Re: question about "object"

[Per Bothner <per at bothner dot com>]

Doug Orleans <dougo@ccs.neu.edu> writes:

> Unfortunately, two lists which are equal? might not have the same hash 
> code:

Could you try the cvs version?  It seems to work there.

> Although it would be nice if hash codes were in fact equal for equal
> lists,

It is more than nice - it is required.

> I still need to override the equals method because the keys are
> actually lists of objects which might be circular data structures
> (which causes equal? to go into an infinite loop).

That may be more difficult.  In general, it might be quite expensive
to handle self-referential structures  correctly.  The general trick
is you keep a table of the objects you've seen, and when you see
an object in the table, you know you have a cycle.  For hashCode we
can handle circular lists by taking the hashCode of (say) the
first 10 car value.  Handling equals is trickier - it might be possile
to use a variant of the list-length algorithm.

> #|kawa:27|# (define ht (make <java.util.HashMap>))
> #|kawa:28|# (invoke ht 'put (hashtable-key '(x y z)) 'xyz)
> #!null
> #|kawa:29|# (invoke ht 'containsKey (hashtable-key '(x y z)))
> #f
> 
> Can anyone figure out what's going wrong here?  I'm not sure how to
> debug this.

Did you verify that:
  (equal? (hashtable-key '(x y z)) (hashtable-key '(x y z)))
and
  (invoke (hashtable-key '(x y z) 'equals (hashtable-key '(x y z))))
-- 
	--Per Bothner
per@bothner.com   http://www.bothner.com/~per/