, SetPage NewPage gets dispatched twice. This causes unexpected behavior whenever one wants to trigger a redirect after performing an action, i.e. create a new resource and then navigate to the corresponding view.

[Tarmil]

Yeah, we can’t really detect that a URL change was caused by the same message that it triggers. So it’s best to avoid dispatching it directly. Instead you can either have a separate message like you did, or directly set { model with page = page } in the update for DoAThing.

Maybe the documentation should recommend calling this message something like PageChanged, to emphasize that it’s not meant to be dispatched directly.

[Martin521]

I have not used routing myself, but as far as I understand it, the SetPage message is indeed not meant to be used by you, but only by the router. It is only a way to flexibly configure the router to manipulate the model in the right way.