在 2020年1月3日 +0800 PM1:59，Eli Zaretskii <eliz@gnu.org>，写道：
>
> How do you write a useful Lisp application for a thread if you have no
> way of displaying any messages?

Workers are background workers. They don’t display messages directly. They post the error messages to the UI part of the lisp application.

For a lisp application such as an email client, it is splitted to two parts: the UI part and the worker part. The UI part may has two callbacks(or event handlers):
1. on_new_email
2. on_error

If the worker fetches an email successfully, it sends an event to the UI part and the on_new_email callback of the UI part will be called. If the worker fails, it sends an error to the UI part and the on_error callback will be called. The on_error callback can display the error message to user.