Verifies cross-module function calls according to defined boundaries.

This compiler reports all cross-boundary function calls which are not permitted, according to the current definition of boundaries. For details on defining boundaries, see the docs for the Boundary module.

Usage

Once you have configured the boundaries, you need to include the compiler in mix.exs:

defmodule MySystem.MixProject do
  # ...

  def project do
    [
      compilers: [:boundary] ++ Mix.compilers(),
      # ...
    ]
  end

  # ...
end

When developing a library, it's advised to use this compiler only in :dev and :test environments:

defmodule Boundary.MixProject do
  # ...

  def project do
    [
      compilers: extra_compilers(Mix.env()) ++ Mix.compilers(),
      # ...
    ]
  end

  # ...

  defp extra_compilers(:prod), do: []
  defp extra_compilers(_env), do: [:boundary]
end

Warnings

Every invalid cross-boundary usage is reported as a compiler warning. Consider the following example:

defmodule MySystem.User do
  def auth() do
    MySystemWeb.Endpoint.url()
  end
end

Assuming that calls from MySystem to MySystemWeb are not allowed, you'll get the following warning:

$ mix compile

warning: forbidden reference to MySystemWeb
  (references from MySystem to MySystemWeb are not allowed)
  lib/my_system/user.ex:3

Since the compiler emits warnings, mix compile will still succeed, and you can normally start your system, even if some boundary rules are violated. The compiler doesn't force you to immediately fix these violations, which is a deliberate decision made to avoid disrupting the development flow.

At the same time, it's worth enforcing boundaries on the CI. This can easily be done by providing the --warnings-as-errors option to mix compile.