From: Stefan Monnier <monnier@iro.umontreal.ca>
To: "Lennart Borgman \(gmail\)" <lennart.borgman@gmail.com>
Cc: Emacs Devel <emacs-devel@gnu.org>
Subject: Re: Why is not end-of-defun-function buffer local?
Date: Sat, 08 Dec 2007 21:45:55 -0500 [thread overview]
Message-ID: <jwv63z8egth.fsf-monnier+emacs@gnu.org> (raw)
In-Reply-To: <475B48A7.2030509@gmail.com> (Lennart Borgman's message of "Sun, 09 Dec 2007 02:45:11 +0100")
>>> Looking at some code that is a bit older it looks like some of it uses
>>> make-local-variable where it is not needed since the variables in question
>>> are always buffer local. From that I draw the conclusion that the code in
>>> Emacs uses make-variable-buffer-local more often now. Is not that the case?
>>
>> make-variable-buffer-local has the following downsides:
>> 1 - it cannot be reverted.
>> 2 - it may be done too late.
>> 3 - when you see `setq' it's not obvious that the setting is buffer-local
>> unless you remember seeing the call to make-variable-buffer-local.
>> The second problem may also explain what you're seeing: some code may
>> set a variable before the make-variable-buffer-local gets run.
>> It's actually "common" to introduce bugs this way, because people see
>> "this is automatically buffer-local" in the C-h v info, so they just use
>> `setq' without realizing that the setq may occur before the package
>> gets loaded.
>> make-variable-buffer-local is not evil, but make-local-variable is much
>> tamer and more explicit, and it works just as well in most cases.
> Thanks, that was a good explanation. Why not add this to the doc string of
> make-variable-buffer-local?
Oh, and since I've been looking at the low-level code that handles
variable lookup and things like that, there's another reason:
make-variable-buffer-local has a very subtle semantics which requires
pretty ugly and debatable C code.
More specifically, the problem is to decide *when* to make a variable
buffer-local. I.e. Setting the variable via `setq' should make it
buffer-local, but setting it with `let' shouldn't. But
(let ((var 1))
(setq var 2))
should not make `var' buffer-local either, because the `setq' is
"protected" within a let. OTOH
(let ((var 1))
(with-current-buffer <otherbuf>
(setq var 2)))
should make `var' buffer-local in <otherbuf> unless the code is itself
run within a `let' which was itself done in <otherbuf>. Yuck!
So every `setq' on a variable that has been make-variable-buffer-local
may require walking up the current list of `let' bindings to decide
whether to make the variable buffer-local. Yup, that's right:
the (setq var 2) will take time proportional to the stack depth :-(
And in order to be able to walk up the stack and decide which let
binding might be relevant, the runtime representation of some
let-bindings requires an extra cons-cell, which is not used for
anything else.
Stefan
next prev parent reply other threads:[~2007-12-09 2:45 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2007-12-08 1:02 Why is not end-of-defun-function buffer local? Lennart Borgman (gmail)
2007-12-08 2:18 ` Stefan Monnier
2007-12-08 18:04 ` Lennart Borgman (gmail)
2007-12-08 20:16 ` Stefan Monnier
2007-12-09 1:45 ` Lennart Borgman (gmail)
2007-12-09 2:45 ` Stefan Monnier [this message]
2007-12-09 3:04 ` Lennart Borgman (gmail)
2007-12-09 13:02 ` martin rudalics
2007-12-10 5:13 ` Stefan Monnier
2007-12-11 12:53 ` martin rudalics
2007-12-11 14:58 ` Stefan Monnier
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