class-un_answer

class <-> un_answer: bool

Hint for the incremental garbage collector. By default, fresh created instances (either by new/2 or some get method that generates an instance) are stored in a special table. As these instances are answers to methods, this table is called the answer table internally.

If an object is made an attribute of another object, it will be removed from the answer table and reclaimed by the garbage collector it its reference count drops to zero. Object remaining in the answer table are reclaimed if the context in which they where created is ended. The main event-loop starts/ends such a context and likewise do all method executions.

Now consider the code below, which produces an alphabetically ordered list of class-names:

?- new(Chain, chain),
   send(@classes, for_all,
                message(Chain, append, @arg1)),
   send(Chain, sort).

With the above schema, Chain will be an answer object after creation. It will be made a slot of the message object, thus loosing its answer status. After the chain->for_all, PCE will garbage collect the message. The reference count of the chain drops to zero and thus the chain is reclaimed too.

This situation is common and therefore class code sets the class<-un_answer flags to @off. This tells variable->send not to remove the chain from the answer table.