unbound-value-indicator

Purpose
Represent an unbound value.

Package   :gbbopen-tools

Module   :gbbopen-tools

Value type   A keyword symbol

Value   :---unbound---

See also
    define-unit-class

Example
Define a slot-reader function that returns the value of my-slot or unbound-value-indicator if the slot is unbound:

  (defun safe-my-slot-of (instance)
    (if (slot-bound-p instance 'my-slot)
        (slot-value instance 'my-slot)
        unbound-value-indicator))

unbound-value-indicator