How to use add-to-list on a list returned by function

Question

I have a function that return a list. It may return a blank list or a number list. I would like to apply add-to-list to the return value. Is it possible?

     (defun return-list () body....)
     (setq test (add-to-list (return-list) 1) )

Answer 1

Function add-to-list operates on variables, not lists. E.g.:

(defvar test (return-list))
(add-to-list 'test 1)

If you are adding to a list unconditionally, use macro push which operates on places:

(push 1 test)

In your case, however, you can do even simpler:

(setq test (cons 1 (return-list)))

If you want to add an element only if it is not there yet, use the macro cl-pushnew which operates on places too:

(pushnew 1 test)
;; `test' is now (1)
(pushnew 1 test)
;; `test' is still (1)