Lisp Programming Trick #2

Recursive Condition

This little trick hit me when I was perusing PCL. Specifically, the when selector.

(defun rec-cond (key)
  (cond
    ((eq key :before) (format nil "executing before" ))
    ((eq key :during) (format nil "executing during" ))
    ((eq key :after) (format nil "executing after" ))

    ((eq key :set-up) (rec-cond :before))
    ((eq key :init) (rec-cond :before))

    ((eq key :teardown) (rec-cond :after))

    (t (error (format nil "unknown command: ~A" key))))

This overloads keyword symbols so that you more than one keyword can execute the same code.