[Caml-list] Lazy recomputing

[Hans Ole Rafaelsen <hans@simula.no>]

Hi,

I have a hash table which contains values, of which some are dependent on 
other values in the table.  I store the contents as lazy values. The problem 
I'm having is that once a dependent value have been computed, it will not get 
recomputed when the values it depends on changes.

let _ =
  List.iter
    (fun (k,v) -> Hashtbl.add tbl k v)
    [
      ("a", lazy 1);
      ("c", lazy ((Lazy.force (Hashtbl.find tbl "b")) + (Lazy.force 
(Hashtbl.find tbl "a"))));
      ("b", lazy 4);
    ];;

# let _ = Lazy.force (Hashtbl.find tbl "b");;
- : int = 4
# let _ = Lazy.force (Hashtbl.find tbl "c");;
- : int = 5
# let _ = Hashtbl.replace tbl "b" (lazy 6);;
- : unit = ()
# let _ = Lazy.force (Hashtbl.find tbl "b");;
- : int = 6
# let _ = Lazy.force (Hashtbl.find tbl "c");;
- : int = 5
# 

Is there some trick to get it recomputed, other that keep a list of all 
dependent values, and "refresh" the table whenever some value is updated?
From the documentation of Lazy it does not seem that such behavior is 
supported.

-- 
Hans Ole Rafaelsen

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