Hi Neeraj:

please can you work further on a language for expressing
patterns using APPCs. This is still pretty rough but contains a good start.

-- Karl
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NullObject pattern expressed using a strategy:

NullObject Pattern:

Short description:
If you are testing for absence of a target before sending it a message,
consider using a special object to indicate absence.

// in Demeter/Java syntax, except the caller transportation
Container {
  void Something() from caller:Container to {Target,EmptyTarget}
    { // inlined visitor
      before Target (@ host.Continue(); @)
      before EmptyTarget (@ caller.NullDetected(); @)
    }
}

EmptyTarget is the special object.

If a Container-object does not contain a Target-object, NullDetetced
will be called. Assumes that Target and EmptyTarget are mutually exclusive
and that several Target-objects may exist but at most one
EmptyTarget-object.

The simplest class graph would be:

Container : Target | EmptyTarget.

but infinitely many more are possible.

In APPC syntax:

APPC NULL_OBJECT 
  Interface
    class valued variables
      C,
      T : IT // T has method Continue()
  Behavior
    C {
      // new method
      void Something() from caller:C to {T,EmptyTarget}
      { // inlined visitor
	before T (@ host.Continue(); @)
        before EmptyTarget (@ caller.NullDetected(); @)
      }
    }

usage:
NULL_OBJECT(Container,Target)

The logging pattern from last night:

APPC LOGGING_PATTERN
  Interface
    class valued variables
      A : Loggable // A is a class that fulfills the Loggable personality
		   // tells what to log
    method valued variables
      A.[f1 ... fn]
    strategies
      from A to log:Logging
  Behavior
    modifies A.[f1 ... fn]
    A {
      void [f1 ... fn](...) {next();
	log.register(what_to_log([f1 ... fn]));
      }
    }

usage:
LOGGING_PATTERN(Account; deposit(Amount), withdraw(Amount);
what_to_log(deposit,Amount,"deposit")
what_to_log(withdraw,Amount,"withdraw")

-- Karl