What does `%{%v:lua.…%}` mean in an option?

Question

Currently trying to understand my neovim setup better.

So, looked into what a few classical options are set to.

:set statusline

yields

  statusline=%{%v:lua.require'heirline'.eval_statusline()%}

:help %tab was not very helpful.

So, could someone help me pick this apart? I found out %{%…%} is a lambda expression which gets re-evaluated as statusline expression.

v: would suggest we're entering visual mode, but that makes no sense in the context.

So, I feel completely noobish, here.

Answer 1

v:lua is used to call Lua functions from Vimscript.

Quoting the v:lua-call help subject (lua.txt) from Neovim v0.10.0:

From Vimscript the special v:lua prefix can be used to call Lua functions which are global or accessible from global tables.

The expression

call v:lua.func(arg1, arg2)

is equivalent to the Lua chunk

return func(...)

where the args are converted to Lua values.

The expression

call v:lua.somemod.func(args)

is equivalent to the Lua chunk

return somemod.func(...)