Forwarding a match pattern to a custom macro

Questions / Help
, , ,
1

Hey,

I’m currently trying to DRY up some tests which have a lot of boilerplate code in the form of:

test "user is updated", %{user_id: user_id} do
  pid = spawn(fn ->
    EventStore.subscribe(UserUpdated)
    assert_receive event = %UserUpdated{aggregate_id: ^user_id}
    EventStore.acknowledge(event)
  end)

  ref = Process.monitor(pid)

  # the actual test

   assert_receive {:DOWN, ^ref, _, _, _}
end

Basically I’m checking if the event UserUpdated is fired in our custom event_store. I tried to implement a custom macro like this:

defmacro assert_event(event, do: block) do
  quote do
    pid =
      spawn(fn ->
        EventStore.subscribe(unquote(event).__struct__)
        assert_receive event = unquote(event)
        EventStore.acknowledge(event)
      end)

    ref = Process.monitor(pid)

    unquote(block)

    assert_receive {:DOWN, ^ref, _, _, _}
  end
end

Which can then be used like this:

test "user is updated", %{user_id: user_id} do
  assert_event %UserUpdated{aggregate_id: user_id} do
    # the actual test
  end
end

But this results in the following warning:

warning: variable “user_id” is unused (there is a variable with the same name in the context, use the pin operator (^) to match on it or prefix this variable with underscore if it is not meant to be used)

But pinning user_id results in the following error:

cannot use ^user_id outside of match clauses

So, my question is (after reading the complicated source code of assert_receive), is there a way to mark some code as a pattern and just forward it to the assert_receive macro inside my custom assert_event macro?

2

Why not keep it simple and use a function?

def assert_event(%event_mod{} = event, func) do
  pid = spawn(fn ->
    EventStore.subscribe(event_mod)
    assert_receive ^event
    EventStore.acknowledge(event)
  end)

  ref = Process.monitor(pid)

  func.()

  assert_receive {:DOWN, ^ref, _, _, _}
end
2 Likes
3

Yeah, a function would have been sufficient, I think I just got carried away with the nicer syntax with a do block instead of an inline func (sorry, still a lot of Ruby influences here :wink:). However the problem still persists. I cannot forward the match to the function. The problem is, I need to work with a match here, as I don’t know all the data inside the emitted event, and therefore can’t use a simple == comparison.

4

Pattern matches are a compile time structure in code. You cannot „pass“ a pattern at runtime, so the only way to do it is using macros. When using macros you‘ll need to read up on macro hygine, so variables within generated code become accessible to code not being generated.

1 Like
5

Yeah, that’s probably the topic I will have to deep dive into :blush: Thx a lot!