Generic parameter with parameter of that generic type

[JonSnowbd]

Here is a small snip of code for context:

---@class Human
---@field name string
---@field age integer
local Human = {}
Human.name = "Charles"
Human.age = 30

---@generic T
---@generic Obj : `T`
---@param type T
---@param overrides Obj
function Make(type, overrides)
    local new = {}
    for k, v in pairs(type) do
        new[k] = overrides[k] or v
    end
    return new
end

Make(Human, {age = 50})

I want to make Make take in anything as T, and provide intellisense when the overrides parameter is being filled in. For example when {age=50} is being written, age and name would show up.

It's not a huge issue if not possible, but would be great QoL for a library im working on.

---@generic T
---@generic Obj : `T`
---@param type T
---@param overrides Obj
function Make(type, overrides)

Signature does not work

and neither does

---@generic T
---@param type T
---@param overrides T
function Make(type, overrides)

Any help would be appreciated thank you for your time.

[tomlau10]

This seems to me impossible 😕 because when you specify a param with a generic type T, you are capturing its type, not enforcing its type. In order to have autocompletion for the overrides object, you must first specify the actual type of it in the function param, not generic type.

---

To achieve this, what I can come up with is to modify the Make() syntax:

• Create a MakeFactory(type) => return a closure function that accepts a param of type type
• And to avoid creating a closure function variable every time, you may also cache it by type

---@class Human
---@field name string
---@field age integer
local Human = {}
Human.name = "Charles"
Human.age = 30


local factories = {}
---@generic T
---@param type T
---@return fun(obj: T): T
function MakeFactory(type)
    local factory = factories[type]
    if factory == nil then
        factory = function (overrides)
            local new = {}
            for k, v in pairs(type) do
                new[k] = overrides[k] or v
            end
            return new
        end
        factories[type] = factory
    end
    return factory
end

-- then you can have the following syntax
MakeFactory(Human) {
    age = 50,
    --[[ field auto completion here ]]
}

-- or even use a local var for the factory function
local MakeHuman = MakeFactory(Human)
MakeHuman({ age = 50, --[[ auto completion here ]] })

[JonSnowbd]

This is a great solution, thank you