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bug#5971: 23.1.95; `delete' modifies default value instead of buffer-loc

From: Drew Adams
Subject: bug#5971: 23.1.95; `delete' modifies default value instead of buffer-local value
Date: Mon, 19 Apr 2010 10:03:09 -0700

> > Nothing in the doc states that they share list structure.
> There is no buffer-local value before you set it.

Yes, I know that. And I noted that explicitly.
> The doc is pretty clear that a buffer-local value only starts to exist
> the first time it is set:
> Make VARIABLE become buffer-local whenever it is set.
> At any time, the value for the current buffer is in effect,
> unless the variable has never been set in this buffer,
> in which case the default value is in effect.
>                   ^^^^^^^^^^^^^^^^^^^^^^^^^^

Yes, and I explictly said that too. Please read what I wrote.

I am not questioning the fact that the default value is used if no buffer-local
value is set. And that behavior is adequately documented.

What is _not_ documented AFAICT is that using `delete' to set the buffer-local
value in buffer X also modifies the default value. There is no reason to expect
that, from reading the doc.

Once the default value has been modified, it is clear that buffer Y will (and
should) pick it up if you have not yet set a buffer-local value in Y. What is
not clear is why the _default_ value gets modified, just by setting the
_buffer-local_ value (in X).

And as I noted, using `nconc' (which is destructive like `delete') does not have
this effect. The same code uses `nconc' to set the buffer-local value in X, and
that does not also modify the default value.

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