Re: Lua 5.2: Can you explain this behaviour? (Dirk Laurie)

Since 'w' never appears outside 'loadstring', the local 'w' is
irrelevant, and you can get rid of the '_ENV'. (Once you do that,
Lua 5.1 also runs the snippet.)

I get it now; _ENV basically acts like the _G table. (but does something else as well).

I was looking for a way to use _ENV to do the same setfenv functionality from lua 5.1, and did it wrong.

I searched for it on the 'net, found this "loadin" function, but its not in my lua 5.2 beta!

> print(loadin)

nil

If there's no such function I'd have to do something like:

local pairs = pairs

local function copy(from, to)

for k, v in pairs(from) do to[k] = v end

end

local function clear(t)

for k, v in pairs(t) do t[k] = nil end

end

local function use(env, fun)

local _env = {}

copy(_ENV, _env)

clear(_ENV)

copy(env, _ENV)

local result = {fun()}

copy(_ENV, env)

clear(_ENV)

copy(_env, _ENV)

return table.unpack(result)

end

On 15/09/2011, at 9:00 PM, lua-l-request@lists.lua.org wrote:

On Thu, Sep 15, 2011 at 08:36:09AM +0200, Eric Man wrote:

If you run the code snippet, it'll print "= a<a<". I'd have thought it would be "= a<<.>a<<.>>".

If you toggle line 4 off, though, it will print "= a<<.>a<<.>>" as expected.

...
local function test(bool)
local w, t
if bool
--or true --- toggle this line
then
   t = "a"
   w = function (s) t = t .. tostring(s) end
   _ENV.w = w
else
   t = "."
end

Since 'w' never appears outside 'loadstring', the local 'w' is
irrelevant, and you can get rid of the '_ENV'. (Once you do that,
Lua 5.1 also runs the snippet.)

The above code simplifies to

   local function test (bool)
     local t
     if bool then
       t = "a"
       function w(s) t = t .. tostring(s) end
     else
       t = "."
     end

Whereas the code below, since the 'or true' means the condition
always holds

local function test(bool)
local w, t
if bool
or true --- toggle this line
then
   t = "a"
   w = function (s) t = t .. tostring(s) end
   _ENV.w = w
else
   t = "."
end

simplifies to

   local function test (bool)
     local t = "a"
     function w(s) t = t .. tostring(s) end

Dirk