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Weak hash table for attaching extra data to an object
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Date: 2007-08-14 (11:48)
From: Till Varoquaux <till.varoquaux@g...>
Subject: Re: [Caml-list] Weak hash table for attaching extra data to an object
I have a small library that could do the trick. It is built a a
hashtable whose keys  are weak, you need to recompact it from time to
time in order to free associated values. One strategy would be to
recompact when n values are freed.
I tried to limit the performance cost by storing collision values in a
separate table.
The code can be browsed online at:
There's a very small dependency on another file, i use Option.get
(defined as in extlib:
  let get = function
   | Some v -> v
   | None -> raise No_value
It is licensed under lgpl+special linking clause. If you need a more
permissive license let me know.
This can be downloaded via darcs:
darcs get http://till.varoquaux.free.fr/projects/mini_js/WeakHt.ml.html
or directly at:
There no mli file (I'm lazy).
Constructive criticism is welcome.
If you need automatic collection of values and have a hard time
implementing I can work on it.
P.S.: I wasn't aware of HWeak when I did this library, I don't know
how it compares performance wise. Having never had to profile this
library I actually don't know how bad it really is. This can certainly
also be worked on.
On 8/14/07, Thomas Fischbacher <tf@functionality.de> wrote:
> Richard Jones wrote:
> > I have a module which I can't edit[1].  This module, let's call it
> > Document, stores a representation of an XML tree in internal node
> > objects:
> >
> >   type intnode
> >
> > What I want to do is -- completely outside Document -- attach my own
> > extra information to these nodes.
> Ah, so the situation really is somewhat common, and not something
> just I am running into occasionally. (The problem in particular
> also arises occasionally when writing symbolic algebra code when one
> wants to attach meta-data (e.g. on typesetting) to terms without
> touching the underlying term representations in any way.)
> > The key point is that if the
> > garbage collector collects an intnode, then I want my extra
> > information to be garbage collected too.
> >
> > I wonder if anyone has done this, or can suggest a better way to solve
> > this problem?  The issue of attaching extra data to some existing
> > object would seem to be a fairly common one ...
> You might be able to "fake" a weak hash table by using a trick: for a
> key-value entry k1 -> v1, you take k1 and v1 and rebuild the outermost
> data structure to get k2 with k2 = k1 and k2 != k1 (oh hey, now that
> looks confusing, right?), as well as v2 = v1 and v2 != v1. Now, you
> enter k2 -> v2 into an ordinary hash table and register finalizers for
> k1 and v1 which remove this k2 -> v2 entry from your hash. That might
> do.
> --
> best regards,
> Thomas Fischbacher
> tf@functionality.de
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