I do still hope though, that you reconsider your current API to something more explicit and less side-efficty despite the compiler-switch…

Why in a variable at all?

response = say("foo")
assert "sayed foo" = response

This snippet will totally obfuscate your assertion when ExUnit prints it in the failure case. Instead of how the actual value was produced you’ll only get printed response didn’t match, where by using the function call directly you will be told that say("foo") didn’t match the expectation, which in my opinion is much more obvious.

Besides of that, assuming purity, it does not make sense to what you did there. Either you care for the returned value and assert against it, or you are only interested in the side effect which you then mark explicitely with an underscore(d name), or you are interested in both, this is usually when assert against the return value first and then against the expected effects.

Why is it inconvenient to replace response with _ or _response in the cases where it is not used? It would make your code easier to read and understand for other developers.

In this case, I’d really rethink the API.

You seem to modify a global, invisible something with each call. When you do this from multiple processes this will probably be a big shoot in your own foot.

I really think its better than to do it like this:

response = init_say()
|> say("a")
|> say("b")

assert_text response, "a, b"

Or even:

init_say()
|> say("a")
|> say("b")
|> assert_text("a, b")

Building the value this way is much more “thread”-safe and less likely to “race”.

iex(2)> defmodule Blah do
...(2)>   def bloop, do: a = 42
...(2)> end
warning: variable "a" is unused
  iex:3

{:module, Blah,
 <<70, 79, 82, 49, 0, 0, 3, 124, 66, 69, 65, 77, 65, 116, 85, 56, 0, 0, 0, 83,
   0, 0, 0, 8, 11, 69, 108, 105, 120, 105, 114, 46, 66, 108, 97, 104, 8, 95, 95,
   105, 110, 102, 111, 95, 95, 9, 102, ...>>, {:bloop, 0}}
iex(3)> defmodule Blahs do     
...(3)>   def bloop, do: unquote(quote generated: true, do: a) = 42
...(3)> end
{:module, Blahs,
 <<70, 79, 82, 49, 0, 0, 3, 148, 66, 69, 65, 77, 65, 116, 85, 56, 0, 0, 0, 84,
   0, 0, 0, 8, 12, 69, 108, 105, 120, 105, 114, 46, 66, 108, 97, 104, 115, 8,
   95, 95, 105, 110, 102, 111, 95, 95, 9, ...>>, {:bloop, 0}}

You could always wrap it up in a macro too quite easily so you could do something like ignore_warnings(a) = 42 or so.