(define (standart-deviation-generator)
  (let ((nums '()))
    (lambda (x)
      (set! nums (cons x nums))
      (let* ((mean (/ (apply + nums) (length nums)))
      (mean-sqr (lambda (y) (expt (- y mean) 2)))
      (variance (/ (apply + (map mean-sqr nums)) (length nums))))
    (sqrt variance)))))

(let loop ((f (standart-deviation-generator))
           (input '(2 4 4 4 5 5 7 9)))
  (unless (null? input)
      (display (f (car input)))
      (newline)
      (loop f (cdr input))))