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tried to clarify the ephemeron docs by setting the stage for the precise definition that was already there a little bit.

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Robby Findler 2010-11-28 16:04:13 -06:00
parent c123a8cc5d
commit 62f3d7e254
1 changed files with 12 additions and 5 deletions
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17
collects/scribblings/reference/memory.scrbl
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@ -37,13 +37,20 @@ Returns @racket[#t] if @racket[v] is a weak box, @racket[#f] otherwise.}
@;------------------------------------------------------------------------
@section[#:tag "ephemerons"]{Ephemerons}
An @deftech{ephemeron} is similar to a weak box (see
@secref["weakbox"]), except that
An @deftech{ephemeron} is a generalization of a weak box (see
@secref["weakbox"]). Instead of just containing one value, it holds two,
the value of the ephemeron, plus a key. Like a weak box, the
value in the ephemeron may be replaced by @racket[#f], but it does
this when the key is no longer reachable. In addition, the memory
manager specially treats links from the value to the key.
A weak box can be seen as a specialization of an ephemeron where
the key and value are the same.
More precisely,
@itemize[
@item{an ephemeron contains a key and a value; the value can be
extracted from the ephemeron, but the value is replaced
@item{the value in an ephemeron is replaced
by @racket[#f] when the automatic memory manager can prove that
either the ephemeron or the key is reachable only through weak
references (see @secref["weakbox"]); and}
@ -57,7 +64,7 @@ An @deftech{ephemeron} is similar to a weak box (see
]
In particular, an ephemeron can be combined with a weak hash table
On particularly common use of ephemerons is to combined them with a weak hash table
(see @secref["hashtables"]) to produce a mapping where the memory
manager can reclaim key--value pairs even when the value refers to the
key.