Ledger

type PlayerData = { Gold: number, Items: { string } }

type PlayerOps = {
	AddGold: { Amount: number },
	SpendGold: { Amount: number },
	BuyItem: { Item: string, Price: number },
}

local PlayerStore = Ledger.New<<PlayerData, PlayerOps>>({
	Name = "PlayerData",
	Keys = "Player",
	Default = { Gold = 0, Items = {} },
	MustExist = false,
	Erasable = true,
	Balances = {
		Gold = { Credit = "AddGold", Debit = "SpendGold" },
	},
	Reducer = function(Data, Op)
		if Op.Kind == "BuyItem" then
			if Data.Gold < Op.Price or table.find(Data.Items, Op.Item) then
				return nil
			end

			local Items = table.clone(Data.Items)
			table.insert(Items, Op.Item)

			local New = table.clone(Data)
			New.Gold -= Op.Price
			New.Items = Items
			return New
		end

		return nil
	end,
})

local Session = PlayerStore:Load(Player)
if Session then
	local Ok, Why = Session:Apply({ Kind = "BuyItem", Item = "Sword", Price = 50 })
	if not Ok then
		warn("Couldn't buy the sword:", Why)
	end
end