How to debug ExUnit tests with :debugger?

Can you call :debugger.start from within the failing test / or setup block? Or before the module?

I don’t see how I would set breakpoints then, before the test has finished.

I meant something like

:debugger.start()
:int.ni(DebugTest)
:int.break(DebugTest, 16) # needs to be an executable line http://erlang.org/doc/apps/debugger/debugger_chapter.html#breakpoints
# or use http://erlang.org/doc/man/int.html#break_in-3
# :int.break_in(DebugTest, :hello, 0)

defmodule DebugTestTest do
  use ExUnit.Case
  doctest DebugTest

  test "greets the world" do
    result = DebugTest.hello()
    assert result == :world
  end
end

and then mix test --trace.

If you only want to “look around” in the test block, I think it’d be a bit simpler with IEx.pry

defmodule DebugTestTest do
  use ExUnit.Case
  doctest DebugTest
  require IEx

  test "greets the world" do
    result = DebugTest.hello()
    IEx.pry()
    assert result == :world
  end
end

and then iex -S mix test --trace.

Thanks! The first approach works well.

(Although I was surprised to see a lot of metadata and mangled names for parameters. I was assuming the Elixir layer was more immediate above the Erlang..? But that’s tangential.)

With the second suggesting I didn’t see a way to step through the running code. Are you using that? For me that is pretty essential, in cases where I want to debug at all.

Yeah, there is no way to “step through” with IEx.pry. I still use it, yes, usually right before the assert macros to have a “look around”.

yep, gotcha. In my case it’s about an algorithm that is recursive, so it would hit the line in code multiple times. That is a bit annoying if there isn’t any conditional break feature. That’s the reason I prefer a full debugger here.