A delegate is a helper object used by another object. The delegator may send the delegate certain messages, and provide a default implementation when there is no delegate or the delegate does not respond to a message. This pattern is heavily used in Cocoa framework on Mac OS X. See also wp:Delegation pattern.

Objects responsibilities:

Delegator:

 Keep an optional delegate instance.
 Implement "operation" method, returning the delegate "thing" if the delegate respond to "thing", or the string "default implementation".
Delegate:

 Implement "thing" and return the string "delegate implementation"
Show how objects are created and used. First, without a delegate, then with a delegate that does not implement "thing", and last with a delegate that implements "thing".

(class +Delegator)
# delegate
 
(dm operation> ()
   (if (: delegate)
      (thing> @)
      "default implementation" ) )
 
 
(class +Delegate)
# thing
 
(dm T (Msg)
   (=: thing Msg) )
 
(dm thing> ()
   (: thing) )
 
 
(let A (new '(+Delegator))
   # Without a delegate
   (println (operation> A))
 
   # With delegate that does not implement 'thing>'
   (put A 'delegate (new '(+Delegate)))
   (println (operation> A))
 
   # With delegate that implements 'thing>'
   (put A 'delegate (new '(+Delegate) "delegate implementation"))
   (println (operation> A)) )