How to alias a module in __using__

You‘ll need to pull out the module in the macro body and not in the quoted return. It needs to look like this:

alias unquote(module)

Anything where the alias macro doesn‘t receive the module name directly, but e.g. the ast of a variable won‘t work.

To be honest, I did not understand, I know it doesn’t work even with unquote

this is whole my code:

defmacro __using__(opts) do
    quote(bind_quoted: [opts: opts]) do
      use GenServer, restart: :transient
      require Logger
      alias MishkaInstaller.{PluginState, Hook}
      module_selected = Keyword.get(opts, :module)
      initial_entry = Keyword.get(opts, :initial)
      behaviour = Keyword.get(opts, :behaviour)
      event = Keyword.get(opts, :event)

      @ref event
      alias unquote(behaviour) # doesn't work
      @behaviour behaviour

      # Start registering with Genserver and set this in application file of MishkaInstaller
      def start_link(_args) do
        GenServer.start_link(unquote(module_selected), %{id: "#{unquote(module_selected)}"}, name: unquote(module_selected))
      end

      def init(state) do
        {:ok, state, 300}
      end

      # This part helps us to wait for database and completing PubSub either
      def handle_info(:timeout, state) do
        if is_nil(Process.whereis(MishkaHtml.PubSub)) do
          {:noreply, state, 100}
        else
          unquote(module_selected).initial(unquote(initial_entry))
          {:noreply, state}
        end
      end
    end
  end

If I understand a little about what you said, I can not get module name from a list in using and pass it in alias ! Yes!?, so what is your suggestion in this code!!

One thing people usually have a hard time to grok is that macros only deal with AST – or “code”. Even if things look like a variable they don’t really represent the value assigned to the variable.

defmacro __using__(opts) do
  quote(bind_quoted: [opts: opts]) do
    behaviour = Keyword.get(opts, :behaviour)
    alias behaviour
  end
end

Doing this means alias (also a macro) will receive the AST for the behaviour variable (something like this: {:behaviour, [], Elixir}). So it has no idea what module it’s supposed to alias.

defmacro __using__(opts_ast) do
  behaviour = select_module_from_opts(opts_ast)
  quote do
    alias unquote(behaviour)
  end
end

This on the other hand pulls out the module out of the AST your macro received and creates AST (again a representation of code), where alias receives the module as an argument instead of a variable.

The tricky part here is select_module_from_opts, because again opts is AST not necessarily a plain value to consume. Some values represent themselves in AST – no difference between the AST and the value represented – so they’re rather easy to deal with (e.g. many keyword lists with simple values) , but others are not as simple, e.g. maps or anything containing other variables.

Thank you for your complete explanation. The subject is beyond my comprehension, and I’ve refrained from using alias.