Re: In praise of globals

[Steve Litt <slitt@...>]

On Wed, 17 Apr 2013 22:42:38 +0200
Dirk Laurie <dirk.laurie@gmail.com> wrote:

> 2013/4/17 Steve Litt <slitt@troubleshooters.com>:
> 
> 
> > Is there any way to protect every last bit of _ENV? If so, I could
> > run the program and see every place where a global is being set.
> 
> Sure, see Programming in Lua by Roberto Ierusalimschy,
> 
>    13.4.4 – Tracking Table Accesses.
> 
> http://www.lua.org/pil/13.4.4.html

Thanks Dirk,

That didn't work out quite as well as I'd hoped. It works just fine
with a global table named t, but when I change it to _G, it doesn't
work. Here's the program:

#!/usr/bin/lua

globalfname='Steve'
globallname='Litt'
    
-- keep a private access to original table
local _forbidden = _G

-- create proxy
_G = {}

-- create metatable
local mt = {
  __index = function (_G,k)
    print("*access to element " .. tostring(k))
    return _forbidden[k]   -- access the original table
  end,

  __newindex = function (_G,k,v)
    print("*update of element " .. tostring(k) ..
                         " to " .. tostring(v))
    _forbidden[k] = v   -- update original table
  end
}
setmetatable(_G, mt)

print('1=====================')

print(globalfname)
print(globallname)
print(garbage)

print('2=====================')

globalfname = 'Stephano'
globallname = 'Littatto'
garbage = 'Basura'

print('3=====================')

print(globalfname)
print(globallname)
print(garbage)

print('4=====================')

for k, v in pairs(_G) do
	print(k, ':', v)
	print " "
end

print('5=====================')


and here's the output:

slitt@mydesk:~$ ./testglobals.lua 
1=====================
Steve
Litt
nil
2=====================
3=====================
Stephano
Littatto
Basura
4=====================
5=====================
slitt@mydesk:~$ 

You don't see the metatable side-effects when working with _G.

Thanks,

SteveT

Steve Litt                *  http://www.troubleshooters.com/
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