bug#56110: 27+; switching from line-mode to char-mode

[Stefan Monnier via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

>> > Simply speaking I meant one has to figure out whether one wants to
>> > `add-function' to var or to 'var.
>> `add-function` expects a "place", like `setf`.
> Thanks, I had an error in my mental model, partly induced by the SYMBOL
> without quote vs. (local 'SYMBOL) with quote thing.

Right, that's because for the argument to `local`, we can actually
accept any expression (i.e. an rvalue) as long as it returns a symbol
(just like for `(setf (car EXP) ..)` where can accept an expression EXP
as long as it returns a cons-cell).

I did hesitate to restrict the syntax to (local SYMBOL) because of the
asymmetry it between "SYMBOL" for the global case and "(local 'SYMBOL)"
for the local case, but there is no technical reason to restrict it
this way.

> Do we want a let-like environment (`let-function'?) that does the same
> as `add-function' temporarily?

I'm not sure it's common enough to be worthwhile.  But if we do, then
we'd also want to add something for the `add/remove-hook` version of
the problem.


        Stefan