endpoint.ex

defmodule MyAppWeb.Endpoint do
  # All the stuff we just added
  
  @session_options ...
  socket ...
  
  def www_redirect(conn, _options) do
    if String.starts_with?(conn.host, "www.#{host()}") do
      conn
      |> Phoenix.Controller.redirect(external: "https://#{host()}")
      |> halt()
    else
      conn
    end
  end

  # Redirect all www requests to the root url
  plug :www_redirect
  
  plug ...
  plug ...
  
end