Can a library find all modules implementing a Behaviour?

I have a library that wants to find all modules for a given behaviour. I found some description that showed how to do this off of :code.all_loaded() which was neat. Then I moved the library out of the application it was living in because it really isn’t supposed to be an application on its own. And things started breaking intermittently during testing.

So that’s when I found out about Elixir doing lazy loading of modules through this thread. But the build_embedded stuff only really seems to apply to an application.

Should I just use another approach for this? I want to find all modules that are widgets. I was doing this at runtime, building the list when it was first requested and persisting it to the Application environment. I don’t really care if the initial list is made at compile time or runtime. Ideally it should also be able to register new Widgets if they are loaded at runtime or come from outside the library.

Is there a common pattern recommended here that I’m just not familiar with? I realize this is state but the library is mostly pure functions so avoiding needing to start a supervision tree for the lib would be more elegant.

I’ve tried to do similar things in the past and tbh the best/most flexible thing you can do it let your users provide a list somewhere. All other options have downsides like needing global state or your case of a certain embedded mode, …

I messed around with metaprogramming using module compilation hooks to achieve a way to track what modules use other modules a few years ago. It could be applied here to track behavior usage.

You have to own the module being used, and clients have to use use instead of @behaviour. But it will let the library track all modules at compile time, across other libraries and umbrella apps.

Not that this probably isn’t a great idea, one of the whole points of having behaviours is to define an interface that others can call dynamically with arbitrary modules.

Also note I haven’t tried this out for years, so may not work verbatim on recent elixirs.

Thanks, I’ve gone for an Agent storing the list for now. So that needs to be loaded at startup. It’ll do.