Re: _ENV vs _G

[Sean Conner <sean@...>]

It was thus said that the Great Tim Hill once stated:
> 
> On Aug 12, 2014, at 8:11 AM, Steven Degutis <sbdegutis@gmail.com> wrote:
> 
> > This is my current understanding of how _G and _ENV work based on my
> > reading of the manual and Pil^3 along with Roberto's response to my
> > first email:
> > 
> > When you create a new Lua state, its registry (i.e. the table at
> > LUA_REGISTRYINDEX) contains a special table somewhere which is
> > henceforth known as "the global environment".
> > 
> > When you load a new chunk for Lua to execute, something essentially
> > equivalent to the following pseudocode happens:
> > 
> >    local _ENV = lua_gettable(L, LUA_REGISTRYINDEX, <global environment's key>)
> >    _ENV._G = _ENV
> > 
> > That is, _G is a "global" (i.e. a field on the key _ENV) referring to
> > the initial value of _ENV, which is "the global environment”.
> > 
> 
> 
> Read my earlier post. I don’t think the _ENV_G assignment happens when you
> do a load (after all, this would have the effect of resetting _G after
> every call to load()). My reading is that setting _G happens ONCE only for
> the distinguished environment (when the Lua_State is initialized I
> presume), and never again.

  _G is only set if you load the base library into the Lua state, either
with luaopen_base() or luaL_openlibs().  Lua itself does not use _G as far
as I can see (checked both 5.1 and 5.2), and I suspect it's there just for
convenience.

  -spc