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Re: expose XHASH [patch]

From: Stefan Monnier
Subject: Re: expose XHASH [patch]
Date: Thu, 31 Mar 2016 17:52:51 -0400
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/25.1.50 (gnu/linux)

> Please don't write that the example is stupid, it is just an
> example after all.

FWIW, I have bumped into various cases where I also needed to define
a hash for an equality function that's defined for example as (and (eq
(car x) (car y)) (eq (cdr x) (cdr y))).  Worse yet: you say that "sxhash
_will_ work" but in reality it won't if the objects are later modified
by side-effects.

See for example:

               ;; FIXME: Since the fields of `generic' are modified, this
               ;; hash-table won't work right, because the hashes will change!
               ;; It's not terribly serious, but reduces the effectiveness of
               ;; the table.
               (gethash (cons generic methods)

from cl-generic.el.  I wrote "It's not terribly serious", but IIRC there
can be cases where it causes real problems.

So a big "yes please" from me.


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