Ephemeron(3o) OCaml library Ephemeron(3o)
NAME
Ephemeron - Ephemerons and weak hash tables.
Module
Module Ephemeron
Documentation
Module Ephemeron
: sig end
Ephemerons and weak hash tables.
Ephemerons and weak hash tables are useful when one wants to cache or
memorize the computation of a function, as long as the arguments and
the function are used, without creating memory leaks by continuously
keeping old computation results that are not useful anymore because one
argument or the function is freed. An implementation using Hashtbl.t is
not suitable because all associations would keep the arguments and the
result in memory.
Ephemerons can also be used for "adding" a field to an arbitrary boxed
OCaml value: you can attach some information to a value created by an
external library without memory leaks.
Ephemerons hold some keys and one or no data. They are all boxed OCaml
values. The keys of an ephemeron have the same behavior as weak point-
ers according to the garbage collector. In fact OCaml weak pointers are
implemented as ephemerons without data.
The keys and data of an ephemeron are said to be full if they point to
a value, or empty if the value has never been set, has been unset, or
was erased by the GC. In the function that accesses the keys or data
these two states are represented by the option type.
The data is considered by the garbage collector alive if all the full
keys are alive and if the ephemeron is alive. When one of the keys is
not considered alive anymore by the GC, the data is emptied from the
ephemeron. The data could be alive for another reason and in that case
the GC will not free it, but the ephemeron will not hold the data any-
more.
The ephemerons complicate the notion of liveness of values, because it
is not anymore an equivalence with the reachability from root value by
usual pointers (not weak and not ephemerons). With ephemerons the no-
tion of liveness is constructed by the least fixpoint of: A value is
alive if:
-it is a root value
-it is reachable from alive value by usual pointers
-it is the data of an alive ephemeron with all its full keys alive
Notes:
-All the types defined in this module cannot be marshaled using out-
put_value or the functions of the Marshal module.
Ephemerons are defined in a language agnostic way in this paper: B.
Hayes, Ephemerons: A New Finalization Mechanism, OOPSLA'97
Since 4.03.0
module type S = sig end
The output signature of the functor Ephemeron.K1.Make and
Ephemeron.K2.Make . These hash tables are weak in the keys. If all the
keys of a binding are alive the binding is kept, but if one of the keys
of the binding is dead then the binding is removed.
module type SeededS = sig end
The output signature of the functor Ephemeron.K1.MakeSeeded and
Ephemeron.K2.MakeSeeded .
module K1 : sig end
Ephemerons with one key.
module K2 : sig end
Emphemerons with two keys.
module Kn : sig end
Emphemerons with arbitrary number of keys of the same type.
module GenHashTable : sig end
Hash tables on generic containers with notion of death and aliveness.
OCamldoc 2023-02-12 Ephemeron(3o)
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