Hi,
2009/4/30 Stuart Sierra <the.stuart.sie...@gmail.com>:
>
> On Apr 29, 5:58 pm, Stu Hood <stuh...@gmail.com> wrote:
>> Instead of attaching the metadata directly to the object, what if the
>> metadata was stored outside the object, in a global map of {object metadata,
>> ...}? In order to handle garbage collection, something similar to Java's
>> WeakHashMap could be used, with the object itself as the key.
>
> Interesting idea.
>
> There might be performance penalities, depending on how often metadata
> is used. Metadata guarantees atomic updates (alter-meta!) so the
> WeakHashMap might need to be wrapped in a synchronized map, at a
> further penalty.
I guess this would not be sufficient: WeakHashMap are not side-effect
free: they mutate every time one of their key is removed :
"When a key has been discarded its entry is effectively removed from
the map, so this class behaves somewhat differently than other Map
implementations."
, making them improper to be used within STM transactions and such.
Regards,
--
Laurent
>
> I've never needed metadata on non-Clojure types. Do you have a use in
> mind?
>
> This could be a library, too, sort of like:
>
> (defn my-meta [object]
> (if (instance? clojure.lang.IMeta object)
> (meta object)
> (*global-metadata* object)))
>
> -SS
> >
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