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

Thank you for the time you explain this, but I need to convert atom inside sub_field macro, and user just put an atom

Code:

  defmodule TestNestedStruct do
    use GuardedStruct

    guardedstruct do
      field(:title, String.t())
      field(:subject, String.t())

      sub_field(:oop, String.t(), enforce: true) do
        field(:title, String.t())
        field(:fam, String.t())
      end

      field(:site, String.t())
    end
  end

And

Error

error: MishkaDeveloperToolsTest.GuardedStructTest.TestNestedStruct.Oop.__struct__/0 is undefined, cannot expand struct MishkaDeveloperToolsTest.GuardedStructTest.TestNestedStruct.Oop. Make sure the struct name is correct. If the struct name exists and is correct but it still cannot be found, you likely have cyclic module usage in your code
  test/guarded_struct_test.exs:534: MishkaDeveloperToolsTest.GuardedStructTest."test nested macro field"/1

I think it just accept a Tuple for creating submodule

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

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

    module_name = Module.concat(Elixir, name)

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

If I put sub_field(Oop, String.t(), enforce: true) do, it works

I am improving my macro

Credit where it’s due: I just copied it from @shahryarjb’s post above mine!