https://elixirforum.com/t/7660

errors.ex

defmodule Test.ErrorView do
  use Phoenix.View, root: ""

  def render(path, _) do
    [code, format] = path |> String.split(".")
    
    IO.puts "Rendering #{format}"
    
    format
  end
end

route.ex

defmodule Test.Routes.Sample do
  use Phoenix.Controller

  defexception plug_status: 500

  def index(conn, params) do
    IO.inspect conn.private # should contain phoenix_format: "json"

    raise __MODULE__, plug_status: 400
  end
end

router.ex

  defmodule Test.Router do
  use Phoenix.Router

  pipeline :api do
    plug :accepts, ["json"]
  end

  scope "/", Test.Routes do
    pipe_through :api

    get "/", Sample, :index
  end
end