#lang racket

;; Don't worry about the details of the procedure definition for now.
;; We'll learn how to write such programs later.  For now, just look
;; at the answers it provides.  For fun, you can load the definition
;; into Dr. Racket and try your own procedure calls.  Observe (1) how
;; fast it is! and (2) how it does not overflow the system stack!

(define factorial
  (lambda (n)
    (letrec ((accumulate
              (lambda (n answer)
                (if (zero? n)
                    answer
                    (accumulate (- n 1) (* n answer))))))
      (accumulate n 1))))