(top-repl): suggestions for a demo?

[Eduardo Ochs]

Hi all,

I am a very basic user of lilypond - I never did anything more
advanced than simple piano pieces with lyrics - but a few weeks ago I
found a way to use REPLs to learn Tikz, and I recorded a video about
that, that is here...

  http://angg.twu.net/eev-tikz.html

Its two main ideas can be adapted to Lilypond, and I am going to ask
for some help with the second.

The first, and main, idea of the video is to use a very simple macro
language in which "<<expr>>"s in strings are replaced by the result of
evaluating expr in Lua in a certain way. This is trivial to adapt, I
just need to change the "<<>>"s by other delimiters.

The second idea, that is explained in an exercise at the end of the
video - starting here:

  https://www.youtube.com/watch?v=d7nIzpXcV6c#t=1h27m34s

is a way to a) make lualatex run a Lua REPL that is much better than
debug.debug(), and b) use that REPL to inspect definitions from the
calling lualatex with token.get_meaning, with a wicked trick to
inspect the values of pgfkeys.

Anyway, let me go back to Lilypond...

I know that I can get a Scheme REPL from Lilypond by running this,

  (top-repl)

but I've never been able to use it for anything useful. I _guess_ that
competent Scheme users should be able to use that to ask Lilypond what
it knows about what it has already processed of the current .ly
file...

Can anyone give me some examples of how to use the REPL to inspect
some interesting variables and data structures? I am hoping to get
something like a 5-line demo that I can use as a starting point...

  Thanks in advance!
    Eduardo Ochs
    http://angg.twu.net/eepitch.html