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

[Stefan Monnier]

>>> 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