how to inform luals a table with __call() metamethod is also a function?

[haolian9]

hi folks, i have such usecase

  local foo = setmetatable({}, { __call = function() print("foo") end })

  ---@param fn fun()
  local function bar(fn) fn() end

  bar(foo)

and luals 3.6.21 warns me at bar(foo) line

Diagnostics:
1. Cannot assign `table` to parameter `fun()`.
   - `table` cannot match `fun()`
   - Type `table` cannot match `fun()` [param-type-mismatch]

is there any workaround? thanks in advance.

[C3pa]

Something like this works fine:

---@class foo: function
local foo = setmetatable({}, { __call = function() print("foo") end })

---@param fn fun()
local function bar(fn) fn() end

bar(foo)

[haolian9]

thanks, it worked!
one thing i still dont understand: why this function can be used here? is there any document backing of it?
i checked out the annotation doc, and know the class can inherit/extends. but still, to be honest, this approach looks like a black magic to me.

[C3pa]

All of the basic types under the hood are annotated as classes. See here: https://github.com/LuaLS/lua-language-server/blob/master/meta/template/builtin.lua#L16

That allows us to create classes inheriting from them.

[haolian9]

it makes perfect sense, thank again!

[carsakiller]

If you need to define parameters and returns, it is recommended to use @overload

---@overload fun(a: string): boolean
local foo = setmetatable({}, {
	__call = function(a)
		print(a)
        return true
	end,
})

local bool = foo("myString")

[haolian9]

thanks! this way is more practical as it can provide more details about the signature.

(if you dont mind, the first argument passed to __call() is the table itself, so there could be a typo in the given example.)

[carsakiller]

Ah yes, good catch, thanks!