the.lisp

;; Define a simple structure                                                                     
(defstruct point
  x
  y)

(defun make-point (x y)
  (make-instance 'point :x x :y y))

;; Normal Lisp style function                                                                    
(defun add-points (p1 p2)
  (make-point (+ (slot-value p1 'x)
                 (slot-value p2 'x))
              (+ (slot-value p1 'y)
                 (slot-value p2 '))))

;; With typechecks                                                                               
(defun add-points ((the point p1) (the point p2))
  (make-point (+ (slot-value p1 'x)
                 (slot-value p2 'x))
              (+ (slot-value p1 'y)
                 (slot-value p2 '))))

;; With typechecks + destructuring, named points                                                 
(defun add-points ((the point p1 :x x1 :y y1)
                   (the point p2 :x x2 :y y2))
  (make-point (+ x1 x2) (+ y1 y2)))

;; With typechecks + destructuring, anonymous points                                             
(defun add-points ((the point :x x1 :y y1)
                   (the point :x x2 :y y2))
  (make-point (+ x1 x2) (+ y1 y2)))

;; With destructuring, works for any objects with X and Y slots                                  
(defun add-points ((the :x x1 :y y1)
                   (the :x x2 :y y2))
  (make-point (+ x1 x2) (+ y1 y2)))