Neovim Lua integration

Question

I'm trying to understand how can I use Lua to extend Neovim, following a baby steps approach. Something like:

-- ~/.config/nvim/lua/test.lua

local api = vim.api
local firstmodule = {}

function firstmodule.hello_world()
  api.nvim_command('echo "hello world"')
end

return firstmodule

Then in my init.vim I import the module to a command:

command! Greet lua require 'test'.hello_world()

And I get a command :Greet that prints hello world.

---

So far so good, but how can I get/use a string returned by a lua function, let's say:

function firstmodule.hello_string()
  return 'hello world'
end

I try to call this function in my init.vim:

let g:Myvar = v:lua.test.hello_string()

When I source the file, I get the errors:

E121: Undefined variable: v:lua
E15: Invalid expression: v:lua.test.hello_string()

How can I use that string returned by a lua function, let's say in my statusline?

Answer 1

The v:lua interface was only introduced to NeoVim in PR #11338, which has a milestone of 0.5, so it looks like it's not generally available in released versions of NeoVim just yet. (NeoVim 0.5 is still in Beta as of early May 2020.)

You should use the luaeval() Vimscript function to evaluate a Lua expression (such as a function call) and use the returned value back in Vimscript.

In your case, it seems to me this should work:

let g:Myvar = luaeval('require("test").hello_string()')

---

Inside 'statusline', you can use %{...} to evaluate a Vim expression, which could be a global variable such as g:Myvar or even a luaeval(...) function call. Just keep in mind that 'statusline' is evaluated quite often, so the overhead of using a Lua function call from there might become quite noticeable and produce lag while editing.

A solution that uses some caching should be used to avoid that, either a Vim global variable updated from an autocmd (such as CursorHold, CursorMoved, CursorMovedI, SafeState, etc.) or a Vim function that evaluates whether conditions changed and require a new call to the Lua code or not.