how can i annotate a function that returns optional multiple value

[haolian9]

hi folks, i'm always having such itch:

local function foo()
    if "nice day" then
        return 1, 2
    else
        return
    end
end

the only way i figured out to annotate the return value of foo is number?,number?, but it's not accurate.

because the above function returns either number,number or nil, while number?,number? implies two more forms: nil,number and number,nil. if possible i'd rather use (number,number)?.

[carsakiller]

The best way to approach this is maybe with an @overload, but I am still not sure that the type will be narrowed properly in more complex cases:

---@return number
---@return number
---@overload fun(): nil
local function foo()
    if "nice day" then
        return 1, 2
    else
        return
    end
end

local one, two = foo()

[haolian9]

thanks for replying! nice approach, though luals 3.6.21 does not give appropriate diagnosic for bar(two)

---@param a number
local function bar(a) return a end

local one, two = foo()

if one == nil then
  print(bar(two))
end

[carsakiller]

Ideally, we wouldn't need to provide an annotation and the language server would be able to infer which case is possible, either two numbers returned, or none. Unfortunately, it is not able to do so yet.

It looks like two is considered to always be a number in my answer, which isn't correct either. Your best bet is probably leaving them as number | nil and confirming the type before calling bar() – alternatively, the a parameter for bar could be made optional.

[haolian9]

yeah, that's exactly what i'm using right now:

---@type fun():number?,number?
local foo

---@type fun(...: number): number
local add

local a, b = foo()

if a and b then print(add(a, b)) end