vim-patch:8.2.1054: not so easy to pass a lua function to Vim
vim-patch:8.2.1084: Lua: registering function has useless code
I think I have also opened up the possibility for people to use these
callbacks elsewhere, since I've added a new struct that we should be
able to use.
Also, this should allow us to determine what the state of a list is in
Lua or a dictionary in Lua, since we now can track the luaref as we go.
compared to vim.api.|nvim_call_function|, this fixes some typing issues
due to the indirect conversion via the API. float values are preserved
as such (fixes#9389) as well as empty dicts/arrays.
Ref https://github.com/norcalli/nvim.lua for the call syntax
Rationale: the purpose of nvim_execute_lua is to simply call lua code with lua
values. If a lua function expects a floating point value, it should be enough
to specify a float as argument to nvim_execute_lua.
However, make sure to preserve the existing roundtripping behavior of
API values when using `vim.api` functions. This is covered by existing
lua/api_spec.lua tests.
