remember_scheme.scm

(define (factorial n)
  ;; tail recursion is fast, and often requires a helper function
  (define (helper n product)
    (if (< n 2)
      product
      (helper (- n 1) (* n product))))
  (helper n 1))

;; Scheme looping structure
(do
  ((n 100 (+ n 100)))   ;; variable, initial value, step
  ((> n 1000) 3)        ;; terminating condition and return value
  (format #t "~s ~s\n" n (factorial n)))    ;; Scheme's printf