How to exit function from inside its `interactive' call?

[Thorsten Jolitz <tjolitz@gmail.com>]

Hi List, 

how can I use catch/throw (or whatever) to exit function foo from inside
its 'interactive call? All combinations I tried so far only exit
'interactive and still execute the function body. 

#+begin_src emacs-lisp
  (defun foo (&optional arg)
    (catch 'exit
      (interactive
       (let ((bar (if current-prefix-arg
                      (prefix-numeric-value)
                    (throw 'exit nil))))
         (list (+ 2 bar)))))
    (message "Arg: %s" arg))
#+end_src

#+results:
: Arg: nil

#+begin_src emacs-lisp
 (foo)
#+end_src

#+results:
: Arg: nil

#+begin_src emacs-lisp
 (foo '(4))
#+end_src

#+results:
: Arg: (4)

Thanks for any hint.



-- 
cheers,
Thorsten