Final version (without types)

Counter.elm

module Counter exposing (..)

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


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


type Action
    = Increment
    | Decrement
    
    
update action state =
    case action of
        Increment ->
            state + 1

        Decrement ->
            state - 1


view counterValue =
    div []
        [ button [ onClick Decrement ] [ text "-" ]
        , span [] [ text (toString counterValue) ]
        , button [ onClick Increment ] [ text "+" ]
        ]