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10.7 Weak Pairs

Weak pairs are a mechanism for building data structures that point at objects without protecting them from garbage collection. The car of a weak pair holds its pointer weakly, while the cdr holds its pointer in the normal way. If the object in the car of a weak pair is not held normally by any other data structure, it will be garbage-collected.

Note: weak pairs are not pairs; that is, they do not satisfy the predicate pair?.

— procedure: weak-pair? object

Returns #t if object is a weak pair; otherwise returns #f.

— procedure: weak-cons car cdr

Allocates and returns a new weak pair, with components car and cdr. The car component is held weakly.

— procedure: weak-pair/car? weak-pair

This predicate returns #f if the car of weak-pair has been garbage-collected; otherwise returns #t. In other words, it is true if weak-pair has a valid car component.

— procedure: weak-car weak-pair

Returns the car component of weak-pair. If the car component has been garbage-collected, this operation returns #f, but it can also return #f if that is the value that was stored in the car.

Normally, weak-pair/car? is used to determine if weak-car would return a valid value. An obvious way of doing this would be:

     (if (weak-pair/car? x)
         (weak-car x)
         ...)

However, since a garbage collection could occur between the call to weak-pair/car? and weak-car, this would not always work correctly. Instead, the following should be used, which always works:

     (or (weak-car x)
         (and (not (weak-pair/car? x))
              ...))

The reason that the latter expression works is that weak-car returns #f in just two instances: when the car component is #f, and when the car component has been garbage-collected. In the former case, if a garbage collection happens between the two calls, it won't matter, because #f will never be garbage-collected. And in the latter case, it also won't matter, because the car component no longer exists and cannot be affected by the garbage collector.

— procedure: weak-set-car! weak-pair object

Sets the car component of weak-pair to object and returns an unspecified result.

— procedure: weak-cdr weak-pair

Returns the cdr component of weak-cdr.

— procedure: weak-set-cdr! weak-pair object

Sets the cdr component of weak-pair to object and returns an unspecified result.