Check availability of optional dependencies in Elixir

Question

I made a hex package (Channels) that optionally depends on another one (AMQP). My implementation is based on Ecto's optional dependency on Mariaex.

In Channels' mix.exs:

defp deps do
  [{:amqp, "0.1.4", optional: true}]
end

In the Channels' file that depends on AMQP:

if Code.ensure_loaded?(AMQP) do

  defmodule Channels.Adapter.AMQP do
    ...
  end

end

The application is starting properly on dev and test are also passing (including those that depend on AMQP mix test --include amqp_server).

But when I try to use the package from another project some problems arise:

In my project I'm adding the following to my mix.exs file:

def application do
  [applications: [:amqp, :channels, ...],
   ...]
end

defp deps do
  [{:amqp, "0.1.4"},
   {:channels, "~> 0.1.1"},
   ...]
end

When I try to start the application or run the tests I'm getting the following error:

=INFO REPORT==== 27-May-2016::10:28:35 ===
application: logger
exited: stopped
type: temporary
** (Mix) Could not start application channels: Channels.start(:normal, []) returned an error: shutdown: failed to start child: Channels.Monitor.Supervisor
  ** (EXIT) shutdown: failed to start child: :main
    ** (EXIT) an exception was raised:
      ** (UndefinedFunctionError) undefined function Channels.Adapter .AMQP.connect/1 (module Channels.Adapter.AMQP is not available)
          Channels.Adapter.AMQP.connect([])
          (channels) lib/channels/monitor.ex:42: Channels.Monitor.init/1
          (stdlib) gen_server.erl:328: :gen_server.init_it/6
          (stdlib) proc_lib.erl:240: :proc_lib.init_p_do_apply/3

I'm adding AMQP as a dependency but it seems that Code.ensure_loaded?(AMQP) is returning false and therefore Channels.Adapter.AMQP is not being defined.

I've noticed that in the compilation process Channels is being compiled before AMQP:

...
==> channels
...
Generated channels app
==> rabbit_common (compile)
==> amqp_client (compile)
==> amqp
...
Generated amqp app
...

May this be the cause of my problem? If so, is there a way to decide the compilation order of my dependencies?

Answer 1

As Dogbert pointed out it was my mistake because I forgot to push the latest version of the package to hex. I've done it and worked properly.

Specifying the dependency as optional does change the compilation order.