Re: Recipe: setfenv with Lua 5.2.0 work3

[Roberto Ierusalimschy <roberto@...>]

> 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
  end

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

-- Roberto