Create sub module with atom name

shahryarjb

Hello friends,
I want to create a module inside another module with an atom name:

For example:

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

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

If I use Module name, it is okey and creates a sub module for me like:

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

But If I replace Oop with :opp it has invalid module name error.

I tried to convert atom to string and use Macro.camelize() and convert it to atom again ( :Oop), but it does not accept.

Then I print :opp, it just returns :opp in my macro but if I put module name Opp it returns something like this:

{:__aliases__,
 [
   counter: {MishkaDeveloperToolsTest.GuardedStructTest.TestNestedStruct, 3},
   line: 520
 ], [:Opp]}

now how can use atom to do this?

Thank you in advance

15 comments

#macro

0 1353 15

2023-08-13 07:42:19 UTC

Marked As Solved

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Eiji

Take a look at this code for inspiration:

defmodule Example do
  def sample(module) when is_atom(module) do
    module |> Atom.to_string() |> Macro.camelize() |> String.to_atom()
  end
end

module = Example

[Opp, :Opp, :opp]
|> Enum.map(&Example.sample/1)
|> Enum.map(&Module.concat(module, &1))
|> IO.inspect()

# returns: [Example.Opp, Example.Opp, Example.Opp]

Note: I have tried Module.split/1, but it does not support non-Elixir modules (those having Elixir. prefix when changing to String).

Edit: I have forgot about Macro.camelize/1, so I have updated my code. Thanks @zachallaun!

Post #6

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Eiji

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 …

Post #1

zachallaun

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.

Post #5

Eiji

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

Post #3

Last Post!

Eiji

It’s not a part of public API and therefore no answer guarantees you a working solution using it. Why cant you use %module{…} notation?

First of all why you do this? Much more common is to place it in test/fixtures/my_fixture.ex file …

There was some talk about this issue … I think it was about Elixir scripts i.e. exs files called like elixir my_script.exs. There was a question I think about ecto’s macros not working on same level where module is defined, but working inside some function … Somebody from Elixir’s core team (most probably José Valim) commented it and that it’s known limitation.

Post #15