How can I change a buffer-local glyphless char table?

How can I change a buffer-local glyphless char table?

[Nick Helm]

Occasionally, I find it useful to use
glyphless-char-display-control to visualise normally invisible
characters. I do this with something like:

  (update-glyphless-char-display 'glyphless-char-display-control
    '((c0-control     . empty-box)
      (c1-control     . empty-box)
      (format-control . empty-box)
      (no-font        . hex-code)))

and turn it off again with:

  (update-glyphless-char-display 'glyphless-char-display-control
    (eval (car (get 'glyphless-char-display-control
                    'standard-value))))

However this has a global effect. I would like it to only act on the
current buffer. I've tried creating a buffer-local char table with:

  (make-local-variable 'glyphless-char-display)

and repeating the call to update-glyphless-char-display, but it
affects both local and global values regardless.

If I make glyphless-char-display buffer-local and tweak its value
directly, it works fine.

Any ideas how I can do this more easily?

Re: How can I change a buffer-local glyphless char table?

[Eli Zaretskii]

> From: Nick Helm <nick@tenpoint.co.nz>
> Date: Wed, 06 Jun 2018 14:27:59 +1200
> 
>   (update-glyphless-char-display 'glyphless-char-display-control
>     (eval (car (get 'glyphless-char-display-control
>                     'standard-value))))
> 
> However this has a global effect. I would like it to only act on the
> current buffer. I've tried creating a buffer-local char table with:
> 
>   (make-local-variable 'glyphless-char-display)
> 
> and repeating the call to update-glyphless-char-display, but it
> affects both local and global values regardless.
> 
> If I make glyphless-char-display buffer-local and tweak its value
> directly, it works fine.

What do you mean by "tweak its value directly", and how does that
differ from the above tweaking?

Re: How can I change a buffer-local glyphless char table?

[Stefan Monnier]

>   (make-local-variable 'glyphless-char-display)

This makes the variable buffer-local but its value is still the same
array object, so update-glyphless-char-display will still update the
same object shared by all buffers.

> Any ideas how I can do this more easily?

You might try

    (set (make-local-variable 'glyphless-char-display)
         (copy-sequence glyphless-char-display))


-- Stefan

Re: How can I change a buffer-local glyphless char table?

[Nick Helm]

On Thu, 07 Jun 2018 at 02:50:32 +1200, Eli Zaretskii wrote:

>> From: Nick Helm <nick@tenpoint.co.nz>
>> Date: Wed, 06 Jun 2018 14:27:59 +1200
>> 
>>   (update-glyphless-char-display 'glyphless-char-display-control
>>     (eval (car (get 'glyphless-char-display-control
>>                     'standard-value))))
>> 
>> However this has a global effect.
>>
>> If I make glyphless-char-display buffer-local and tweak its value
>> directly, it works fine.
>
> What do you mean by "tweak its value directly", and how does that
> differ from the above tweaking?

As an experiment, I changed the value of glyphless-char-display as
described above, copied the new value from C-h v, restored the default,
then did setq-local with the copied value in the target buffer.

Re: How can I change a buffer-local glyphless char table?

[Nick Helm]

On Thu, 07 Jun 2018 at 05:45:29 +1200, Stefan Monnier wrote:

>>   (make-local-variable 'glyphless-char-display)
>
> This makes the variable buffer-local but its value is still the same
> array object, so update-glyphless-char-display will still update the
> same object shared by all buffers.

Yes of course, I see now. Thanks.

>> Any ideas how I can do this more easily?
>
> You might try
>
>     (set (make-local-variable 'glyphless-char-display)
>          (copy-sequence glyphless-char-display))

Yes, this works well.

Thanks again.