Making it stick.

I guess I agree with Brian Marick's code reading style more than Richard Gabriel's. Rather than using an optional private parameter in a public function, though, I prefer to use the "named" let, with an accumulator there, for tail recursive functions.

Is it idiomatic? It's fairly common in Scheme. I am not sure why it has not been adopted more widely into Common Lisp.

;; Richard Gabriel's Idiomatic Common Lisp
(defun fact (n)
 (labels ((f (n acc)
	 (if (<= n 1) acc (f (- n 1) (* n acc))))) (f n 1))) ;; My (Idiomatic?) Scheme (define (fact n) (let loop ((n n) (acc 1)) (if (<= n 1) acc (loop (- n 1) (* n acc))))) ;; "Old School" Common Lisp (defun fact(n &optional (acc 1)) (if (<= n 1) acc (fact (- n 1) (* n acc)))