This check/guard will always yield the same result

Well I can put in [""] as the default for one.

Which warning?

Sorry I put it as the title:
This check/guard will always yield the same result

Yes, the compiler is warning you that the guard will always fail, because no input will ever satisfy it.

So ideally should I write the function differently, or for dev environment passing just an empty string in as a possibility is fine?

You can conditionally add the first head:

if @unavailable_countries != [] do
  defp is_available?(%{
         city: %{
           country: %{iso_code: iso_code}
         }
       })
       when iso_code in @unavailable_countries do
    false
  end
end

  defp is_available?(_), do: true

Or simplify it to:

  defp is_available?(%{
         city: %{
           country: %{iso_code: iso_code}
         }
       }), do: iso_code not in @unavailable_countries

  defp is_available?(_), do: true

I will try the first thanks!
I think the simplified version wouldn’t work because I only want it to return if the item is in the list otherwise not, but this could return both true or false.

Edit: the check with the if around the function works, thanks!

I have fixed the “fixed” example to match your needs.