Elm reducer / union type example

model.elm

type alias Model =
  { error: Maybe String
  , message: Maybe String
  , isSubmitting: Bool
  }

type alias Error = 
  { message: String }

type Msg
  = ClearAlerts
  | SubmissionInitiation
  | SubmissionSuccess
  | SubmissionError Error
  | SubmissionMessage String


initModel : (Model, Cmd Msg)
initModel =
  { error = Nothing
  , message = Nothing
  , isSubmitting = False
  }
    ![]

-- Update (Note, I remove the Cmd ![] return value from each of these for brevity sake.
update : Msg -> Model -> (Model, Cmd Msg)
update msg model =
  case msg of
    ClearAlerts ->
      { model | error = Nothing, message = Nothing }
    SubmissionInitiation ->
      { model | isSubmitting = True }
    SubmissionInitiation ->
      { model | isSubmitting = True }
    SubmissionError err ->
      { model | error = err }
    SubmissionMessage message ->
      { model | message = message }