Going by the documentation it seems to fit the xUnit classification of Configurable Test Double.

Testmox.hello calls MyApp.Calculator.Impl.add not MyApp.CalcMock.add. In elixir there nothing, which would allow a library to change code, so Mox cannot dynamically alter what get’s called, so you need to find a way to make the implementation changeable on your own. The options for that are using the app env to configure the implementation, passing the implementation as argument to the function or some compile time magic if you don’t need to change the implementation at runtime.

The other problem is that you’re running the tests before running defmock in your test_helpers.exs.

So, this should work, right?

defmodule CalculatorTest do
  use ExUnit.Case, async: true

  alias Testmox
  alias MyApp.Calculator.Impl, as: Calc

  import Mox

  # Make sure mocks are verified when the test exits
  setup :set_mox_global
  setup :verify_on_exit!

  test "invokes add and mult" do
    Mox.defmock(Calc, for: MyApp.Calculator)

    expect(Calc, :add, &Calc.add/2)

    Testmox.hello()
  end
end

It doesn’t…

I still don’t see any code to make Testmox.hello call the module defined by Mox instead of calling the hardcoded implementation of MyApp.Calculator.Impl.

I think you don’t really get how Mox works yet. It doesn’t matter what you use as callback in expect() for the issues you’re having. Testmox.hello is hardcoded to call MyApp.Calculator.Impl.add, which means it’s never going to not call your actual implementation, which you probably don’t want to happen as you’re trying to mock it.

What Mox.defmock(Calc, for: MyApp.Calculator) is doing is creating a new module with the name you supply as the first argument. Mox can then assert if the functions on this new module are actually called and let it return the values by the callbacks you supply via expect. It does not somehow magically instrument existing implementations to do different things than before.

You last implemenation is does basically override your existing module with a new module, which is not what you want to do.