How to create interactive elisp function with optional arguments

Question

How do you write an elisp function, that should be bound to a key-press, that works without prompting by default, but when preceeded by Ctrl-u prompts the user for an argument. Something similar to (which is wrong syntax, but I hope you get the idea)?

 (defun my-message (&optional (print-message "foo"))
   (interactive "P")
   (message print-message))
 (global-set-key "\C-c\C-m" 'my-message)

Answer 1

Following the same kind of implementation as your example, you could do something like this:

(defun my-message (&optional arg)
  (interactive "P")
  (let ((msg "foo"))
    (when arg
      (setq msg (read-from-minibuffer "Message: ")))
    (message msg)))

Answer 2

The following use a feature of interactive that allows you to use code instead of a string. This code will only be executed when the function is called interactively (which makes this a different answer compared to the earlier answer). The code should evaluate to a list where the elements are mapped to the parameters.

(defun my-test (&optional arg)
  (interactive (list (if current-prefix-arg
                         (read-from-minibuffer "MyPrompt: ")
                       nil)))
  (if arg
      (message arg)
    (message "NO ARG")))

By using this method, this function can be called from code like (my-test) or (my-test "X") without it prompting the user for input. For most situations, you would like to design functions so that they only prompt for input when called interactively.