(define (read/write filename length outport #!optional (bs (* 1024 2)))
  (print "Length : " length ", blocksize " bs )
  (let ((bar (make-progress-bar frame: (string-append filename ": [~a~a~a~a|~a]") width: 40 max: length)))
    (let loop ((size length))
      (let* ((bytes (min bs size)))
        (if (zero? size) #t
            (let ((input (read-string bytes)))
              (if input
                  (write-string input bytes outport)
                  (when (> bytes 0)
                    (error "Missing ~a from input" (pretty-filesize bytes))))
              (advance-progress-bar! bar bytes)
              (loop (- size bytes))))))
    (finish-progress-bar! bar)))