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Re: new `obarray` type

From: Alan Mackenzie
Subject: Re: new `obarray` type
Date: Tue, 14 Mar 2017 20:14:14 +0000
User-agent: Mutt/1.7.2 (2016-11-26)

Hello, Stefan.

On Tue, Mar 14, 2017 at 08:52:59 -0400, Stefan Monnier wrote:
> > It's not more palatable to this user.  It sounds more like "dumbing
> > down".  There are few things more frustrating whilst debugging than
> > having Emacs obfuscating information "for my own good".

> The good thing about having the `obarray` type is that e get to choose
> how to print them.

> I can easily make it so it prints all the symbols in it, would you
> prefer that?

Yes, please!

> > Indeed, why not just print _all_ vectors by printing only their size?

> The reason why the patch I sent only prints the number of entries is
> that currently the way obarrays are printed you only get to see some of
> the symbols but not all.  But I guess you're right: it would be more
> useful to list all the symbols in it.
> [ Time passes... Done!  ]

Apologies for that.  I wasn't really aware that (currently) printing an
obarray only displays some of the symbols.

> > Not rarely, particularly in CC Mode, I will be dealing with obarrays
> > with relatively small numbers of symbols.

> Regardless of what we decide to do with obarrays, I strongly recommend
> you change cc-mode to use hash-tables instead.  My experience with EIEIO
> (where I "recently" moved from obarrays to hash-tables) is that it's
> measurably faster and the code tends to be clearer (tho that's clearly
> subjective).

There are currently four uses of (make-vector LENGTH 0) in CC Mode, at
least one of which, possibly two, genuinely deal with Lisp symbols.
Converting those to hash-tables would probably be a net loss, though
converting the ones which just use obarrays as a string look-up would
surely gain.

> > Of course I want to see these symbols' names when I ask for that
> > obarray to be printed.

> With hash-tables, you'll them see all, properly printed and even `read`able!

> > I'm also not in favour of introducing another vector-like type without a
> > very good reason.

> Obarrays are very weird, currently, because they combine "plain vectors"
> and "plain symbols" in a tricky way.
> - Have you ever tried to do `aref` on an obarray?
> - The printout lists some of the symbols, but not all.  Which ones appear
>   is arbitrary, unpredictable.
> - Have you ever tried to put something else than 0 in an obarray slot?
> - An obarray can lead to unexpected space behavior:

>     (let* ((o (obarray-make))
>            (s1 (intern "s1" o))
>            (s2 (intern "s2" o)))
>       (set s1 (make-list 100000 t))
>       s2)

>   might leave you with a 10000-element list preserved as "reachable" as
>   long as `s2` is reachable, even tho it's clearly not reachable any more.

> BTW, my patch doesn't address this GC problem yet.

> > positions.  Would a new obarray type prevent any vector operations being
> > carried out on it, should any package do such things?  If so, that would
> > be a Bad Thing.

> Currently, I haven't changed `aref` to work on obarrays, no.  I've never
> ever seen code try to do that (I guess in theory it could be potentially
> useful, tho I can't think of any operation you could implement reliably
> using `aref` on obarrays would be `obarray-empty-p`).

> I was planning on allowing `aset` in case some package uses it to do the
> equivalent of `clrhash`, but I haven't yet found any package doing that,
> so I haven't bothered either.

> > This change would create hassle in general for many packages, all of
> > which create obarrays with (make-vector LENGTH 0), and would need
> > changing to use `make-obarray'.

> Of course, (make-vector LENGTH 0) still works.  And there's
> obarray-make, introduced in Emacs-25, IIRC.  But even if we deprecate
> (make-vector LENGTH 0) you won't get any byte-compilation warning for it
> since we can't detect whether a (make-vector LENGTH 0) is meant as an
> obarray or as a normal array that happens to be filled with zeroes.

You could prohibit symbol operations on the "wrong" type of vector.

> > It would mean having to write yet more compatibility macros (for the
> > inevitable day when old style obarrays get removed from Emacs).

> To the extent that we can't detect when make-vector is used for an
> obarray, I expect that it'll take *many* years until we can drop support
> for "old-style" obarrays, so I wouldn't worry about it.
> I suspect that even cc-mode will have switch to hash-tables before
> support for old-style obarrays is dropped.


Thanks for this reply.  I withdraw my objections to the new obarray

>         Stefan

Alan Mackenzie (Nuremberg, Germany).

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