Numeric argument defaulting

[Bill Brodie <wbrodie@panix.com>]

I have several utility functions that take optional numeric arguments and
that I'd also like to make interactive.  There are two ways to do this:

1. Define the function to accept either a raw or a numeric argument.

(defun f (&optional x)
  (interactive "P")
  (cond
   ((numberp x))
   ((null x) (setq x <default value>))
   (t (setq x (prefix-numeric-value x))))
  ...)

-or-

2. Define an interactive wrapper around the function.

(defun f (&optional x)
  (if (null x) (setq x <default value>))
  ...)
(defun f-interactive (x)
  (interactive "P")
  (if x (f (prefix-numeric-value x)) (f)))


Neither of these seems especially clean.  Is there a standard Emacs Lisp
idiom for this?