[DGD] Continuations

[Felix A. Croes]

Within the cloud library, continuations from different objects can now
be used together.  This enables code like:

    c = queue->dequeueCont() >> new ChainedContinuation("process");
    c->runNext();

I.e. queue->dequeueCont() returns a continuation that can be used to
dequeue an item, when one is available.  If the queue is empty, the
continuation can be suspended until a new item is enqueued; at that
point, the item will be forwarded to the next step to process it.  I
include the code of the queue below, as an example of the use of
suspension and reawakening of continuations:

    private object *queue, *suspended;

    static void create()
    {
	queue = ({ });
	suspended = ({ });
    }

    Continuation enqueueCont(object obj)
    {
	return new Continuation("enqueue", obj);
    }

    static void enqueue(object obj)
    {
	object token;

	if (sizeof(suspended) != 0) {
	    token = suspended[0];
	    suspended = suspended[1 ..];
	    token->wakeContinuation(obj);
	} else {
	    queue += ({ obj });
	}
    }

    Continuation dequeueCont()
    {
	return new Continuation("dequeue");
    }

    static object dequeue()
    {
	object obj;

	if (sizeof(queue) != 0) {
	    obj = queue[0];
	    queue = queue[1 ..];
	    return obj;
	} else {
	    suspended += ({ suspendContinuation() });
	    returh nil;
	}
    }

Regards,
Felix Croes