type alias Model =
    { settings : Settings
    ...
    }

type alias Settings =
    { username : String
    , password : String
    ...
    }

update : Msg -> Model -> ( Model, Cmd Msg )
update msg model =
    case msg of
        SetUsername username ->
            ( model |> setUserName username, Cmd.none )

        SetPassword password ->
            ( model |> setPassword password, Cmd.none )

setUserName : String -> Model -> Model
setUserName username model =
    let
        oldUserSettings =
            model.userSettings

        newUserSettings =
            { oldUserSettings | username = username }
    in
        { model | userSettings = newUserSettings }


setPassword : String -> Model -> Model
setPassword password model =
    let
        oldUserSettings =
            model.userSettings

        newUserSettings =
            { oldUserSettings | password = password }
    in
        { model | userSettings = newUserSettings }

setUserSettings : String -> String -> Model
setUserSettings field variable model =
    let
        oldUserSettings =
            model.userSettings

        newUserSettings =
            { oldUserSettings | field = variable }
    in
        { model | userSettings = newUserSettings }

setUserName : String -> Model -> Model
setUserName value model =
  setUserSettings username value

setPassword : String -> Model -> Model
setPassword value model =
  setUserSettings password value