I’m reading the amazing book Functional Web Development with Elixir, OTP, and Phoenix.
I encounter an issue when reading the section Adding a New Dependency in chapter 6 Generate a New Web Interface with Phoenix.

The section adds a project islands_engine to a new phoenix project islands_interface.
Here are the steps:

1. mix phx.new islands_interface --no-ecto
2. add a dependency in mix.exs:

defp deps do
  [
    ...
    {:islands_engine, path: "../islands_engine"}
  ]
end

By running iex -S mix phx.server, it will display {:islands_engine, 'islands_engine', '0.1.0'} after executing :application.which_applications() in the iex.

However, I don’t see the islands_engine in my output.
I’m using Elixir v1.7.2 and Erlang v21, while the book uses Elixir v1.4. Not sure whether it affects my output.

I tried two ways:

1. add included_applications in mix.exs:

def application do
  [
    ...
    included_applications: [:island_engine]
  ]
end

It doesn’t launch islands_engine.I read Erlang’s doc and find that:

The application controller automatically loads any included applications when loading a primary application, but does not start them. Instead, the top supervisor of the included application must be started by a supervisor in the including application.

2. start the islands_engine supervisor tree in islands_interface’s application.ex:

def start(_type, _args) do
  ...
  children = [
    supervisor(IslandsInterfaceWeb.Endpoint, []),
    %{id: IslandsEngine.Application, start: {IslandsEngine.Application, :start, [nil, nil]}}
  ]
  ...
end

The approach can launch islands_engine, but it’s not what I want. The islands_engine supervisor tree would be a child of islands_interface supervisor tree. :application.which_applications still doesn’t output island_engine.

Do I miss anything? What’s the correct way to add a dependency in a phoenix project?