Editing Multiple dispatch (section)

==== Common Lisp ====
In a language with multiple dispatch, such as [[Common Lisp]], it might look more like this (Common Lisp example shown):

<syntaxhighlight lang="lisp">
(defclass asteroid ()  ((size :reader size :initarg :size)))
(defclass spaceship () ((size :reader size :initarg :size)))
(defun space-object (class size) (make-instance class :size size))

; collide-with is a generic function with multiple dispatch
(defmethod collide-with ((x asteroid)  (y asteroid))  "a/a")
(defmethod collide-with ((x asteroid)  (y spaceship)) "a/s")
(defmethod collide-with ((x spaceship) (y asteroid))  "s/a")
(defmethod collide-with ((x spaceship) (y spaceship)) "s/s")


(defun collide (x y)
  (if (and (> (size x) 100) (> (size y) 100))
      "big-boom"
      (collide-with x y)))

(print (collide (space-object 'asteroid 101)  (space-object 'spaceship 300)))
(print (collide (space-object 'asteroid 10)   (space-object 'spaceship 10)))
(print (collide (space-object 'spaceship 101) (space-object 'spaceship 10)))
</syntaxhighlight>

and similarly for the other methods. Explicit testing and "dynamic casting" are not used.

In the presence of multiple dispatch, the traditional idea of methods as being defined in classes and contained in objects becomes less appealing—each ''collide-with'' method above is attached to two different classes, not one. Hence, the special syntax for method invocation generally disappears, so that method invocation looks exactly like ordinary function invocation, and methods are grouped not in classes but in [[generic function]]s.