App's state file

State.elm

init : ( Model, Cmd Msg )
init =
    let
        ( subModuleInitModel, subModuleInitCommand ) =
            SubModule.State.init
        -- repeat for any submodule you have 
        model = Model subModuleInitModel ... -- init your model here
        cmd = Cmd.batch [ subModuleInitCommand, ... ] -- init your commands here
    in
        ( model, cmd )

update : Msg -> Model -> ( Model, Cmd Msg )
update msg model =
    case msg of
        BoardMsg boardMsg ->
            Board.State.update boardMsg model.board
                |> (\( newBoard, _ ) -> ( { model | board = newBoard }, Cmd.none ))

        SetupMsg setupMsg ->
            Setup.State.update setupMsg model.setup
                |> (\( newSetup, cmd ) -> ( { model | setup = newSetup }, Cmd.none ))
        
        SomeOtherModuleMsg theActualMsg -> 
            SomeOtherModule.State.update theActualMsg model.someOtherModule 
            -- here we just feed the submodule's update function our app's "instance" of the model
                |> (\( newSubModuleModel, someAssociatedCommand) -> 
                    ( { model | someOtherModule = newSubModuleModel}, someAssociatedCommand)
        
        Mdl mdlMsg ->
            Material.update Mdl mdlMsg model