Distinguish C-u <key> from C-u 4 <key>

Question

I know that C-u sends the prefix 4. Is there any way to make C-u <key> a special case of a command, while still allowing C-u 4 <key>?

Answer 1

Sure. Use the raw prefix argument instead:

(defun my-prefix-test (arg)
  (interactive "P")
  (cond
   ((consp arg)   (message "Prefix argument"))
   ((numberp arg) (message "Numeric argument"))
   ((eq '- arg)   (message "Negative argument"))
   ((not arg)     (message "No argument"))))

As you can see, current-prefix-arg can be a list, a number, - or nil.