(define len (lambda (lst)
              (if (null? lst)
                 0
               (+ 1 (len (cdr lst))))))

;; (foldr f base (list x-1 ... x-n)) = (f x-1 ... (f x-n base))
;; (foldl f base (list x-1 ... x-n)) = (f x-n ... (f x-1 base))

(define flen (lambda (lst)
               (foldl (lambda (_ n) (+ n 1)) 0 lst)))

;; recursive factorial
(define fact (lambda (n) (if (= n 0) 1 (* n (fact (- n 1))))))

;; enumerate a list from n downto 1
(define enumerate (lambda (n)
                    (if (= 0 n)
                        '()
                        (cons n (enumerate (- n 1)))
                        )))
;; folded factorial
(define ffact (lambda (n) (foldl * 1 (enumerate n))))