Read only iterable table in lua? - lua

Question

I want to have a read only table in my Lua program. If ever a key is removed or a key is associated with a new value, an error must be thrown.
function readonly(table)
local meta = { } -- metatable for proxy
local proxy = { } -- this table is always empty
meta.__index = table -- refer to table for lookups
meta.__newindex = function(t, key, value)
error("You cannot make any changes to this table!")
end
setmetatable(proxy, meta)
return proxy -- user will use proxy instead
end
It works great.
t = { }
t["Apple"] = "Red"
t[true] = "True!"
t[51] = 29
for k,v in pairs(t) do
print(v)
end
t = readonly(t)
t[51] = 30
Prints
Red
True!
29
input:7: You cannot make any changes to this table!
Problem
for k, v in pairs(t) do
print(v)
end
Will print nothing under all circumstances now. That's because the proxy table will never have anything inside of it. pairs apparently never calls index and thus cannot retrieve anything from the actual table.
What can I do to make this readonly table iterable?
I'm on Lua 5.1 and have access to these metamethods:
Lua 5.1 Manual

Answer 1

You can modify standard Lua function pairs to work correctly with your read-only tables.
local function readonly_newindex(t, key, value)
error("You cannot make any changes to this table!")
end
function readonly(tbl)
return
setmetatable({}, {
__index = tbl,
__newindex = readonly_newindex
})
end
local original_pairs = pairs
function pairs(tbl)
if next(tbl) == nil then
local mt = getmetatable(tbl)
if mt and mt.__newindex == readonly_newindex then
tbl = mt.__index
end
end
return original_pairs(tbl)
end
Usage:
t = { }
t["Apple"] = "Red"
t[true] = "True!"
t[51] = 29
for k,v in pairs(t) do
print(k, v)
end
t = readonly(t)
for k,v in pairs(t) do
print(k, v)
end
t[51] = 30

Answer 2

One solution is to create a wholly custom iterator for the table.
function readonly(table)
local meta = { } -- metatable for proxy
local proxy = { } -- this table is always empty
meta.__index = table -- refer to table for lookups
meta.__newindex = function(t, key, value)
error("You cannot make any changes to this table!")
end
local function iter()
return next, table
end
setmetatable(proxy, meta)
return proxy, iter -- user will use proxy instead
end
Usage:
t = { }
t["Apple"] = "Red"
t[true] = "True!"
t[51] = 29
for k,v in pairs(t) do
print(v)
end
t, tIter = readonly(t)
t[51] = 30
for k, v in tIter do
print(v)
end