bug#73538: [PATCH] Add notifications support to 'mpc'

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

>>> This adds support for displaying a notification when the song changes.
>> Oh, wow!  A patch for `mpc.el`!  Thanks!
> I hope you won’t mind a few more.

🙂

>>> +  (when-let (((string= "play" (alist-get 'state mpc-status)))
>>> +             (title (or (alist-get 'Title mpc-status) "Unknown Title"))
>>> +             (body (or (alist-get 'Artist mpc-status)
>>> +                       (alist-get 'AlbumArtist mpc-status)
>>> +                       "Unknown Artist"))
>>
>> Comment: I think it would make sense to use `mpc-format` here with
>> Custom vars to specify what goes in the title and body.
>
> What do you think the custom vars should look like?
> Would it be a FORMAT-SPEC like “%{title} %5{duration}” or something else?

I don't have a strong opinion on what the default format should look like.

> How would it handle the case where you want to try different tags like
> “use Artist or AlbumArtist”?

Ah... Hmm... good point.  We don't support that currently in
`mpc-format`.  Maybe we should add support for this (and take the
opportunity to actually document the % thingies understood by `mpc-format`).

Currently, the main format is `%WIDTH{NAME-POST}` where `-POST` is
optional, and it means to use the empty string if NAME is absent or
else use the concatenation of the content of NAME with the string POST.
Currently `-POST` is used only for `%{Disc--}`.

Maybe we should simply extend that to something like `%{NAME1|NAME2|...-POST}`?

Not sure whether it's worth adding support for a "default string", nor
what format we could use for that.  Maybe simply make it so that if
there's a | then the last element is not a tag name but a literal string
to use, so we could use:

    %{Artist|AlbumArtist|Unknown Artist}

Of course we could combine the two into something like
`%{NAME-POST+DEFAULT}` where DEFAULT would be recursively parsed as
a format string so we could do

    %{Artist+%{AlbumArtist+Unknown Artist}}

but I think this is getting too fancy for my taste (at that point, an
S-exp based format would start looking pretty attractive).


        Stefan