Phoenix content negotiation

content_negotiator.ex

defmodule ContentNegotiator do

  def init(opts), do: opts

  def call(conn, opts) do
    conn
    |> Phoenix.Controller.accepts(Map.keys(opts))
    |> dispatch_by_accept(opts)
  end

  defp dispatch_by_accept(%Plug.Conn{halted: true} = conn, _), do: conn
  defp dispatch_by_accept(conn, opts) do
    plug = Map.fetch!(opts, conn.params["format"])
    plug.call(conn, plug.init([]))
  end

end

router.ex

defmodule Sampler.Router do

  defmodule HTMLPipeline do
    use Plug.Builder
    import Phoenix.Controller

    plug :protect_from_forgery
  end

  defmodule JSONPipeline do
    import Phoenix.Controller

    # can't use `Plug.Builder` here as it expects at least one `plug`
    def init(opts), do: opts
    def call(conn, _), do: conn
  end

  use Sampler.Web, :router

  pipeline :browser do
    plug :fetch_session
    plug :fetch_flash
    plug ContentNegotiator, %{"html" => HTMLPipeline,
                              "json" => JSONPipeline}
  end

  scope "/", Sampler do
    pipe_through :browser

    get "/", PageController, :index
  end
end