Weak Boxes

 

A weak box is similar to a normal box (see section 2.7), but a weak box does not keep its contents from being garbage-collected.

•   (make-weak-box v) returns a new weak box that initially contains v.
•   (weak-box-value wb) returns the value contained in the weak box wb. If the value that was in wb has already been garbage-collected, then #f is returned instead.
•   (weak-box? wb) returns #t if wb is a weak box or #f otherwise.