Lua Read Only Table (or Userdata) module　http://nyaocat.hatenablog.jp/entry/2013/11/09/111856

readonly.lua

--[[
lua readonly module
2013 (c) nyaocat
this program is licensed under NYSL( http://www.kmonos.net/nysl/ )

lua5.1 and lua5.2
]]

local newproxy = newproxy or require("newproxy")  -- for Lua5.2
local type, getmetatable, pairs, assert, error = type, getmetatable, pairs, assert, error
local next, ipairs, pairs = next, ipairs, pairs
local module, getfenv = module, getfenv -- for Lua5.1

local readonly_table_tag = newproxy() -- unique tag
local readonly

if ... and module then
  -- Lua5.1
  module(..., package.seeall)
  readonly = getfenv(1)
else
  readonly = {}
end

function readonly.next(t, index )
  local mt = getmetatable(t)
  if mt and mt.__type == readonly_table_tag then
    return getmetatable(t).__next(t, index )
  else
    return next(t, index)
  end
end

function readonly.pairs(t)
  local mt = getmetatable(t)
  if mt and mt.__type == readonly_table_tag then
    return getmetatable(t).__next, t, nil
  else
    return pairs(t)
  end
end

local function iter(tbl, i)
  i = i + 1
  local var = tbl[i]
  if var then
    return i, var
  end
end

function readonly.ipairs(t)
  local mt = getmetatable(t)
  if mt and mt.__type == readonly_table_tag then
    return iter, t, 0
  else
    return ipairs(t)
  end
end

function readonly.readonly(tbl)
  assert(type(tbl) == "table" or type(tbl) == "userdata",
         "Only user data or table is possible to read-only.")
  local ret = newproxy(true)  -- ※
  local mt = getmetatable(ret)
  local tbl_mt = getmetatable(tbl)
  if tbl_mt then
    for k, v in pairs(tbl_mt) do
      if k:match("^__") then
        mt[k] = function(...) return v(...) end
      end
    end
  end
  mt.__len   = function() return #tbl end
  mt.__index = function(ud, key)
    if ((type(tbl[key]) == "table") or (type(tbl[key]) == "userdata")) then
      local mt_ = getmetatable(tbl[key])
      if mt_ and mt_.__type == readonly_table_tag then
        return tbl[key]
      else
        return readonly.readonly(tbl[key])
      end
    else
      return tbl[key]
    end
  end
  mt.__newindex = function()
    error("value of a read-only table can not be updated.")
  end
  mt.__type = readonly_table_tag
  mt.__next   = function(t, index ) return next(tbl, index) end
  return ret
end

function readonly.readonly2(tbl) -- member function can be updated.
  assert(type(tbl) == "table" or type(tbl) == "userdata",
         "Only user data or table is possible to read-only.")
  local ret = newproxy(true)
  local mt = getmetatable(ret)
  local tbl_mt = getmetatable(tbl)
  if tbl_mt then
    for k, v in pairs(tbl_mt) do
      if k:match("^__") then
        mt[k] = function(...) return v(...) end
      end
    end
  end
  mt.__len   = function() return #tbl end
  mt.__index = function(ud, key)
    if (type(tbl[key]) == "function") then
      return function(self, ...) return tbl[key](tbl, ... ) end
    elseif ((type(tbl[key]) == "table") or (type(tbl[key]) == "userdata")) then
      local mt_ = getmetatable(tbl[key])
      if mt_ and mt_.__type == readonly_table_tag then
        return tbl[key]
      else
        return readonly.readonly2(tbl[key])
      end
    else
      return tbl[key]
    end
  end
  mt.__newindex = function()
    error("value of a read-only table can not be updated.")
  end
  mt.__type = readonly_table_tag
  mt.__next   = function(t, index ) return next(tbl, index) end
  return ret
end

-- tests

if not (...) then
  local print, pcall = print, pcall
  local ret, err = pcall(function()
    arr = readonly.readonly {3, 9, 7, 4}
    assert(pcall(function() local val = arr[2] end) == true )
    assert(pcall(function() arr[1] = 98        end) == false)

    t1 = readonly.readonly {
      val     = 10,
      getVal  = function(self)         return self.val   end,
      setVal  = function(self, newval) self.val = newval end
    }
    assert(pcall(function() t1:setVal(20) end) == false )
    assert(pcall(function() t1:getVal()   end) == true  )
    assert(pcall(function() t1.val = 30   end) == false )

    t2 = readonly.readonly2 {
      val     = 10,
      getVal  = function(self)         return self.val   end,
      setVal  = function(self, newval) self.val = newval end
    }
    assert(pcall(function() t2:setVal(20) end) == true  )
    assert(pcall(function() t2:getVal()   end) == true  )
    assert(pcall(function() t2.val = 30   end) == false )
  end)
  if ret then
    print("tests all ok")
  else
    print("test failed ...", err)
  end
end

return readonly