【高级技巧】哨兵值：nil语义的显式化设计

local SENTINEL = {
    NOT_SET = "\1NOT_SET",
    DENIED = "\1DENIED",
}
local function getConfig(tbl, key, deniedKeys)
    if deniedKeys and deniedKeys[key] then
        return SENTINEL.DENIED
    end
    local v = tbl[key]
    if v == nil then
        return SENTINEL.NOT_SET
    end
    return v
end

local DENY = { masterKey = true }
local function readKey(actor, key)
    actor = getplayerbyname(actor)
    local v = getConfig({ coin = 5000 }, key, DENY)
    if v == SENTINEL.NOT_SET then
        sendmsg(actor, 1, key .. " 未配置，走默认逻辑。")
        return 0
    end
    if v == SENTINEL.DENIED then
        sendmsg(actor, 1, key .. " 为受限字段，拒绝读取。")
        return 0
    end
    return v
end