16.2.6.1 Returning a value from another process

Rather than using global variables to pass values between processes, you can use closures instead. For example:

(defun send-with-result (process function)

  (let ((remote-result :none))

    (flet ((resultp ()

             (listp remote-result))

           (run-it ()

             (setq remote-result

                   (multiple-value-list (funcall function)))))

      (mp:process-send process (list #'run-it))

      (mp:process-wait "Waiting for result" #'resultp)

      (values-list remote-result))))