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

Returns a new ephemeron whose key is key and whose value is initially v.

(ephemeron-value ephemeron) → any

ephemeron : ephemeron?

Returns the value contained in ephemeron. If the garbage collector has proven that the key for ephemeron is only weakly reachable, then the result is #f.

(ephemeron? v) → boolean?

v : any/c

Returns #t if v is an ephemeron, #f otherwise.