Walking Hash Tables

[Jonathan A Rees <JAR@MIT-MC>]

    Date: Wed, 10 Apr 85 17:31:01 PST
    From: Scott Turner <srt at UCLA-LOCUS>

    Is there a way to walk a hash table in T2.8?  Is there a way to 
    specify a predicate other than EQ?.  Will these things be added to 
    T2.9?

["T-Users" is intended to be a one-way channel from implementors to
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Answers: internally in the implementation, but not released; no; and no.
You should check for (I think) TABLE-WALK in the implementation
environment; look at the source.

I didn't think it was a good idea to make "walking" part of the table
abstraction, since: (a) it isn't necessary (it can be simulated); (b) it
complicates the implementation & specification (there's too much lard in
the manual already); and (c) it is potentially pre-emptive of the
possibility of one day making tables be "weak," in the sense that an
entry goes away if its key becomes inaccessible.  Yes, it would be
possible to lock out GC's, make an internal list of the objects which
are keys, unlock GC's, and then apply the procedure to those keys, but
that seems like a lot of trouble for something that isn't even very
well-formed (how do you axiomatize "accessible"?).

To simulate WALK-TABLE, just keep, along with the table, a list of all
the keys.  I guess the disadvantage of this is that deletion can be
expensive; I'll think about it.

Jonathan