Is there a way to annotate the type of a return from a module?

[Agent-E11]

For instance, if I had a class that a few different modules returned:

-- types.lua

---@class MyModule
---@field do_thing fun(): string

And I wanted to declare the table at the top:

---@module 'types'

---@type MyModule
local M = {}

function M.do_thing() return "hello, world" end

I get the following error at the definition of M:

Missing required fields in type `MyModule`: `do_thing`

So, is there a way to annotate the return instead of the object itself?

I have tried all of these methods, but they don't seem to actually enforce that the do_thing field is present:

---@cast M MyModule
return M

---@type MyModule
return M

---@return MyModule
return M

---

P.S. I know I could do this

local function do_thing() return "hello, world" end

---@type MyModule
local M = {
  do_thing = do_thing
}
return M

But... I think it's ugly, and do_thing is repeated 3 times

[tomlau10]

• The ---@type is to cast the next variable as the type you specified. Since local M = {} is just an empty table at this line, therefore a Missing required fields warning is prompted.

Back to the question, I don't quite understand what you are going to achieve. 😕

• Are you trying to enforce the implementation of a module by writing the definition first?
• Or are you just trying to write meta definition for an existing module?

---

I don't think LuaLS is designed to enforce the implementation of a given module.
What it does is the opposite: auto infer the types of a given module.
Given the following code:

---@class MyModule
local M = {}
function M.do_thing() return "hello, world" end

• Now even without specifying a ---@field do_thing fun(): string, LuaLS can auto infer that there is a do_thing function which returns a string. You can hover over the local M to see the preview.
• Actually you don't even need to use ---@class to achieve this, but adding a ---@class will let LuaLS do undefined-field check on this table access.
  For example if there are typos in other places like local s = M.doo_thing()
  => this will trigger an undefined-field warning (only if you have annotated the module table with a ---@class)

---

And if you really want to use it to enforce the module implementation, the only suggestion that I can think of is to inline the function declaration, so that you don't need to repeat do_thing 3 times 🙈

---@type MyModule
local M = {
  do_thing = function ()
    return "hello, world"
  end,
}
return M

Maybe other experts of LuaLS can help achieve your goal using better ways 🙂

[Agent-E11]

Yes, I am trying to enforce the "public api" of this module, and a few other modules, because they should all have the same public API.
I'm not going to mark this as the answer, as I would like a more definite "Yes/No" on whether it is possible, but thank you for the information