#lang racket

;; draw a graph of cos and deriv^3(cos)
(require plot)
(define ((deriv f) x)
  (/ (- (f x) (f (- x 0.001))) 0.001))
(define (thrice f) (lambda (x) (f (f (f x)))))
(plot (list (function ((thrice deriv) sin) -5 5)
            (function cos -5 5 #:color 'blue)))

;; Print the Greek alphabet
(for ([i (in-range 25)])
  (displayln
   (integer->char
    (+ i (char->integer #\u3B1)))))

;; An echo server
(define listener (tcp-listen 12345))
(let echo-server ()
  (define-values (in out) (tcp-accept listener))
  (thread (λ ()
             (copy-port in out)
             (close-output-port out)))
  (echo-server))