access control

1.lua

local raw_setmetatable = setmetatable
local setmetatable = function(t, mt)
    local raw_t = raw_setmetatable(t, mt)
    return raw_setmetatable({}, {
        __index = function(raw, k)
            if not(mt[k]) then
                error("can not visit this member")
            end
            return function(_, ...)
                return raw_t[k](raw_t, ...)
            end
        end,
        __newindex = function(raw, k, v)
            error("can not add new member")
        end,
    })
end

local M = {}
local mt = {}
mt.__index = mt

function mt:f1(m)
    return self.a + m
end

function mt:f2()
    self.aa = 3
end

function mt:f3()
    return self.aa
end

M.new = function()
    return setmetatable({
        a = 1,
        b = 2,
    }, mt)
end
return M

2.lua

local m = require("1")

local t = m.new()

print(t:f1(1))
t:f2()
print(t:f3())
print(t.a)