Main.elm

import Html.App as App
import Html exposing (Html, text)

import Dict exposing (Dict)

type alias Model = ()
type alias Msg = ()

existsInBoth : comparable -> (List a) -> (List a) -> Dict comparable (List a) -> Dict comparable (List a)
existsInBoth key leftValue rightValue dict =
  Dict.insert key (leftValue ++ rightValue) dict

magicMerge : Dict comparable (List a) -> Dict comparable (List a) -> Dict comparable (List a)
magicMerge left right =
  Dict.merge Dict.insert existsInBoth Dict.insert left right Dict.empty

init : (Model, Cmd Msg)
init = ((), Cmd.none)

update : Msg -> Model -> (Model, Cmd Msg)
update _ _ = ((), Cmd.none)

subscriptions : Model -> Sub Msg
subscriptions _ = Sub.none

view : Model -> Html Msg
view model = text <| toString <| magicMerge (Dict.empty |> Dict.insert "a" [1]) (Dict.empty |> Dict.insert "a" [2] |> Dict.insert "b" [3])

main : Program Never
main = App.program {
    init = init,
    update = update,
    subscriptions = subscriptions,
    view = view
  }