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The car of a weak pair holds its pointer weakly, while the cdr holds its pointer strongly. If the object in the car of a weak pair is not held strongly by any other data structure, it will be garbage-collected.
Note: weak pairs can be defeated by cross references among their slots. Consider a weak pair P holding an object A in its car and an object D in its cdr. P points to A weakly and to D strongly. If D holds A strongly, however, then P ends up holding A strongly after all. If avoiding this is worth a heavier-weight structure, See Ephemerons.
Note: weak pairs are not pairs; that is, they do not satisfy the
predicate pair?.
Allocates and returns a new weak pair, with components car and cdr. The car component is held weakly.
This predicate returns
#fif 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.
Returns the car component of weak-pair. If the car component has been garbage-collected, this operation returns
#f, but it can also return#fif 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.