Is there a way to get information of multiple specific modules?

Questions / Help
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1

I’m developing some features for https://github.com/tony612/protobuf-elixir/, but I need a way to know what modules are protobuf modules and even want to store more information for them

A protobuf module is defined like this

defmodule Foo do
  use Protobuf, syntax: :proto3
end

defmodule Bar do
  use Protobuf, syntax: :proto3
end

I want to get the information in runtime/compile time like

> Protobuf.all_messages
[
  {Foo, some_foo_metadata},
  {Bar, some_bar_metadata}
]

Is there any way to achieve this?

A direct way I can think of is to define a function for each protobuf module, and call them when the program starts. I can use ets or persistent_term to build “all_messages”. But I can’t find a callback like this.

I don’t know if this can be implemented in compiling time, but it seems hard because we can’t make sure every protobuf module is compiled every time.

2

If you want to do it dynamically, you could try :code.all_loaded(). It returns a list of tuples, the first element is an atom of each module name, the second is a string of their location. You could stringify and check the first element for a constant module string. So instead of Foo you could do FooProtobuf and check for the Protobuf string. A bit hackey, but it’d work.

As for the metadata, I am unsure.

3

@tony612 Last time I was thinking about really similar case. The best solution I found is to define a separate module:

defmodule Foo do
  use Protobuf, syntax: :proto3
end

defmodule Bar do
  use Protobuf, syntax: :proto3
end

defmodule Example do
  use Protobuf.Agent

  add_proto(Foo)
  add_proto(Bar)
end

Each add_proto/1 call would append value to module attribute which should be registered as accumulate. Based on such attribute use Protobuf.Agent should generate some code which would be executed on before_compile callback. Finally Example should be added to app supervisor tree.

This allows you to:

  1. Define multiple modules like Example which may be helpful in case developer do not want to have all modules from all libraries.
  2. Possibility to further generate __using__/1 macro in order to allow “extend” list of modules.
defmodule Example do
  use Protobuf.Agent
  use MyLib.ProtobufAgent
  use MyOtherLib.ProtobufAgent

  add_proto(Foo)
  add_proto(Bar)
end

Of course there are ways to work on .beam files, persistent attributes and so on, but it adds way too many magic to your code and I would personally avoid such practices.

Just for sure instead of Protobuf.Agent you may have Protobuf.ETS and so on …

4

The difficult thing is it’s hard to let users write add_proto for every module, especially the protobuf modules code are generated.

5

This is interesting. A better way could be checking if the protobuf modules have some functions like def __metadata__(:is_protobuf). And once we have all the protobuf modules, it will be easy to get all metadata, we just need to call their function, like def __meta__() do.