How to annotate this table iterator function?

[AFCMS]

I have added this pairsByKeys function to a project : https://www.lua.org/pil/19.3.html

But it doesn't have annotations yet.

I didn't manage to build a stateless version of the function and I neither managed to get annotations working (hovering on index and value in for loop shows undefined):

---@generic T: table, K, V
---@param t T
---@param f? function
---@return fun(): K, V
function pairsByKeys(t, f)
end

Could someone help me?

[MillhioreBT]

I tested it like this and it works, I just copied the pairs annotations

---@generic T: table, K, V
---@param t T
---@param f? function
---@return fun(table: table<K, V>, index?: K):K, V
---@return T
function pairsByKeys(t, f)
end

[AFCMS]

Somehow works, but I get an error about returned types, since only one function is returned. Plus the returned function parameters are wrong:

And if I remove the ---@return T the types are not applied:

[MillhioreBT]

just add nil after iter

[AFCMS]

That works, but the returned function parameters still do not match what is really returned. 😕

[tomlau10]

I've been testing how to annotate iterator these few days in my project, and in your case I think it should be like this:

---@generic K, V
---@param t table<K, V>
---@param f? function
---@return fun(): K, V
function pairsByKeys(t, f)
end

local t = { ccc=4, aaa=12, bbb=131 }
local iter = pairsByKeys(t)     --- iter: fun() => string, integer

for i, v in pairsByKeys(t) do
    --- i: string, v: integer
end

• you just need to capture the generic K, V directly from the table param t
• then use them directly in the return function type