Weird… when I try this code:

defmodule MyLib do
  defmacro sub_field(name, do: block) do
    quote do
      defmodule unquote(name) do
        unquote(block)
      end
    end
  end
end

defmodule Example do
  import MyLib

  sub_field :opp do
    def sample, do: IO.inspect(__MODULE__)
  end
end

:opp.sample()

then everything is working without any problem. module is print properly and there are no warnings or errors …

On my phone, so apologies if some of this is incorrect as I can’t test it, but I’d try the following:

defmacro sub_field(name, type, opts \\ [], do: block) do
    ast = register_struct(block, opts)

    name =
      name
      |> Atom.to_string()
      |> Macro.camelize()
      |> String.to_atom()

    quote unquote: false, bind_quoted: [name: name, ast: ast] do
      module_name = Module.concat(__MODULE__, name)

      defmodule unquote(module_name) do
        unquote(ast)
      end
    end
  end

I’d really need to test to be sure, and it can be a little tricky to follow with the nested unquote/bind_quotes bit, but essentially you need to access __MODULE__ in the calling module.

Ah, I see … You have tried my example to create a top-level module in another module.

First of all you need to understand something … As same as you can check AST with quote do … end as same you can check how modules are “defined” inside using IO.puts(module).

iex> IO.puts(String)
Elixir.String

iex> IO.puts(Example)
Elixir.Example

iex> IO.puts(:opp)
opp

iex> IO.puts(:Opp)
Opp

With this you should understand why sub_field :Opp do … end does not generates Opp module. However it’s easy to do so. Simply add Elixir. prefix instead of :, so:

defmodule MyLib do
  defmacro sub_field(name, do: block) do
    quote do
      defmodule unquote(name) do
        unquote(block)
      end
    end
  end
end

defmodule Example do
  import MyLib

  sub_field Elixir.Opp do
    def sample, do: IO.inspect(__MODULE__)
  end
end

Opp.sample()

You can also do that within macro using for example Module.concat/2

iex> Module.concat(Elixir, :Opp)
Opp