The operation of default-value

The operation of default-value

[Heime]

I am struggling to understand the operation of "default-value".

The documentation states that

"This is the value that is seen in buffers that do not have their own values
for this variable.  The default value is meaningful for variables with
local bindings in certain buffers."

Take for instance the variable "mode-line-format".  When do buffer variables 
have their own values, and when do they not have them?

How can one figure out what has been set or not (whether buffers have variable
values or not)?.

RE: [External] : The operation of default-value

[Drew Adams]

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> The documentation states that
> 
> "This is the value that is seen in buffers that
> do not have their own values for this variable.
> The default value is meaningful for variables
> with local bindings in certain buffers."

Good to see you are looking at the doc now!

Did you read the nodes previous to that one,
in the same chapter (Buffer-Local Variables)?
I think they answer your questions.

> When do buffer variables have their own values,
> and when do they not have them?

I think node `Creating Buffer-Local' explains this. 

If you don't create/set a buffer-local value in
buffer A for a variable then its global, "default"
value applies in buffer A.  It's as simple as that. 

> How can one figure out what has been set or not (whether buffers have
> variable values or not)?.

Check whether (buffer-local-value THE-VAR) is the
same as (default-value THE-VAR).

Again, this is explained in the node that precedes
the node you quoted from.

I suggest starting at the beginning of chapter
Buffer-Local Variables, and reading from there.

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RE: [External] : The operation of default-value

[Heime]

------- Original Message -------
On Thursday, December 1st, 2022 at 11:16 PM, Drew Adams <drew.adams@oracle.com> wrote:


> > The documentation states that
> > 
> > "This is the value that is seen in buffers that
> > do not have their own values for this variable.
> > The default value is meaningful for variables
> > with local bindings in certain buffers."
> 
> 
> Good to see you are looking at the doc now!
> 
> Did you read the nodes previous to that one,
> in the same chapter (Buffer-Local Variables)?
> I think they answer your questions.
> 
> > When do buffer variables have their own values,
> > and when do they not have them?
> 
> 
> I think node `Creating Buffer-Local' explains this.
> 
> If you don't create/set a buffer-local value in
> buffer A for a variable then its global, "default"
> value applies in buffer A. It's as simple as that.

Meaning that a package can change the buffer-local variable,
with the default-value being different.  Thusly, the variable
used in the buffer is either the default value when the local
value is nil, or the local value when they differ.

Meaning that I cannot rely completely on default-value, because
the local value might be set up differently.

 
> > How can one figure out what has been set or not (whether buffers have
> > variable values or not)?.
> 
> 
> Check whether (buffer-local-value THE-VAR) is the
> same as (default-value THE-VAR).
> 
> Again, this is explained in the node that precedes
> the node you quoted from.
> 
> I suggest starting at the beginning of chapter
> Buffer-Local Variables, and reading from there.

RE: [External] : The operation of default-value

[Drew Adams]

> > I think node `Creating Buffer-Local' explains this.
> >
> > If you don't create/set a buffer-local value in
> > buffer A for a variable then its global, "default"
> > value applies in buffer A. It's as simple as that.
> 
> Meaning that a package can change the buffer-local variable,
> with the default-value being different.

A package, or anything else, can set a buffer-local
value, yes.

> Thusly, the variable used in the buffer is either
> the default value when the local
> value is nil, or the local value when they differ.

You can think of it being a different variable, yes.
Or you can think of it as being the same variable,
with both (1) a default value and (2) a buffer-local
value; that is, with two kinds of _bindings_.  The
doc generally speaks of it in this second way:

  “buffer-local” bindings, which apply only in one
   buffer.  Having different values for a variable
   in different buffers

But it also talks about "buffer-local variables" (not
just buffer-local variable bindings):

   A buffer-local variable has a buffer-local binding
   associated with a particular buffer.

"Buffer-local variable" refers to the buffer-local
binding of a variable.

`setq-local' sets the buffer-local value for the
current buffer.  So does `setq', if the variable
has already been made buffer-local in the buffer.

`make-local-variable' makes a variable's value be
buffer-local in the current buffer.
`make-variable-buffer-local' makes a variable always
be buffer-local, in every buffer.

If you look at the definition of `setq-local' you'll
see that it's just this, since `make-local-variable'
conveniently returns the variable (symbol).

  (set (make-local-variable 'VARIABLE VALUE))

> Meaning that I cannot rely completely on default-value,
> because the local value might be set up differently.

As usual, your meaning isn't clear.  Specify what
you mean by "rely completely on".

Even when a variable has a buffer-local value, you
can always access -- get or set -- its default value.
Functions `default-value' and `setq-default' do that,
respectively.

If this explanation helps, I suggest you reread the
doc that covers this subject.  With the added
understanding you may get more out of it on a second
reading.  If this didn't help, hopefully someone else
can help better.  But you really owe it to yourself,
as well as those who try to help you, to make clearer
what you're asking.

Re: [External] : The operation of default-value

[Emanuel Berg]

Drew Adams wrote:

> `setq-local' sets the buffer-local value for the current
> buffer. So does `setq', if the variable has already been
> made buffer-local in the buffer.

`setq-local' - always local

`setq-default' - always global

`setq' - local if local is present, global if global is
present and local isn't, and if none are present, global,
unless the variable automatically becomes buffer-local when
set?

I agree, pretty confusing ...

-- 
underground experts united
https://dataswamp.org/~incal

Re: [External] : The operation of default-value

[Emanuel Berg]

Nope, it's like this:

`setq-local' - always local

`setq' - local if local is present;
         local if the var is auto-local;
         oterwise global 

`setq-default' - always global

-- 
underground experts united
https://dataswamp.org/~incal