Re: "Locking" global variables?

[Peter Shook <pshook@...>]

=== FAQ ===

Ando Sonenblick wrote:
> Gang,
>
> Is there a way in Lua to lock a global variable after initializing it so
> that trying to reassign it will fail?
>
> We'd like to prevent inadvertent reassignment of key global variables by
> some unaware scriptor...

If you want to make all globals readonly, see lua-5.0/test/readonly.lua

If you want to make only a few variables readonly, try something like this:

$ cat z.lua

local ReadOnly = {
  x = 5,
  y = 'bob',
  z = false,
}

local function check(tab, name, value)
  if ReadOnly[name] ~= nil then
    error(tostring(name) ..' is a read only variable', 2)
  end
  rawset(tab, name, value)  -- tab[name] = value
end

-- make some global variables readonly
setmetatable(_G, {__index=ReadOnly, __newindex=check})


$ lua -lz -i
>
> = x,y,z
5       bob     false
> x = 44
stdin:1: x is a read only variable
stack traceback:
        [C]: in function `error'
        z.lua:10: in function <z.lua:8>
        stdin:1: in main chunk
        [C]: ?
> z = 99
stdin:1: z is a read only variable
stack traceback:
        [C]: in function `error'
        z.lua:10: in function <z.lua:8>
        stdin:1: in main chunk
        [C]: ?
> a,b = 1,2
> = a,b
1       2
>


Here are some similar threads:
http://lua-users.org/lists/lua-l/2003-06/msg00362.html
http://lua-users.org/lists/lua-l/2003-07/msg00298.html

- Peter