1Ephemeron(3)                       OCamldoc                       Ephemeron(3)
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NAME

6       Ephemeron - Ephemerons and weak hash table
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Module

9       Module   Ephemeron
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Documentation

12       Module Ephemeron
13        : sig end
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16       Ephemerons and weak hash table
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24       ===  Ephemerons  and weak hash table Ephemerons and weak hash table are
25       useful when one wants to cache or memorize the computation of  a  func‐
26       tion,  as long as the arguments and the function are used, without cre‐
27       ating memory leaks by continuously keeping old computation results that
28       are  not  useful anymore because one argument or the function is freed.
29       An implementation using .t is not  suitable  because  all  associations
30       would keep in memory the arguments and the result.  Ephemerons can also
31       be used for adding a field to an arbitrary boxed ocaml value:  you  can
32       attach an information to a value created by an external library without
33       memory leaks.  Ephemerons hold some keys and one or no data.  They  are
34       all boxed ocaml values. The keys of an ephemeron have the same behavior
35       than weak pointers according to the garbage collector.  In  fact  ocaml
36       weak pointers are implemented as ephemerons without data.  The keys and
37       data of an ephemeron are said to be full if  they  point  to  a  value,
38       empty  if the value have never been set, have been unset, or was erased
39       by the GC. In the function that accesses the keys  or  data  these  two
40       states  are  represented by the option type.  The data is considered by
41       the garbage collector alive if all the full keys are alive and  if  the
42       ephemeron  is  alive. When one of the keys is not considered alive any‐
43       more by the GC, the data is emptied from the ephemeron. The data  could
44       be  alive  for another reason and in that case the GC will not free it,
45       but the ephemeron will not hold the data anymore.  The ephemerons  com‐
46       plicate  the notion of liveness of values, because it is not anymore an
47       equivalence with the reachability from root  value  by  usual  pointers
48       (not  weak  and not ephemerons). With ephemerons the notion of liveness
49       is constructed by the least fixpoint of: A value is alive if: - it is a
50       root value - it is reachable from alive value by usual pointers - it is
51       the data of an alive ephemeron with all its full keys  alive  Notes:  -
52       All  the  types defined in this module cannot be marshaled using Perva‐
53       sives.output_value or the functions of the Marshal module.   Ephemerons
54       are  defined  in  a  language  agnostic  way  in  this paper: B. Hayes,
55       Ephemerons: a New Finalization Mechanism, OOPSLA'9 ===
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58       module type S = sig end
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61       The   output   signature   of   the   functor   Ephemeron.K1.Make   and
62       Ephemeron.K2.Make .  These hash tables are weak in the keys. If all the
63       keys of a binding are alive the binding is kept, but if one of the keys
64       of the binding is dead then the binding is removed.
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67       module type SeededS = sig end
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70       The   output  signature  of  the  functor  Ephemeron.K1.MakeSeeded  and
71       Ephemeron.K2.MakeSeeded .
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74       module K1 : sig end
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79       module K2 : sig end
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84       module Kn : sig end
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89       module GenHashTable : sig end
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972018-04-14                          source:                       Ephemeron(3)