(define (derivative f)
  (lambda (x)
    (/ (- (f (+ x delta))
          (f (- x delta)))
       (* 2 delta))))

(define delta 0.0001)

;; ----------------------------------------
;; The following code uses the "gui.ss" teachpack,
;; which is distibuted with DrScheme in
;; the "htdp" subdirectory

;; mk-greet : str -> (event -> bool)
(define (mk-greet what)
  (lambda (evt)
    (draw-message greet-msg (string-append
                             what
                             ", "
                             (text-contents name-field)))))

(define name-field
  (make-text "Name:"))
(define hi-button
  (make-button "Hello" (mk-greet "Hi")))
(define bye-button
  (make-button "Goodbye" (mk-greet "Bye")))
(define greet-msg
  (make-message "______________________________"))

(create-window (list (list name-field)
                     (list hi-button bye-button)
                     (list greet-msg)))