A way for interactive to modify a let-bound variable?

[Kaushal Modi <kaushal.modi@gmail.com>]

Hi,

The below snippet will of course not work but I would like to achive
something like that:

(defun my-dummy-fn ()
  (let (msg)
    (interactive (progn
                   (if (use-region-p)
                       (setq msg "Region")
                     (setq msg "No region"))
                   nil))
    (message msg)))


This is a very simplified example. But I need to change a let-bound
variable in the interactive form based on some condition and then use that
variable in the defun body.

Is that possible?

--
Kaushal Modi