The case matches that pertain to retrieving the categories

main.elm

- update drives the changes in the UI and interaction with the server.
update : Msg -> Model -> ( Model, Cmd Msg )
update msg model =
    
    case msg of 
        DataReceived (Ok categories) -> 
            ({ model | categories = categories, errorMsg = Nothing }, Cmd.none)

        DataReceived (Err httpError) ->
            ({ model | errorMsg = Just (createErrorMessage httpError) }, Cmd.none)
       
       ... pretend the rest of the case options are here ...