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".

module Delegates
 
export Delegator, Delegate
 
struct Delegator{T}
    delegate::T
end
 
struct Delegate end
 
operation(x::Delegator) = thing(x.delegate)
thing(::Any) = "default implementation"
thing(::Delegate) = "delegate implementation"
 
end  # module Delegates