Modeling impossible state is an anti-pattern in Elm, so instead we need to come up with new approaches to model our application. If we want to model a list of items such that one is always selected, we can use the zipper data structure. It enforces that one item is always selected, and is the perfect complement to many UI patterns. In this episode, we use Richard Feldman’s selectlist to clean up problematic code.

Examples Main.elm

``` update : Msg -> Model -> ( Model, Cmd Msg ) update msg model = case msg of SelectTab id -> ( { model | tabs = SelectList.select (\u -> u.id == id) model.tabs }, Cmd.none )

```

View.elm

``` view : Model -> Html Msg view model = let befores = model.tabs |> SelectList.before |> List.map (tabView False)

```

Links * rtfeldman/selectlist * Zipper Data Structure