I switched the interface to the one I mentioned in the second half of this post. Basically, now we can do:
defmodule MySystemWeb do
use Boundary, deps: [MySystem, Ecto.Changeset]
Which will restrain usage of ecto to only Ecto.Changeset boundary. If the external app defines its own boundaries, the specs of those boundaries will define what can be invoked. Otherwise, if the app doesn’t define any boundary (like in this case), an implicit boundary is created which will contain all modules from the given namespace (like e.g. Ecto.Changeset.Foo.Bar). An implicit boundary always exports all of its modules.
More details are available in this part of the moduledoc.
As I suggested in the aforementioned post, I also plan to add some support for strict mode, which will require us to explicitly allow the usage of any external boundary. I’m not completely clear on the details, but the rough idea is to allow a per-boundary setting (e.g. via externals: :strict), as well as global default (e.g. by providing externals: :strict in mix project config).
