Chat.elm

port autoScroll : Signal ()
port autoScroll = Signal.map (always ()) scrollMailbox.signal

scrollMailbox : Signal.Mailbox Action
scrollMailbox  = Signal.mailbox FromEffect

{- configuration for the start app -}
config : StartApp.Config Model Action
config =
  { init = (initialModel, initialEffect)
  , update = update
  , view = view
  , inputs = [ {- Some more signals here -}, scrollMailbox.signal] {- Added the last one -}
  }

update action model =
  case action of
    NewMessages items ->
      (
        { model | items = items }
      , Signal.send scrollMailbox.address NoOp |> kickOff {- kickoff converts the taks to an effect of the right type -}
      )

index.html

<script>
      chat.ports.autoScroll.subscribe(function (event) {
        var div = document.getElementsByClassName('chat-history')[0];
        div.scrollTop = div.scrollHeight
      });
<script>