Lua table hack

table.lua

local function protect_table(tbl)
    return setmetatable ({}, {
        __index = tbl,
        __newindex = function (t, n, v)
            error ("attempting to change constant " .. 
            tostring (n) .. " to " .. tostring (v), 2)
        end
    })
end 

local function copy_on_write_table(tbl)

    return setmetatable ({}, {
        __index    = tbl,
        __newindex = function(t, n, v)

            setmetatable(t, {})

            for i, v in pairs(tbl) do
                t[i] = v
            end

            t[n] = v
        end 
    })
end

table.copy_on_write = copy_on_write_table
table.protect       = protect_table