Programming in Lua : 13.4.5

Part II. Tables and Objects Chapter 13. Metatables and Metamethods

13.4.5 - Read-Only Tables

It is easy to adapt the concept of proxies to implement read-only tables. All we have to do is to raise an error whenever we track any attempt to update the table. For the __index metamethod, we can use a table---the original table itself---instead of a function, as we do not need to track queries; it is simpler and quite more efficient to redirect all queries to the original table. This use, however, demands a new metatable for each read-only proxy, with __index pointing to the original table:

    function readOnly (t)
      local proxy = {}
      local mt = {       -- create metatable
        __index = t,
        __newindex = function (t,k,v)
          error("attempt to update a read-only table", 2)
        end
      }
      setmetatable(proxy, mt)
      return proxy
    end

(Remember that the second argument to error, 2, directs the error message to where the update was attempted.) As an example of use, we can create a read-only table for weekdays:

    days = readOnly{"Sunday", "Monday", "Tuesday", "Wednesday",
             "Thursday", "Friday", "Saturday"}
    
    print(days[1])     --> Sunday
    days[2] = "Noday"
    stdin:1: attempt to update a read-only table