On Fri, Mar 17, 2017 at 4:16 AM, Andreas Röhler
<andreas.roehler@online.de> wrote:

--8<---------------cut here---------------start------------->8---
(defun my-command (beg end)
  (interactive "r")
  (if (use-region-p)
      (my-command-region beg end)
    (my-command-non-region)))
--8<---------------cut here---------------end--------------->8---

AFAIK, you cannot use the (interactive "r") form for this "dwim-ish"
kind of behavior.

Why that? If a region is set, why should (use-region-p) fail?

(use-region-p) doesn't fail, the "r" in interactive fails, when there
is no mark (i.e., when a region is not and has never been set). It's
basically equivalent to this:

(defun my-command (beg end)
  (interactive (sort (list (point) (mark)) #'<))
  (if (use-region-p)
      (my-command-region beg end)
    (my-command-non-region)))