A practical use for change-class!

Previously only thought useful for metaprogramming, the amazing change-class finds a use in the hallowed realm of application programming.

(defwidget maybe-pagination (weblocks:pagination)
  ((show-pred :initarg :show-predicate :accessor show-paginator-predicate
              :initform (constantly t)
              :documentation "One-arg proc accepting self answering
              whether to show the widget contents."))
  (:documentation "Hide the paginator sometimes."))

(defmethod weblocks:render-widget-body
    ((self maybe-pagination) &key &allow-other-keys)
  "Iff `show-paginator-predicate''s value answers NIL on myself, don't
send to super."
  (when (funcall (show-paginator-predicate self) self)
    (call-next-method)))

(defmethod initialize-instance :after
    ((self my-listedit-subclass) &key &allow-other-keys)
  (change-class (weblocks:dataseq-pagination-widget self) 'maybe-pagination
    :show-predicate (f_% (typep (weblocks:authenticatedp) 'admin-account))))

Wow, huh?

(The above makes it so that only admins can see the subwidget that
shows the current page with links for paging through the results.)