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How to get a list of tracks current playlist, then play one? (or, why do

From: hugo
Subject: How to get a list of tracks current playlist, then play one? (or, why doesn't my attempt work)?
Date: Sun, 19 Dec 2021 14:46:05 -0000 (GMT)
User-agent: SquirrelMail/1.4.8-4.0.1.el5.centos.2

I'm writing a consult-based interface for EMMS. consult--read is a
function much like completing-read. I want to write a function which
presents the current playlist in a completing-read like interface, and the
jumps to the track the user selects. I intend this with embark actions for
more operations (moving/killing tracks etc.)

So, I need to get a list of tracks in the current playlist, in such a way
that I can then write a function which plays the selected one. The basic
data structure of EMMS is a buffer with a list of tracks, so I decided to
get an alist of track names and the value of point where that track is
(this makes it pretty simple to write a function which visits the right
bffer, goes to that char, then plays the track at point). I have this:

 (lambda (cell) (null (car cell)))
 (with-current-buffer emms-playlist-buffer
     (cl-loop initially do (goto-char (point-min))
              until (eobp)
              collect (cons
                       (emms-track-get (emms-playlist-track-at) 'info-title)
              do (next-line)))))

Breaking this down a bit, the idea:
- in the current playlist buffer (`(with-current-buffer
- goto to the begining (`initially do (goto-char (point-min))`)
- push a cons of the track title and value of point into a list, then go
to the next line (`collect (cons (emms-track-get (emms-playlist-track-at)
'info-title) \n do (next-line)`)
- repeat until end of buffer (`until (eobp)`)
- Finally, return resuling alist, and remove any cells with a null car
(`(cl-remove-if (lambda (cell) (null (car cell)))`. This deals with lines
in the playlist which are not actually tracks (e.g. lines with album art).

This works -- mostly. For some reason, I can run it and get the list I
expect (of say, the 173 tracks I have in my current playlist right now),
then run it again and get a much shorter list (of say 97 songs), alway a
subset of the 'correct' list. In cases of the first kind, my function for
playing the selected song (goto the char, then play the track at point)
always works, but in the second case (innacurate list) it often plays the
wrong track, suggesting that the list is being constructed wrong.

But how? I can't work out what could be different between the two
invocations -- the difference between then can be literally a few seconds.
If I then visit the EMMS playlist buffer (say, by runing
`emms-smart-browse`), the function always works again as expected.

I have two questions then:
- What is happening with my implementation above? Why does it sometimes
give me the wrong list?
- Is there just a better way to achieve what I want? (I want to aviod the
XY problem -- no point solving problems I don't need to!)

Any help greatly appreciated.

Blue skies, Hugo

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