module Main exposing (..)

import Html exposing (Html, button, text)
import Html.Events exposing (onClick, onBlur)


main : Program Never Model Msg
main =
    Html.program
        { init = init
        , update = update
        , subscriptions = always Sub.none
        , view = view
        }


type alias Model =
    { page : Page }


type Page
    = A
    | B


type Msg
    = ShowPageB
    | AMsg AMsg
    | BMsg BMsg


type AMsg
    = AClick
    | ABlur


type BMsg
    = NoOp


init : ( Model, Cmd Msg )
init =
    { page = A } ! []


update : Msg -> Model -> ( Model, Cmd Msg )
update msg model =
    case Debug.log "msg" msg of
        AMsg msg ->
            case msg of
                AClick ->
                    update ShowPageB model

                ABlur ->
                    Debug.crash "Never gets here because ABlur message is mapped as `BMsg ABlur` instead of `AMsg ABlur`"

        ShowPageB ->
            ( { model | page = B }
            , Cmd.none
            )

        BMsg msg ->
            case msg of
                NoOp ->
                    Debug.log "Gets here because no match for `BMsg ABlur` so last match runs by default." <|
                        ( model, Cmd.none )


view : Model -> Html Msg
view model =
    case model.page of
        A ->
            button [ onClick AClick, onBlur ABlur ] [ text "Show Page B" ]
                |> Html.map AMsg

        B ->
            -- NOTE this is where Page A's on blur message is getting mis-tagged!!!
            text "Page B"
                |> Html.map BMsg