How to run tests in Elixir IEx shell

Hey! Have you ever wondered about tests running inside the IEx shell? For a long time, I was convinced that it’s not really possible. And as it turns out - that’s not really straightforward. You won’t easily find information about that in the documentation.

But! If you dig deep enough in elixir-lang repository at github, you might find a Mix.Tasks.Test module, which is responsible for the mix test task running.

Fortunately, there is a very useful function, which we can use for our own purpose:

def run(args) do
    ...
end

The arguments it receives are just the same as when you run the mix test causally but written as string values List. e.g

Mix.Tasks.Test.run(["test/path_to/some_test.exs:666", "--seed", "420"])

is equivalent to

$ mix test test/path_to/some_test.exs:666 --seed 420

Just last one thing that is to remember - if you want to run tests inside IEx, you have to set your MIX_ENV to test for environment.

$ MIX_ENV=test iex -S mix

Let's put it together

$ MIX_ENV=test iex -S mix
iex(1)> Mix.Tasks.Test.run(["test/path_to/some_test.exs"])
....
Finished in 1.0 seconds
4 tests, 0 failures

Randomized with seed 69090
:ok
iex(2)>

Use case example

Of course, somebody could ask: Hey, but why would I even do that? I'm pretty sure, that there are lots of use cases. My favorite is to combine it with break!/4.

Let's check it out:

$ MIX_ENV=test iex -S mix
iex(1)> break! SomeModule, :function_used_in_test, 1
iex(2)> Mix.Tasks.Test.run(["test/path_to/some_test.exs"])

Request to pry #PID<0.716.0> at SomeModule.function_used_in_test/1 (lib/some_module.ex:309)
309:   def function_used_in_test(args)
Allow? [Yn] Y

pry(1)> args
["debugging", "like", "that", "is", "wonderful"]
pry(2)> continue
....
Finished in 1.0 seconds
4 tests, 0 failures

Randomized with seed 69090
:ok
iex(3)>

That's so cool!