This paper explains how to implement them and gives example code:

http://community.haskell.org/~simonmar/papers/weak.pdf


I'm not aware of a package that does it for you, but I implemented one as part 
of my own work so that may be able to provide another example.


Cheers,
-Trev


On 11/08/2013, at 6:03 AM, Alessandro Vermeulen <a.vermeu...@students.uu.nl> 
wrote:

> There is supposed to be some weak hash map implementation somewhere.
> 
> However, if you can't find it you can easily create it yourself. However, you 
> will need to use the map in IO so I suggest that you use the hashtables[1] 
> package in combination with the weak pointers mentioned by Erik.
> 
> - Alessandro
> 
> [1]: http://hackage.haskell.org/package/hashtables
> 
> On 10 aug. 2013, at 21:08, Erik Hesselink <hessel...@gmail.com> wrote:
> 
>> I'm not sure, but there are weak pointer [0], though I have never used them.
>> 
>> Erik
>> 
>> [0] 
>> http://www.haskell.org/ghc/docs/latest/html/libraries/base/System-Mem-Weak.html
>> 
>> On Sat, Aug 10, 2013 at 7:13 PM, Aleksey Uymanov <s9gf4...@gmail.com> wrote:
>>> Hello, haskellers.
>>> 
>>> Is there any package implementing magic hash tables weak in value? I
>>> mean when the value is garbage collected, then this key+value
>>> authomatically removes from the hash table.
>>> 
>>> --
>>> Aleksey Uymanov <s9gf4...@gmail.com>
>>> 
>>> _______________________________________________
>>> Haskell-Cafe mailing list
>>> Haskell-Cafe@haskell.org
>>> http://www.haskell.org/mailman/listinfo/haskell-cafe
>> 
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