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further refinments to the ephemeron description

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Matthew Flatt 2010-11-28 18:05:59 -07:00
parent 37d16cf8f8
commit 64979a5480
2 changed files with 31 additions and 19 deletions
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32
collects/scribblings/reference/memory.scrbl
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@ -37,15 +37,26 @@ Returns @racket[#t] if @racket[v] is a weak box, @racket[#f] otherwise.}
@;------------------------------------------------------------------------
@section[#:tag "ephemerons"]{Ephemerons}
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.
An @deftech{ephemeron} @cite{Hayes97} is a generalization of a
@tech{weak box} (see @secref["weakbox"]). Instead of just containing
one value, an emphemeron holds two values: one that is considered the
value of the ephemeron and another that is the ephemeron's key. Like
the value in a weak box, the value in and ephemeron may be replaced by
@racket[#f], but when the @emph{key} is no longer reachable (except
possibly via weak references) instead of when the value is no longer
reachable.
A weak box can be seen as a specialization of an ephemeron where
the key and value are the same.
As long as an ephemeron's value is retained, the reference is
considered a non-weak reference. References to the key via the value
are treated specially, however, in that the reference does not
necessarily count toward the key's reachability. A @tech{weak box} can
be seen as a specialization of an ephemeron where the key and value
are the same.
On particularly common use of ephemerons is to combine 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.
More precisely,
@itemize[
@ -64,11 +75,6 @@ More precisely,
]
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.
@defproc[(make-ephemeron [key any/c] [v any/c]) ephemeron?]{

18
collects/scribblings/reference/reference.scrbl
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@ -111,12 +111,18 @@ The @racketmodname[racket] library combines
#:title "A Generalization of Exceptions and Control in ML-like Languages"
#:location "Functional Programming Languages and Computer Architecture"
#:date "1995")
(bib-entry #:key "Hieb90"
#:author "Robert Hieb and R. Kent Dybvig"
#:title "Continuations and Concurrency"
#:location "Principles and Practice of Parallel Programming"
#:date "1990")
(bib-entry #:key "Hayes97"
#:author "Barry Hayes"
#:title "Ephemerons: a New Finalization Mechanism"
#:location "Object-Oriented Languages, Programming, Systems, and Applications"
#:date "1997")
(bib-entry #:key "Hieb90"
#:author "Robert Hieb and R. Kent Dybvig"
#:title "Continuations and Concurrency"
#:location "Principles and Practice of Parallel Programming"
#:date "1990")
(bib-entry #:key "L'Ecuyer02"
#:author "Pierre L'Ecuyer, Richard Simard, E. Jack Chen, and W. David Kelton"