I try to reproduce the “Context” example in the phoenix guide. But instead of defining authenticate_user in the router.ex file, I did it in another module called SessionAuth under the web directory which imports only Plug.Conn with a few aliases MyApp.Router.Helpers, Phoenix.Controller and MyApp.Accounts.

in the router:

scope "/cms", MyAppWeb.CMS, as: :cms do
pipe_through [:browser, &MyApp.SessionAuth.authenticate_user/2]
resources "/pages", PageController
end

Then I get

= Compilation error in file lib/my_app_web/router.ex ==
** (ArgumentError) argument error
    :erlang.atom_to_list(&MyAppWeb.SessionAuth.authenticate_user/2)
    (plug) lib/plug/builder.ex:186: Plug.Builder.init_plug/1
    (plug) lib/plug/builder.ex:181: anonymous fn/4 in Plug.Builder.compile/3
    (elixir) lib/enum.ex:1811: Enum."-reduce/3-lists^foldl/2-0-"/3
    (plug) lib/plug/builder.ex:181: Plug.Builder.compile/3

when authenticate_user is defined and called from the router, there is no error.

when

import MyAppWeb.SessionAuth, only: [authenticate_user: 2]
pipe_through [:browser, :authenticate_user]

there is no error