List of all modules implementing a behaviour in elixir

What could be the shortest possible way to get a list of all the modules implementing a behaviour?

e.g.

defmodule X do
  @callback foo()
end

defmodule A do
  @behaviour X
  def foo() do
    "hello"
  end
end

defmodule B do
  @behaviour X
  def foo() do
    "world"
  end
end

I would like to get a list as [A,B]. I tried loading all the modules and then filtering using the key attribute behaviour but I think it's going to be very slow. Is there any other way?

No, there is no better way. Behaviours are not indexed in any way, as they are a compile-time feature used to check that all required callback functions have been defined. So something like this is as good as it gets:

for {module, _} <- :code.all_loaded(),
    X in (module.module_info(:attributes)[:behaviour] || []) do
        module
end