Write a table in Lua?

local tbl = write_once_tbl()
tbl["a"] ='foo'
tbl["b"] ='bar'
tbl["a"] ='baz' - asserts false

function write_once_tbl()
 local T={} 
 return setmetatable({},{
 __index=T,
 __newindex=
 function (t,k,v)
 if T[k]==nil then 
 T[k]=v
 else
 error("table is write-once")
 end
 end,
 __pairs = function (t) return pairs(T) end,
 __ipairs = function (t) return ipairs(T) end,
 })
end

I want to use a write-once table in Lua (especially LuaJIT 2.0.3), like this:

local tbl = write_once_tbl( )
tbl["a"] ='foo'
tbl["b"] ='bar'
tbl["a"] ='baz' - asserts false

Ideally, this will run like regular tables (pairs() and ipairs() work).

__newindex is basically the opposite of how I want to achieve it, I don’t know any Use pair() and ipairs() to make the proxy table mode work.

You need to use a proxy table, that is, one that captures all access rights to the actual table Empty table:

function write_once_tbl()
 local T={}
 return setmetatable({},{
 __index =T,
 __newindex=
 function (t,k,v)
 if T[k]==nil then
 T[k]=v
 else< br /> error("table is write-once")
 end
 end,
 __pairs = function (t) return pairs(T) end,
 __ipairs = function (t ) return ipairs(T) end,
 })
end

Please note that __ pairs and __ipairs are only applicable to Lua 5.2 and later.