Understanding __using__, with a practical problem

Hi all,

I’m trying to solve a practical problem - make an ExUnit “data case” with async true as default, instead of false.

So if a user does this:

use MyTestCase

Then async will be true, but if the user specifies use MyTestCase, async: false then it will be respected.

It seems to me that it should be fairly easy to inject code into the ExUnit.CaseTemplate to make it happen, but my understanding of Elixir metaprogramming is limited and have been stuck.

In my datacase, the first thing I do is to use ExUnit.CaseTemplate. This will import some callbacks and assertions and inject an __using__, overridable macro. As can be seen here: elixir/case_template.ex at v1.13.4 · elixir-lang/elixir · GitHub

Now, I thought I could do something like:

defmodule MyTestCase
  use ExUnit.CaseTemplate

  using opts do quote do
    # modify opts keyword list, put async true if not present
  end
end

But the __proxy__ function is called before evaluating the provided block (it’s the using/1 helper function in ExUnit.CaseTemplate that I’m leveraging). This means that the opts will have already been processed so I can’t know, at that point, whether the user had specified async: true or async: false.

What am I missing? Can I work with this or do I need to essentially drop the use ExUnit.CaseTemplate line and replicate what is does but in the way I need?

You likely want this:

defmodule MyTestCase
  use ExUnit.CaseTemplate

  defmacro __using__(opts) do
    opts
    |> Keyword.put_new(:async, true)
    |> super()
  end
end

For overwriting a overwritable __using__ macro, you need to implement exactly that macro. To call the injected implementation with your changes you can use super(). There’s no need to fiddle with quote in this case as you never need to convert text of code into AST.

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