(* * This file defines our persistent counter, which we use in the * CountPersistent command.
*)
(* * At its core, our persistent counter looks exactly the same as * our non-persistent counter (with a different name to prevent collisions):
*) let counter = Summary.ref ~name:"persistent_counter" 0
(* * The difference is that we need to declare it as a persistent object * using Libobject.declare_object. To do that, we define a function that * saves the value that is passed to it into the reference we have just defined:
*) let cache_count v =
counter := v
(* * We then use declare_object to create a function that takes an integer value * (the type our counter refers to) and creates a persistent object from that * value:
*) let declare_counter : int -> Libobject.obj = letopen Libobject in
declare_object
{
(default_object "COUNTER") with
cache_function = cache_count;
load_function = (fun _ -> cache_count);
classify_function = (fun _ -> Keep);
} (* * See Libobject for more information on what other information you * can pass here, and what all of these functions mean. * * For example, if we passed the same thing that we pass to load_function * to open_function, then our last call to Count Persistent in Count.v * would return 4 and not 6.
*)
(* * Incrementing our counter looks almost identical:
*) let increment () =
Lib.add_leaf (declare_counter (succ !counter)) (* * except that we must call our declare_counter function to get a persistent * object. We then pass this object to Lib.add_leaf.
*)
(* * Reading a value does not change at all:
*) let value () =
!counter
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