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.

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.

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