[ cl-cookbook-Feature Requests-619966 ] Question/suggestion for hash-table page

Feature Requests item #619966, was opened at 2002-10-07 15:07
Message generated for change (Comment added) made by nobody
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Category: None
Group: None
Status: Open
Priority: 5
Submitted By: Robert P. Goldman (rpgoldman)
Assigned to: Nobody/Anonymous (nobody)
Summary: Question/suggestion for hash-table page

Initial Comment:
I stumbled across this while looking for an answer to a
poignant hash-table question.  Namely, how does one
make a hash-table whose entries are CLOS objects?  The
spec only requires that hash-tables accept eq, eql,
equal, equalp as tests.  Unfortunately, these are all
equivalent to EQ when applied to CLOS objects.  And
none are generic functions.  An answer to this question
would sure be helpful!

If I find a good one, I'll submit it.

Cheers,
R

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Comment By: Nobody/Anonymous (nobody)
Date: 2006-04-07 22:53

Message:
Logged In: NO 

Implementation dependant.  In CMUCL, do something like
(ext:define-hash-table-test 'foo #'foo-t #'foo-h), and then
(make-hash-table :test 'foo) makes a hash table that uses
(foo-t x y) to compare two items, and (foo-h x) to generate
the hash value.  [Of course, you have to make sure that
(foo-t x y) implies (= (foo-h x) (foo-h y))]

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Comment By: Robert P. Goldman (rpgoldman)
Date: 2002-10-08 07:15

Message:
Logged In: YES 
user_id=275595

I wasn't confusing CLOS objects with the ability to use
Generic Functions as a test in a hash table.  But there is
an obvious connection:  since none of the existing equality
tests are sufficient to test CLOS objects, a generic
function is a logical  way to implement an
equality/equivalence test for CLOS objects in a hash table.

Actually, it turns out that I'm NOT stuck.  For CMUCL, one
can make special-purpose hash tables using the (not
well-documented) function define-hash-table-test, which
allows you to provide a special hash-function/equality test
combination for new kinds of hash tables.

[I'm not sure what the situation is for Allegro, since I
don't have a license any more, but I'm pretty sure I was
able to make hash tables of objects there.]

The CMUCL solution is actually just the Right Thing, because
it allows you to provide two pretty simple pieces of
information, and then use the hash-table functions as they
are.  Much better than having to do all kind of monkeying
around with making lists out of objects, handling buckets
and collisions yourself, etc.

After all, if I wanted to write C, I could write C.  I'm
writing CL because (1) I need to do rapid prototyping and
experiment with different data structures and (2) my time is
more important than my Athlons'!  So, with all due respect,
*I* would prefer to be the judge of whether my hash table is
going to be too inefficient to be usable.  Not only do I
want to use CL instead of C, I want to use CL instead of
Pascal, the language that nags me! :-)

I could write up instructions about how to use hash-tables
for more ambitious things in CMUCL, but probably I would be
better off offering to help fix the CMUCL documentation for
this.

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Comment By: Nobody/Anonymous (nobody)
Date: 2002-10-07 23:16

Message:
Logged In: NO 

I think you might be confusing CLOS objects with the ability to use GFs, so I want to ask: do you want a 
hash-table whose equality test is a GF, or a table which distinguishes CLOS objects by some means other than 
the four equality tests you (correctly) listed?

If it's the first of these, then you're stuck. If it's the other you could write some code to extact the information you 
want out of a CLOS structure into - say - a list in such a form that equal would serve you.

Hash-tables have to be restricted in this way, otherwise they'd be too inefficient to be usable. Maybe you shouldn't 
be using a hash-table?

What are you trying to achieve? What would your equality test look like?

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