There are no private modules, only private functions. It might be possible to do something of the sort with macro / metaprogramming trickery, but there is no standard feature for it. There is a library called Boundary that provides some guarantees to prevent using modules you shouldn’t.

To hard-restrict calling macros from allowed places only, one might use __CALLER__ special form inside macro.

@allowed_callers Foo.Bar, Foo.Baz
defmacro any_macro do
  raise unless __CALLER__.module in @allowed_callers
  ...
end

Love this solution. I would mark it as the solution in this thread but I can’t apparently because of the discussion type I chose.

One question, is there a way to check if __CALLER__.module is a submodule or child module of another module – i.e. ModA.ModB is child of ModA?

I made the change to the topic and set the solution as requested.

There is no such thing as “submodule” or “child module” in Elixir. Foo.Bar, despite it shares the first part of the name with Foo is absolutely not related to Foo by any means.

If you meant “to check whether a module shares a namespace,” that’d be easy to achieve with Module.split/1 and List.starts_with?/2, like

{m1, m2} = {Foo.Bar.Baz, Foo.Bar}

[m2, m1] |> Enum.map(&Module.split/1) |> Enum.reduce(&List.starts_with?/2)
#⇒ true
[m1, m2] |> Enum.map(&Module.split/1) |> Enum.reduce(&List.starts_with?/2)     
#⇒ false