objc:objc-exception-reason

(objc:objc-exception-reason condition)

The reason of the exception an objc:objc-exception carries, a string, or nil when it has none (a thrown object that is not an NSException has none). A reader of a condition of the macOS-only objc package; see the macOS GUI guide.

MY-APP> (defvar *e* (handler-case (invoke (invoke "NSArray" "array") "objectAtIndex:" 5)
                      (objc-exception (e) e)))
*E*
MY-APP> (objc-exception-reason *e*)
"*** -[__NSArray0 objectAtIndex:]: index 5 beyond bounds for empty array"