Re: hash tables and GC

[Jeff Dalton <jeff%aiai.edinburgh.ac.uk@NSS.Cs.Ucl.AC.UK>]

> From: goldman@edu.isi.vaxa
> Subject: hash tables and GC
> Date: Thu, 21 Jul 88 09:33:27 PDT

> > Apropos of hash tables, one feature of Pop(Log) that I sometimes want
> > to have is "temporary properties".  They are essentially hash tables
> > such that being in them does not prevent being garbage collected. 

> I'm not sure just what you mean by "being in" a hash table.  I have
> commonly seen the following case -- is it what you have in mind?
> 
> I have an EQ hash table H.  My program will never apply the MAPHASH accessor
> to H.  Therefore, the fact that H is accessible to my program does NOT
> imply that any of the keys or values are accessible.  If the pair [K V]
> is in the table, then if K is independently accessible, V is also
> accessible.  But if K is not independently accessible, it is garbage, and
> so is V unless V is independently accessible.

Yes, that's what I had in mind, except for one thing:

Even if you do a MAPHASH, there is no way for you to tell K should be
in the table (or even that it exists) unless you have independent
access to it.  So, the possibility of MAPHASH would still allow K
and V to be removed.  Of course, you could gather various statistics
with MAPHASH that would change if K were removed (e.g., the number of
keys would decrease), but I think that's OK.

Indeed, T had something called "populations" (now called "weak sets")
that did not prevent GC of their elements and that supported an
operation call walk-population.  They are somewhat less useful that
what I have in mind, though, because all you can ask is whether
something's in the population or not (i.e., whether it's a K --
there's no associated V).

-- Jeff