Snapshot of MIT Scheme 9.1 documentation:
An ephemeron is an object with two weakly referenced components
called its key and datum. The garbage collector drops an
ephemeron's references to both key and datum, rendering the ephemeron
broken, if and only if the garbage collector can prove that
there are no strong references to the key. In other words, an
ephemeron is broken when nobody else cares about its key. Note that
an ephemeron's reference to its datum may be dropped even if the datum
is still reachable; all that matters is whether the key is reachable.
Once broken, ephemerons never cease to be broken; setting the key or
datum of a broken ephemeron with set-ephemeron-key! or
set-ephemeron-datum! has no effect.
Ephemerons are considerably heavier-weight than weak pairs, because garbage-collecting ephemerons is more complicated than garbage-collecting weak pairs. Each ephemeron needs five words of storage, rather than the two words needed by a weak pair. However, while the garbage collector spends more time on ephemerons than on other objects, the amount of time it spends on ephemerons scales linearly with the number of live ephemerons, which is how its running time scales with the total number of live objects anyway.
Allocates and returns a new ephemeron, with components key and datum.
Returns
#tif the garbage collector has dropped ephemeron's references to its key and datum; otherwise returns#f.
These return the key or datum component, respectively, of ephemeron. If ephemeron has been broken, these operations return
#f, but they can also return#fif that is the value that was stored in the key or value component.
These set the key or datum component, respectively, of ephemeron to object and return an unspecified result. If ephemeron is broken, neither of these operations has any effect.
Like weak-pair/car?, ephemeron-broken? must be used with
care. Ephemeron-broken? on an ephemeron guarantees that any
prior call to ephemeron-key or ephemeron-datum on the
same ephemeron yielded the key or datum that was stored in the
ephemeron, but it makes no guarantees about subsequent calls to
ephemeron-key or ephemeron-datum. Thus, the correct
idiom to fetch an ephemeron's key and datum and use them if the
ephemeron is not broken is
(let ((key (ephemeron-key ephemeron))
(datum (ephemeron-datum ephemeron)))
(if (ephemeron-broken? ephemeron)
... broken case ...
... code using key and datum ...))