I’ve learned the hard way that if you ever have more than one optional argument its a smell and you should refactor it to an opts list

You can also use functions from the Keyword module

def foo(opts \\ []) do
    arg1 = Keyword.get(opts, :arg1, "default_value") 
    ...
end

Absolutely never do this. Your colleagues or even your own future self will forget or be in a hurry just once and introduce a bug that would be difficult to track down.

Either only one optional argument and it must 99% of the time be the last one – unless the differences in type will make it completely obvious if something goes wrong – or use a keyword list as others have said. That approach also allows you to easily name stuff f.ex.

do_stuff(repo, changeset, username: "amir", password: "nope", dob: ~D[2000-01-01])