Elm run-time exceptions

Exception.elm

import Html exposing (beginnerProgram, div, span, button, text)
import Html.Events exposing (onClick)

type Msg 
  = Increment Int
  | Decrement Int
  

type alias Model = 
  { action : Int -> Msg 
  , currentValue : Int
  }


view model =
  div [] 
   [ span [] [ text "Current value: ", text (toString model.currentValue) ]
   , button [ onClick (Increment 7) ] [text "Click me!"]
   ]


flipMsg model =
  if model.action == Increment then Decrement else Increment


update msg model =
  case msg of 
    Increment n -> {model | currentValue = model.currentValue + n, action = flipMsg model}
    Decrement n -> {model | currentValue = model.currentValue - n, action = flipMsg model}


  
main = Html.beginnerProgram 
  { model = Model Increment 0
  , view = view
  , update = update
  }