Erlang: How to "Do Nothing" in true branch of if statement

Erlang does not have the notion of nothing like void or unit. I would suggest returning another atom like not_ok (or even void or unit.)

The best answer is don't use if, just use case.

case A of
   B -> ok;
   C -> throw({error,a_doesnt_equal_b_or_whatever_you_want_to_do_now})
end

typically ok or undefined or noop are returned as atoms which mean essentially, nothing.