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:

Delegator := Object clone do(
    delegate ::= nil
    operation := method(
        if((delegate != nil) and (delegate hasSlot("thing")),
            delegate thing,
            "default implementation"
        )
    )
)

Delegate := Object clone do(
    thing := method("delegate implementation")
)

a := clone Delegator
a operation println

a setDelegate("A delegate may be any object")
a operation println

a setDelegate(Delegate clone)
a operation println