(define $main
  (lambda [$world $argv]
    (do {[$world (write-string world "input char : ")]
         [$world (flush world)]
         [[$world $c] (read-char world)]
         [$world (write-char world '\n')]
         [$world (write-string world "output : ")]
         [$world (write-char world c)]
         [$world (write-char world '\n')]}
        world)))