Lua: setfenv() vs. _ENV

Question

What is the big deal with Lua switching from setfenv() to _ENV?

In various "What's New" sources, this move is mentioned as one of the most important changes between Lua versions 5.1 and 5.2.

However, the examples given in PIL and elsewhere can be trimmed to the following:

-- Lua 5.1                   -- Lua 5.2
function myfunc()            function myfunc()
    setfenv(1, {})               _ENV = {}
end                          end

So far, what we gained here is we have saved five key strokes. (I believe the situation is not much different from the C side.) Moreover, if I got it right, setfenv() can be used both from the outside and from the inside of a function, whereas _ENV can be accessed only from the inside of a function. (Of course, when using the C API, one can access the upvalues directly.) From what I wrote, the 5.2 approach seems much less flexible.

In his The Novelties of Lua 5.2, Roberto writes:

"Being a syntactic sugar, it is much simpler than the old environments"

Where's the simplicity? What have I overlooked?

I believe this topic is worth better treatment that it is given in the 5.2 User Manual.

Nicol Bolas Nicol Bolas · Accepted Answer · 2012-08-19T02:29:51

Where's the simplicity?

It depends on how you define "simplicity".

In Lua 5.1, the environment was a magical, mystical setting that was unlike any other setting in the system. It didn't have an explicit place and it could only be set by using standard library features.

In Lua 5.2, the environment is a variable, just like any other. It has a name which you can use. So it's simpler in that it's more obvious what's going on.

Furthermore, in Lua 5.1, the environment of a function can be changed dynamically.

In Lua 5.2, outside of direct upvalue manipulation, once a function has an environment, that is the environment it will have in perpetuity. The environment for a function is inherited, being lexically scoped like a regular local variable. So if you look at your code, you can easily see what environment a function is in. If there is no local _ENV in the scope of that function's creation, then the environment shall be the environment of the chunk (which is defined by the load call).

Lua: setfenv() vs. _ENV

2 Answers