I'm beginning to use web sockets with an Elm frontend and a Elixir/Phoenix backend. The Phoenix server needs to tell the Elm app what uri to connect to. (E.g., `ws://localhost:4000/socket/websocket` in the development environment, but something else in the production environment.) 

I was hoping there was something the equivalent of `static_url`, but I can't find it. Is there? 

(I know I could hardcode the possibilities, but it seems like Phoenix must already know them.)