Editing Generic function (section)

=== Common Lisp ===
Define a generic function with two parameters object-1 and object-2. The name of the generic function is ''collide''.
<syntaxhighlight lang="lisp">
 (defgeneric collide (object-1 object-2))
</syntaxhighlight>

Methods belonging to the generic function are defined outside of classes.
Here we define a method for the generic function ''collide'' which is specialized for the classes asteroid (first parameter object-1) and spaceship (second parameter object-2). The parameters are used as normal variables inside the method body. There is no special namespace that has access to class slots.
<syntaxhighlight lang="lisp">
 (defmethod collide ((object-1 asteroid) (object-2 spaceship))
   (format t "asteroid ~a collides with spaceship ~a" object-1 object-2))
</syntaxhighlight>

Calling the generic function:
<syntaxhighlight lang="lisp">
? (collide (make-instance 'asteroid) (make-instance 'spaceship))
asteroid #<ASTEROID 4020003FD3> collides with spaceship #<SPACESHIP 40200048CB>
</syntaxhighlight>

Common Lisp can also retrieve individual methods from the generic function. FIND-METHOD finds the method from the generic function ''collide'' specialized for the classes ''asteroid'' and ''spaceship''.
<syntaxhighlight lang="lisp">
? (find-method #'collide nil (list (find-class 'asteroid) (find-class 'spaceship)))
#<STANDARD-METHOD COLLIDE NIL (ASTEROID SPACESHIP) 4150015E43>
</syntaxhighlight>