Subject: Re: bug#39529: 28.0.50; Metahelp does not contain help text

From: Paul Eggert <[email-masked]>
Date: Tue, 18 Feb 2020 17:21:58 -0800

On 2/18/20 11:56 AM, Pip Cet wrote:

> if we rely on the precise
> definitions of XHASH etc. that were in place prior to the January
> changes, no "quick fix" was needed; it was simply the case that the
> predicate used by hash tables created with :test equal diverged from
> the built-in `equal' predicate in one more way than was already
> documented. (Those documented deviations are that mutations of hash
> keys won't cause rehashing, and that equal will sometimes signal for
> (equal a b) but not for (equal b a)).

I'm not aware of these deviations - where are they documented?

What I see in the Emacs 27 documentation, under "Defining Hash 
Comparisons", is the statement that the hash and equality functions' 
"behavior should depend on only on properties of the keys that do not 
change". This is true of 'equal' and 'sxhash-equal' if one does not 
mutate any key that's in the hash table. So from what I can see, the 
predicates are supposed to be the same, it's just that one can't mutate 
keys.

Also, I don't understand what is meant by "the predicate used by hash 
tables created with :test equal diverged from the built-in `equal' 
predicate". cmpfn_equal simply calls Fequal, so how can their behaviors 
diverge?

>>> The pure-cons hash, and many other places, assume "equal" means
>>> "equivalent" in some way. That's not true for bytecode objects, where
>>> a function always returning `nil' can be equal to one always returning
>>> `t'.
>>
>> Could you give an example of this? When I byte-compiled this:
> 
> (defun f () (let ((x '(#1=(nil) . #1#)))
>            (eq (car x) (cdr x))))
> (defun g () (let ((x '((nil) . (nil)))
>            (eq (car x) (cdr x))))
> 
> That's somewhat contrived; more realistic examples where this might
> actually be a problem would use string properties.

Although this example is a good one that illustrates a bug (or at least 
a poorly-documented feature) in dumping, it hasn't changed because of 
the January 7 changes to sxhash. The same issue exists in Emacs 27 (and 
in Emacs 26 for that matter). So I'd rather address this issue 
separately, perhaps simply by documenting it for now.

> I'll propose one more thing, which sounds horrible at first but might
> be the least horrible option: accept that `equal', the function, and
> `:test equal', the hash table predicate, have diverged significantly
> already.
Again I'm not following, because the two functions haven't diverged as 
far as I can see.