You can use xref to help better understand how your modules are connected. There can be a bit of a learning curve but some useful commands are:

mix xref graph --format cycles --label compile-connected

and

mix xref graph --label cycles --sink lib/path/to/file/that/causes/many/recompilations.ex

Cycles aren’t 100% avoidable. For example in has_many and belongs_to relationships, A is going to refer to B and B is going to refer to A. But you should avoid calling functions in this scenario, ie, avoid this:

defmodule A do
  def a, do: B.b()
end

defmodule B do
  def b, do: A.a()
end

This is a bit of a deep topic and there are a lot of threads about this on this forum (search for “transitive dependencies”) as well as some blog posts. Here’s a good one. But it’s very hard to give specific actionable advice without seeing some code (though it would require seeing a lot of code which is of course probably not possible and not something I’d really wanna do, lol).

Some brief tips, though:

• Avoid defining macros in files that change often.
• Avoid having modules that you sometimes use in some modules, and sometimes just call random functions (or import) in other modules. Split those up.
• Avoid calling functions in a module’s body. For example, if you have @foo Foo.bar(), change that to def foo, do: Foo.bar(). This can be ok, but these are brief tips so it’s easier to say Just Don’t Do It.
• Sometimes you want to avoid even referring to a module in another module’s body (though I believe this isn’t as much of a problem anymore)
• Along the lines of avoiding cycles: don’t (ever) call any Web functions from your domain modules/resources modules.

That’s all I got. If you have more questions I’m happy to try and answer. I’m not a huge expert here though have helped squash compiletime deps in a few projects and enjoy the topic.

Thank you everyone for your contributions! My mistake was writing inline action hooks within my Ash resources. Moving them into their own modules fixed the issue. I was even experiencing slow compilation times when changing code in my Phoenix controllers, because they referenced Ash domains, which in turn pointed to resources that referenced other modules in the inline hook implementations—creating the so-called transitive dependencies. Breaking those action hooks into separate modules completely solved the problem. Also worth mentioning @zachdaniel for his support in the Ash Discord community. Thanks again!

I can’t help you with the Ash side of things, but you can use this tool to get a more concrete view of your dependency graph if that a concern: