-- Example 10. Control global variable access and assignment

setmetatable(_G, {

__index = function(t, key)

local errmsg = "Unknown global '%s'"

error(string.format(errmsg, key))

end,

__newindex = function(t, key, value)

local errmsg = "Attempt to create global '%s'. \z

Use rawset() instead."

error(string.format(errmsg, key))

end

})

--8<----------------------------------------------------------------------------

xpcall(function() print(x) end, print)

xpcall(function() x = 1 end, print)

--8<----------------------------------------------------------------------------