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2010/5/20 Roberto Ierusalimschy <>
> With Lua 5.1, I have the following code :
>     setfenv(level, M)
> With Lua 5.2.0 work2, I rewrite it as following (compatible with Lua 5.1) :
>     debug.setfenv(debug.getinfo(level, 'f').func, M)
> Now, with Lua 5.2.0 work3, it becomes :
>     if debug.setupvalue(debug.getinfo(level, 'f').func, 1, M) ~= '_ENV' then
>         setfenv(level, M) -- Lua 5.1
>     end
> The value 1 seems magical, but I think _ENV is alway the first upvalue.

Not necessarily. If the function is a main function (that is, the direct
result of load) then _ENV is always the first (and only) upvalue and it
is always present. Other functions have _ENV just like any other upvalue.
For instance, consider the following code:

 local a
 function foo ()
   return a + b

In function foo, the first upvalue is 'a' and the second is '_ENV'.

Thanks for this information.
I'll write a more robust code.

-- Roberto