undefined_bug.elm

module Main exposing (..)

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


main : Program Never
main =
    beginnerProgram { model = init, view = view, update = update }


type alias Model =
    Maybe (List Int)


init : Model
init =
    Nothing


view : Model -> Html Msg
view model =
    div []
        [ button [ onClick Add ] [ text "add" ]
        , div [] [ text (toString model) ]
        ]


type Msg
    = Add


update : Msg -> Model -> Model
update msg model =
    case msg of
        Add ->
            case model of
                Just l ->
                    let
                        i =
                            Maybe.withDefault 42 (Maybe.map (\i -> i + 1) (List.head l))

                        l =
                            i :: l
                    in
                        Just l

                Nothing ->
                    Just [ 1 ]