Better than "Understanding OOP"

systems.el

(defvar systems '((usg-unix-v "UNIX" "good")
                  ;; `string-equal' converts symbols to string automagically
                  ((lambda (sys) (string-equal "gnu/linux" sys)) "UNIX" "good")
                  ("^windows" "Windows" "bad")
                  (darwin "Mac OS" "far superior")))

(defun assoc-match (key alist)
  "Return non-nil if KEY matches the `car' of an element of ALIST.
The `car' of each element of ALIST can be a:

string - Compared to KEY with `string-match'

function - Passed KEY as an argument. It should return non-nil if
it considers KEY to match

symbol - Compared to KEY with `eq'

The value is actually the first element of ALIST whose `car' matches KEY."
  (when (consp alist)
   (let* ((elt (car alist))
          (elt-cmp (car elt)))
     (if (cond
          ;; `string-match' accepts only strings
          ((stringp elt-cmp) (string-match elt-cmp (format "%s" key)))
          ((functionp elt-cmp) (funcall elt-cmp key))
          ((symbolp elt-cmp) (eq elt-cmp key)))
         elt
       (assoc-match key (cdr alist))))))

(let ((box (assoc-test system-type systems)))
  (if box
      (message "This is a %s box and therefore %s." (cadr box) (caddr box))
    (message "This is not a box.")))