How to write a generic function that merges tables and keeps the type of all

[danielo515]

Hello.
I'm trying to write a generic function that is able to keep track of the properties of all the tables it merges. Currently I was not able to find a way, so I would love some help here.

Let me write the simplest example of what I want:

local foo = { a = true }
local bar = { b = true }
-- merge is the function I'm trying to type
local fooBar = merge(foo, bar)

I want fooBar to have type {a: boolean, b: boolean}. Ideally I want the function to accept any number of tables to merge, but I will be fine if I have to write a set of fixed overloads to accept up to X table merges. Any hint will be greatly appreciated.

[danielo515]

This is what I tried so far:

---@generic T
---@generic A { [string]: T }
---@generic O { [string]: T }
---@param ... O|A
---@return O
function M.merge(...)
  local sources = { ... }
  local result = {}
  for _, source in ipairs(sources) do
    for key, value in pairs(source) do
      result[key] = value
    end
  end
  return result
end

[danielo515]

Small attempts that didn't work

---Assigns the properties of one or more objects to another object
---@generic X
---@generic Y
---@generic A { [string]: X }
---@generic B { [string]: Y }
---@generic O { [string]: X | Y }
---@param a A
---@param b B
---@return O
function M.assign(a, b)end

---Assigns the properties of one or more objects to another object
---@generic X
---@generic Y
---@param a { [string]: X }
---@param b { [string]: Y }
---@return { [string]: X | Y }

[danielo515]

Any way of doing this?

[sumneko]

Not fully supported yet, but you can try:

---@generic A, B
---@param a A
---@param b B
---@return A|B
local function merge(a, b)
end

[danielo515]

That is something I tried too, but it does not work well either.
It seems the object gets the right props (but I think with me method that also, happens, not sure) but the types and the docs are lost.
See the below two screenshots to compare the difference between accessing the resulting option and using a require direct to the file: