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Will upvalue 1 work if the loaded file doesn't reference any globals?

On 20.05.2010 23:15, Roberto Ierusalimschy wrote:
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

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'.

-- Roberto


Best regards,
Sergey Rozhenko