Opaque Enums

[Nerixyz]

I want to declare an enum where the values are opaque (i.e. left to the implementation). Users should go through the enum to get to the actual value. The best I could come up with is to define opaque types for each enum variant. Is there a proper way to define an enum like this?

The following is a minimal example:

---@meta

local _private_water
local _private_fire

---@enum EventType
EventType = {
    Water = _private_water,
    Fire = _private_fire,
}

---@class WaterEvent
---@class FireEvent

--- Listen to some event
---@param ty EventType.Water
---@param cb fun(event: WaterEvent)
---@overload fun(ty: EventType.Fire, cb: fun(event: FireEvent))
function listen(ty, cb) end

-- intented use
listen(EventType.Water, function(event) end)
listen(EventType.Fire, function(event) end)
-- unintended use
listen("Water", function(event) end)
listen("Fire", function(event) end)
listen("foo", function(event) end)

[tomlau10]

I know a hacky way to do so:

---@meta

---@enum EventType
EventType = {
    Water = {}, ---@type EventType.Water
    Fire = {},  ---@type EventType.Fire
}

---@class WaterEvent
---@class FireEvent

--- Listen to some event
---@param ty EventType
---@param cb function
---@overload fun(ty: EventType.Water, cb: fun(event: WaterEvent))
---@overload fun(ty: EventType.Fire, cb: fun(event: FireEvent))
function listen(ty, cb) end

-- cb params can type narrow correctly
listen(EventType.Water, function(event) end)
listen(EventType.Fire, function(event) end)

-- all of below will throw `param-type-mismatch`
listen("Water", function(event) end)
listen("Fire", function(event) end)
listen("Wind", function(event) end)

---

Credits: the gist link inside additional notes of this issue comment: #2758 (comment)

AFAIK when declaring an @enum, luals will create the underlying enum value types for each of the values using their key names. But seems they are not supposed to be used externally. And modders of Factorio modding community found this hacky way for defining opaque enums. 😄

[Nerixyz]

Thank you, that's good enough for my use-case!