Re: Why is not end-of-defun-function buffer local?

["Lennart Borgman (gmail)" <lennart.borgman@gmail.com>]

Stefan Monnier wrote:
>>>> 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.


Perhaps make-variable-buffer-local var could be treated like this:

1) When entering (let ((var 1)) make a buffer local copy of the variable 
just as if (make-local-variable 'var) was called before let.

2) When leaving (let ((var 1))...) delete the buffer local copy of the 
variable if it has the default value.

That is of course a slightly different semantic, but I wonder if it matters.

The advantage is that var could be treated just as if it was made buffer 
local with make-local-variable.

I might be misunderstanding something, of course, since I do not know 
this code.