bug#62117: 29.0.60; cl-letf on a map place has side-effects

[Augusto Stoffel <arstoffel@gmail.com>]

On Sun, 12 Mar 2023 at 01:00, Michael Heerdegen wrote:

> Augusto Stoffel <arstoffel@gmail.com> writes:
> This is a symptom of a general limitation of `cl-letf'.  Currently you can't
> rely on a "no side effect" behavior.  There are other examples like that
> (`alist-get') and cases that are worse (binding `buffer-local-value' of
> a variable in a buffer with no buffer local binding doesn't remove the
> buffer-localness - that's one reason why that gv had been deprecated).

Right, so let's drop this bug.

>> Of course it's usual to treat a nil entry and no entry as equivalent in
>> Lisp, but this behavior can be a problem e.g. when constructing data to
>> pass to other programs.
>
> I would say: if it is a problem, map.el is the wrong abstraction for
> your case.  That's the genuine idea of map.el: that the inner structure
> of a map doesn't matter.

This is not right.  map.el doesn't abstract away the nil entries.  They
make a difference all over, e.g. in map-length, map-do, map-empty-p...