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Re: [MIT-Scheme-devel] weak hash table data

From: Chris Hanson
Subject: Re: [MIT-Scheme-devel] weak hash table data
Date: Tue, 22 Dec 2009 02:04:03 -0800

This is incorrect.  GC marks every table as needing rehash; the next
access to that table forces a rehash, which cleans the table.

On Mon, Dec 21, 2009 at 10:09 PM, Taylor R Campbell <address@hidden> wrote:
> Key-weak hash tables are never cleaned automatically, not even in a
> secondary GC daemon; unless you call HASH-TABLE/CLEAN! or cause them
> to be rehashed, broken entries will retain their data (but not their
> keys) strongly.
> (let ((a (make-eq-hash-table))
>      (b (make-eq-hash-table)))
>  (let ((p (list 'p))
>        (q (list 'q))
>        (r (list 'r)))
>    (hash-table/put! a p q)
>    (hash-table/put! b q r))
>  (gc-flip)
>  (list (hash-table/datum-list a)
>        (hash-table/datum-list b)))
> ;Value: (() ((r)))
> The manual doesn't claim that *associations* are deleted from weak
> hash tables if their keys are reclaimed -- it says only that the hash
> table holds its keys weakly --, but this is not the behaviour I expect
> for an object called a `weak hash table'.  Does this behaviour worry
> anyone else?
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