[plt-scheme] Weak hash tables

From: Robby Findler (robby at cs.uchicago.edu)
Date: Thu Sep 9 13:20:08 EDT 2004

Weak hash tables are weak in the keys, not the values (use weak-boxes
if you need the values to be held onto weakly). Probably you meant to
run this test:

  (define wht (make-hash-table 'weak))
  (define strong "strong")
  (hash-table-put! wht strong strong)
  (hash-table-put! wht "weak" "weak")
  (hash-table-for-each wht
                       (lambda (k v)
                         (display (format "key: ~a value: ~a\n" k v))))

which prints like this for me:

  key: strong value: strong

even with the conservative collector.

(I'm not sure when interned symbols are collected. I think Matthew may
have posted something about that here a while back.)


At Thu, 9 Sep 2004 10:06:24 -0700, Don Felgar wrote:
>   For list-related administrative tasks:
>   http://list.cs.brown.edu/mailman/listinfo/plt-scheme
> I expected this:
>   (define wht (make-hash-table 'weak))
>   (define strong "bar")
>   (hash-table-put! wht 'strong strong)
>   (let ((foo "value"))
>     (hash-table-put! wht 'key foo))
>   (collect-garbage)
>   (hash-table-for-each wht
>                        (lambda (k v)
>                          (display (format "key: ~a value: ~a\n" k v))))
> to print:
>   key: strong value: bar
> but it prints:
>   key: strong value: bar
>   key: key value: value
> I presume this is an artifact of conservative gc.
> I wrote the above test to determine if a weak hash table shrinks
> during gc, as opposed to accumulating k/v pairs with a #f value.  If
> the latter case is true I could periodically purge the hash table, but
> I guess adding my own garbage collector wouldn't be much harder.
> Any thoughts?
> --Don

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