bug#30056: 25.3; battery-mode-line-string missing leading space

[Allen Li <darkfeline@felesatra.moe>]

Lars Ingebrigtsen <larsi@gnus.org> writes:

> Allen Li <vianchielfaura@gmail.com> writes:
>
>> The cause of this odd behavior is that display-battery-mode appends
>> and removes its symbol in global-mode-string when it is toggled on or
>> off, while display-time-mode only appends its symbol and does not
>> remove it when it is toggled off.  The reason display-battery-mode
>> comes first after Emacs starts is because of how the custom file
>> works; user options are sorted alphabetically and display-battery-mode
>> comes first, so it is appended first.
>>
>> Naturally, this has some implications for whether each display uses
>> leading, trailing, or no space.
>
> Indeed.  Just a random idea: Would it make sense to add a mode line
> construct like "%S" to mode-line-format that means "put a space here if
> there isn't one already"?  Then battery could put "%S<current string"
> into the list?
>
> Would that work?

I find that idea intriguing.  It would work, and it's a useful feature
to have as an Emacs Lisp developer, because it provide a robust solution
to the problem of "I want to have a space separation in the mode line,
but I don't know what comes before/after me".

Although personally, I feel a little dirty adding a new %-construct just
for this.