Script to check that you use local variables inside functions

I found this nifty code example on the Lua mailing list. It was written by Roberto Ierusalimschy, the developer of Lua.


--
-- strict.lua
-- checks uses of undeclared global variables
-- All global variables must be 'declared' through a regular assignment
-- (even assigning nil will do) in a main chunk before being used
-- anywhere or assigned to inside a function.
--

local mt = getmetatable(_G)
if mt == nil then
  mt = {}
  setmetatable(_G, mt)
end

mt.__declared = {}

mt.__newindex = function (t, n, v)
  if not mt.__declared[n] then
    local w = debug.getinfo(2, "S").what
    if w ~= "main" and w ~= "C" then
      error("assign to undeclared variable '"..n.."'", 2)
    end
    mt.__declared[n] = true
  end
  rawset(t, n, v)
end
  
mt.__index = function (t, n)
  if not mt.__declared[n] then
    error("variable '"..n.."' is not declared", 2)
  end
  return rawget(t, n)
end

What this does is use a metatable on the global (_G) table to make sure that setting variables, or reading them, is only done to "declared" variables. That is, variables that are explicitly mentioned on the global script space.

This helps prevent common errors like misspelling variable names in functions, or accidentally assigning to a global variable when a local one would be more sensible.

Let's assume you save that to disk as "strict.lua".

Some example code:


require "strict"

function f ()
 a = 2 --> error: assign to undeclared variable 'a'
 
 print (b)  --> error:  variable 'b' is not declared
 
 function g ()  --> error:  assign to undeclared variable 'g'
 end -- g
 
end -- f

f ()

A sensible strategy in these cases would be to put "local" in front of variables and functions that are really supposed to only exist inside the scope of the local function.