From: Stefan Monnier via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>
To: Eli Zaretskii <eliz@gnu.org>
Cc: ulm@gentoo.org, 73886@debbugs.gnu.org
Subject: bug#73886: 29.4; Confusing info about void function cells in Emacs Lisp manual
Date: Sun, 20 Oct 2024 12:56:03 -0400 [thread overview]
Message-ID: <jwv5xpm7mpz.fsf-monnier+emacs@gnu.org> (raw)
In-Reply-To: <86frorjqs5.fsf@gnu.org> (Eli Zaretskii's message of "Sun, 20 Oct 2024 08:18:34 +0300")
>> Oops. Looks like I missed this part when I changed it back around
>> Emacs-24.4:
>>
>> ** In 'symbol-function', nil and "unbound" are indistinguishable.
>> 'symbol-function' does not signal a 'void-function' error any more.
>> To determine if a symbol's function definition is void, use 'fboundp'.
>
> Could you explain the rationale for that change? I tried to look for
> relevant discussions around that date, but came up empty-handed.
I can't remember discussing it, no. It was a kind of "executive
decision".
Having a special "void" (`Qundefined`) non-value for the `symbol-value`
is needed for `boundp` since variables can contain *any* value, but not
for the `symbol-function` part where we can use any normal value (I
chose `nil`) to play this role as long as it doesn't collide with values
normally held in the `symbol-function` slot, like function names,
function values, cons cells, vectors, ...
The upside was a simplification in various chunks of code which used to
do things like `(and (fboundp SYM) (symbol-function SYM))` which can now
be simplified to `(symbol-function SYM)`.
I remember two "motivators", i.e. places where the need to pay attention
to the special void case annoyed me enough to look into this and make
the change, one was `nadvice.el` and the other was `cl-letf`.
[ So, it was no accident that the change happened in the same release as
the addition of `nadvice.el`. ]
In both cases the issue is that we want to deal with "places"
(generalized variables) and that abstraction works well for those places
which *always* contain a value, but not as well for those special places
that can be "unbound", so removing the "unbound" case from
`symbol-function` resulted in a welcome simplification.
For the same reason I dislike EIEIO's notion of `slot-boundp` and have
already considered marking it obsolete.
Stefan
next prev parent reply other threads:[~2024-10-20 16:56 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-10-19 14:37 bug#73886: 29.4; Confusing info about void function cells in Emacs Lisp manual Ulrich Müller
2024-10-19 17:45 ` Eli Zaretskii
2024-10-20 2:12 ` Stefan Monnier via Bug reports for GNU Emacs, the Swiss army knife of text editors
2024-10-20 5:18 ` Eli Zaretskii
2024-10-20 16:56 ` Stefan Monnier via Bug reports for GNU Emacs, the Swiss army knife of text editors [this message]
2024-10-27 11:17 ` Eli Zaretskii
2024-10-27 13:40 ` Ulrich Müller
2024-10-27 14:08 ` Eli Zaretskii
2024-10-27 14:59 ` Stefan Monnier via Bug reports for GNU Emacs, the Swiss army knife of text editors
2024-10-27 16:50 ` Eli Zaretskii
2024-10-27 17:10 ` Stefan Monnier via Bug reports for GNU Emacs, the Swiss army knife of text editors
2024-10-27 17:20 ` Eli Zaretskii
2024-10-27 18:53 ` Stefan Monnier via Bug reports for GNU Emacs, the Swiss army knife of text editors
2024-10-19 19:08 ` Andreas Schwab
2024-10-19 21:07 ` Ulrich Müller
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