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Re: hash tables and GC
- To: gz%spt.entity.com@NSS.Cs.Ucl.AC.UK, firstname.lastname@example.org
- Subject: Re: hash tables and GC
- From: Jeff Dalton <jeff%aiai.edinburgh.ac.uk@NSS.Cs.Ucl.AC.UK>
- Date: Fri, 22 Jul 88 18:52:15 BST
- Cc: email@example.com, jeff <@NSS.Cs.Ucl.AC.UK:firstname.lastname@example.org>
> Date: Fri, 22 Jul 88 10:00:21 PDT
> From: John Rose <email@example.com>
> Sometimes that's "OK", sometimes not. For example, if a hash table is
> being used as a relational database (with a primary key indexed by the
> hash table), you probably don't want tuples therein to be GC-able.
True. There are cases where the "independent reference" is in the
user's head. Some value has been associated with a string key, say,
and it should remain in case the user types it again. So, I'm not
whether "weak retention" makes sense for EQUAL tables; but perhaps
it does nonetheless.
> Therefore, it's wise not to present them as hash tables.
I don't mind calling them something else.
> Similarly, if hash table keys are being used to store information,
> as well as merely access it, they shouldn't be GC-ed.
The question is whether you have any way of getting that key object.
If nothing has a pointer to X, there's no way to know X exists much
less that it's in some table. (I'm thinking EQ here.)
> Putting weak links to keys in hash tables would make the EQUAL semantics
> I proposed impossible, since an isomorphism test depends strongly
> on MAPHASH. (Or, before EQUAL is applied to test for isomorphism,
> normalize the two tables by performing a full GC! :@)
I'm willing to have "EQUAL uncertainty" for "weak tables" if it's
decided that EQUAL traverses such things at all.
> Weak pointers are probably more useful than EQUAL isomorphism tests,
> but a reliable MAPHASH also seems indispensible. Sounds to me
> like weakly-linked keys want to be another option to
Just so. I did not want all tables to be that way, just for it to
possible to have some that were.
Jeff Dalton, JANET: J.Dalton@uk.ac.ed
AI Applications Institute, ARPA: J.Daltonfirstname.lastname@example.org
Edinburgh University. UUCP: ...!ukc!ed.ac.uk!J.Dalton