Returns the first object in a mailbox.
mailbox-peek mailbox => result, value-p
Any object or
mailbox-peek returns the first object in the mailbox without removing it. If the mailbox is empty,
nil is returned.
If the mailbox mailbox is not empty, the function
mailbox-peek returns the first object in the mailbox without removing it. The second returned value value-p is
If mailbox is empty, both return values result and value-p are
Since another process may modify the mailbox at any point, the result is not necessarily the next object that the next call to mailbox-read will read, unless no other process is reading from the mailbox.
mailbox-peek needs to lock the mailbox, which means it is significantly slower than mailbox-not-empty-p, and also may affect other processes. In most cases, mailbox-not-empty-p is sufficient and hence is preferable.
LispWorks User Guide and Reference Manual - 20 Sep 2017