Problem Statement

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: Delegate: 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".

Let's start with the solution:

class Non-Delegate  { }

class Delegate {
	method thing {
		return "delegate implementation"
	}
}

class Delegator {
	has $.delegate is rw;

	method operation {
		$.delegate.^can( 'thing' ) ?? $.delegate.thing
		!! "default implementation"
	}
}

my Delegator $d .= new;

say "empty: "~$d.operation;

$d.delegate = Non-Delegate.new;

say "Non-Delegate: "~$d.operation;

$d.delegate = Delegate.new;

say "Delegate: "~$d.operation;