1Ephemeron(3)                     OCaml library                    Ephemeron(3)
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NAME

6       Ephemeron - Ephemerons and weak hash tables
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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 tables
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24       Ephemerons  and  weak hash tables are useful when one wants to cache or
25       memorize the computation of a function, as long as  the  arguments  and
26       the  function  are  used, without creating memory leaks by continuously
27       keeping old computation results that are not useful anymore because one
28       argument or the function is freed. An implementation using Hashtbl.t is
29       not suitable because all associations would keep the arguments and  the
30       result in memory.
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32       Ephemerons  can also be used for "adding" a field to an arbitrary boxed
33       OCaml value: you can attach some information to a value created  by  an
34       external library without memory leaks.
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36       Ephemerons  hold some keys and one or no data. They are all boxed OCaml
37       values. The keys of an ephemeron have the same behavior as weak  point‐
38       ers according to the garbage collector. In fact OCaml weak pointers are
39       implemented as ephemerons without data.
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41       The keys and data of an ephemeron are said to be full if they point  to
42       a  value,  or empty if the value has never been set, has been unset, or
43       was erased by the GC. In the function that accesses the  keys  or  data
44       these two states are represented by the option type.
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46       The  data  is considered by the garbage collector alive if all the full
47       keys are alive and if the ephemeron is alive. When one of the  keys  is
48       not  considered  alive  anymore by the GC, the data is emptied from the
49       ephemeron. The data could be alive for another reason and in that  case
50       the  GC will not free it, but the ephemeron will not hold the data any‐
51       more.
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53       The ephemerons complicate the notion of liveness of values, because  it
54       is  not anymore an equivalence with the reachability from root value by
55       usual pointers (not weak and not ephemerons). With ephemerons  the  no‐
56       tion  of  liveness  is constructed by the least fixpoint of: A value is
57       alive if:
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59       -it is a root value
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61       -it is reachable from alive value by usual pointers
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63       -it is the data of an alive ephemeron with all its full keys alive
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65       Notes:
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67       -All the types defined in this module cannot be  marshaled  using  out‐
68       put_value or the functions of the Marshal module.
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70       Ephemerons  are  defined  in  a language agnostic way in this paper: B.
71       Hayes, Ephemerons: A New Finalization Mechanism, OOPSLA'97
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73       module type S = sig end
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76       The   output   signature   of   the   functor   Ephemeron.K1.Make   and
77       Ephemeron.K2.Make .  These hash tables are weak in the keys. If all the
78       keys of a binding are alive the binding is kept, but if one of the keys
79       of the binding is dead then the binding is removed.
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82       module type SeededS = sig end
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85       The   output  signature  of  the  functor  Ephemeron.K1.MakeSeeded  and
86       Ephemeron.K2.MakeSeeded .
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89       module K1 : sig end
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94       module K2 : sig end
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99       module Kn : sig end
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104       module GenHashTable : sig end
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112OCamldoc                          2021-01-26                      Ephemeron(3)