Re: elisp from CL transition guide

[Tassilo Horn]

Tamas Papp <tkpapp@gmail.com> writes:

Hi Tamas,

> One thing I don't understand yet is namespaces in Elisp: if I set
> fill-column in a buffer (setq fill-column 80), that doesn't seem to
> affect fill-column in other buffers.

Yes, `fill-column' is always buffer-local when set.

,----[ C-h v fill-column RET ]
| fill-column is a variable defined in `C source code'.
| Its value is 72
| Local in buffer *followup to Tamas Papp on gnu.emacs.help*; global value is 79
| Automatically becomes buffer-local when set in any fashion.
| 
| 
| This variable is safe as a file local variable if its value satisfies
| the predicate `integerp'.
| 
| Documentation:
| *Column beyond which automatic line-wrapping should happen.
| Interactively, you can set the buffer local value using C-x f.
`----

To set its default value for all buffers use `setq-default'.

> But if I set some other variable eg (setq foo 12), foo will evaluate
> to 12 in other buffers.

If you do

  (set (make-local-variable 'foo) 12)

I'll be twelve only in that buffer.

> What would be a correct mental model for this?

Some variables like `fill-column' always are buffer-local, but every
variable can be buffer-local.

See

,----[ (info "(elisp)Buffer-Local Variables") ]
| Global and local variable bindings are found in most programming
| languages in one form or another.  Emacs, however, also supports
| additional, unusual kinds of variable binding: "buffer-local"
| bindings, which apply only in one buffer, and "frame-local" bindings,
| which apply only in one frame.  Having different values for a variable
| in different buffers and/or frames is an important customization
| method.
`----

Bye,
Tassilo
-- 
If programmers deserve to  be rewarded for creating innovative programs,
by the same  token they deserve to be punished if  they restrict the use
of these programs. (Richard M. Stallman)