How to set the value of 'foldexpr' to be a lua function?

Question

I have read the documentation on foldexpr, and I have read through Possible to set `foldexpr` using a function reference?.

I have tried this:

vim.o.foldexpr = function()
  return "1"
end

which clearly does not work, giving the following error:

Error detected while processing ftplugin/markdown.lua:
E5113: Error while calling lua chunk: ftplugin/markdown.lua:13: Invalid 'foldexpr': expected Integer/Boolean/String, got Function
stack traceback:
        [C]: in function '__newindex'
        ftplugin/markdown.lua:13: in main chunk

I see this: https://www.jmaguire.tech/posts/treesitter_folding/ which tells me to do

vim.opt.foldexpr = "nvim_treesitter#foldexpr()"

using the opt dict instead of the o one, and setting it to the function via a string - however, this will only work when the function is defined in vimscript not in lua. Do I need to have a shim and define my function in vim (even if the vim function is just one :lua command defining a function??) or can I use a pure lua reference.

Answer 1

Cross-posted with https://www.reddit.com/r/neovim/comments/193n9fx/how_to_reference_local_function_for_foldexpr/

(1) Export the function to the global namespace (_G):

_G.get_my_foldlevel = function() ... end

vim.o.foldexpr = 'v:lua.get_my_foldlevel()`

(2) or write in a lua module, and require:

--- e.g. config/folding.lua
local M = {}
function M.get_my_foldlevel() ... end
return M

vim.o.foldexpr = 'v:lua.require("config.folding").get_my_foldlevel()'

Personally I don't prefer global variables, so I chose an approach similar to (2) in my config.