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15.2 Ephemerons

An ephemeron is similar to a weak box (see Weak Boxes), except that

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.