; factorial-count! : ('init or 'count or natnum) -> (void or natnum)
(define factorial-count!
  (local [; factorial-counter : natnum
          ; Purpose: Contains the count of the number of times that
          ;          factorial has been computed since the last
          ;          (re)initialization.
          (define factorial-counter 0)

          ; factorial : natnum -> natnum
          ; Purpose: Returns the factorial of the given number.
          (define (factorial n)
            (cond [(zero? n)     1]
                  [(positive? n) (* n (factorial (sub1 n)))]))]
    
    (lambda (message)
      (cond [(and (symbol? message) (symbol=? message 'init))
             (set! factorial-counter 0)]

            [(and (symbol? message) (symbol=? message 'count))
             factorial-counter]

            [(number? message)
             
             ; factorial could be defined equally well in a local here
             ; Stylistically, placing all major definitions in one giant
             ; local (as above) is more common.
             
             (begin
               (set! factorial-counter (add1 factorial-counter))
               (factorial message))]))))