import Html exposing (..)
import Html.Attributes exposing (..)
import Html.Events exposing (..)
import StartApp.Simple as StartApp
import String


main =
  StartApp.start { model = initialModel, view = view, update = update }


-- MODEL

initialModel =
  { topField = ""
  , bottomField = ""
  }


-- UPDATE

type Action = Top String | Bottom String


update action model =
  case action of
    Top str ->
      { model | topField <- str }

    Bottom str ->
      { model | bottomField <- str }


-- VIEW

view address model =
  div []
    [ input
        [ value model.topField
        , on "input" targetValue (Signal.message address << Top)
        ]
        []
    , input
        [ value model.bottomField
        , on "input" targetValue (Signal.message address << Bottom)
        ]
        []
    , input
        [ value (toSum model)
        , on "input" targetValue (Signal.message address << Bottom)
        , readonly True
        ]
        []
    ]


toSum {topField, bottomField} =
  case Result.map2 (+) (String.toInt topField) (String.toInt bottomField) of
    Err _ ->
      ""

    Ok n ->
      toString n