(module hello racket
  (provide hello)
  (define (hello) (displayln "Hello world!"))

(module+ test 
  (require rackunit)
  (check-equal? (with-output-to-string hello) "Hello world!\n")
  (check-equal? (with-output-to-string hello) (begin (sleep 3) "Hello world!\n")))

(module+ main
  (hello)))