TwoCounters.elm

module TwoCounters exposing (init, update, view)

import Html exposing (..)
import Html.App exposing (..)
import Counter exposing (Model, init, update, view)

type Slot = Top | Bottom

type alias Model = Slot -> Counter.Model

initialModel : Model
initialModel = \_ -> 0

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

-- UPDATE

type alias Msg = (Slot, Counter.Msg)

update : Msg -> Model -> (Model, Cmd Msg)
update msg model =
  case msg of
    (slot, counterMsg) ->
      let
        counterModel = model(slot)
      in
        transformUpdate model slot (Counter.update counterMsg counterModel)

transformUpdate : Model -> Slot -> (Counter.Model, Cmd Counter.Msg) -> (Model, Cmd Msg)
transformUpdate model slot counterUpdate =
  (
    \mslot ->
      if mslot == slot then
        fst counterUpdate
      else
        model mslot
  , Cmd.map (\cmdMsg -> (slot, cmdMsg)) (snd counterUpdate)
  )


-- VIEW

view : Model -> Html Msg
view model =
  div []
    [
      transformView model Top
    , transformView model Bottom
    ]

transformView : Model -> Slot -> (Html Msg)
transformView model slot =
  model(slot)
    |> Counter.view
    |> Html.App.map (\counterMsg -> (slot, counterMsg))

I bet this leaks memory b/c old versions of the model are being kept around :)