How to run ExUnit tests with erlang debugger?

Questions / Help
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1

Background

I have a small function with some branching logic and I have some tests for it. I would like to use the erlang debugger when I run a given test to make sure I am executing the correct code.

Code

Lets assume we are testing DebuggerTest.greet/1:

defmodule DebugerTest do
  def greet(greeting) do
    if greeting == "Hola" do
      str = "#{greeting} mundo!"
      IO.puts(str)
      :spanish_greet
    else
      str = "#{greeting} world!"
      IO.puts(str)
      :normal_greet
    end
  end
end

Now, this function has some branching logic, and the code is overall horrible, but it does give us a lot of juicy executable lines that we attach a breakpoint to.

These are the tests I am running:

defmodule DebugerTestTest do
  use ExUnit.Case
  doctest DebugerTest

  test "greets the world" do
    assert DebugerTest.greet("Hello") == :normal_greet
  end

  test "greets the world in spanish" do
    assert DebugerTest.greet("Hola") == :fail_greet
  end
end

As you can see, my spanish greeting test will fail. However, I want to find that out using the erlang debugger:

Running the tests

Given that I am only interested in running the failing test, I change my code to:

@tag :wip
 test "greets the world in spanish" do
    assert DebugerTest.greet("Hola") == :fail_greet
  end

And then I run mix test --only wip.

There, now I am running only the failing test. But I am still not using the debugger.
To try to fix this I tried running the tests via an iex shell iex -S mix test --only wip but the run is automated and it ends before I can setup the debugger:

:debugger.start()
:int.ni(DebugerTest)
:int.break(DebugerTest, 3)

Question

How can I run the erlang debugger when using mix test ?

2

Try iex -S mix test --trace --only wip.

3

Doesnt work:

iex -S mix test --trace
Erlang/OTP 23 [erts-11.2] [source] [64-bit] [smp:12:12] [ds:12:12:10] [async-threads:1] [hipe]

Hello world!
Hola mundo!

DebugerTestTest [test/debuger_test_test.exs]
  * test greets the world (0.03ms) [L#5]
  * test greets the world in spanish (0.02ms) [L#9]


Finished in 0.02 seconds (0.00s async, 0.02s sync)
2 tests, 0 failures
4

Seems to be working for me:

  test "greets the world in spanish" do
    :debugger.start()
    :int.ni(DebugerTest)
    :int.break(DebugerTest, 3)
    assert DebugerTest.greet("Hola") == :fail_greet
  end

and then:

iex -S mix test --trace

waits for the debugger to finish.

2 Likes
5

ElixirLS VSCode extension supports running tests with debugging right from the UI. Navigate to Testing tab and click Debug test or right click an icon next to test definition and select Debug test.

1 Like