15.2 Ephemerons
An ephemeron is similar to a weak box (see Weak Boxes), except that
an ephemeron contains a key and a value; the value can be extracted from the ephemeron, but the value is replaced by #f when the automatic memory manager can prove that either the ephemeron or the key is reachable only through weak references (see Weak Boxes); and
nothing reachable from the value in an ephemeron counts toward the reachability of an ephemeron key (whether for the same ephemeron or another), unless the same value is reachable through a non-weak reference, or unless the value’s ephemeron key is reachable through a non-weak reference (see Weak Boxes for information on weak references).
In particular, an ephemeron can be combined with a weak hash table (see Hash Tables) to produce a mapping where the memory manager can reclaim key–value pairs even when the value refers to the key.
(make-ephemeron key v) → ephemeron? |
key : any/c |
v : any/c |
(ephemeron-value ephemeron) → any |
ephemeron : ephemeron? |
(ephemeron? v) → boolean? |
v : any/c |