Re: _ENV vs _G

[Tim Hill <drtimhill@...>]

On Aug 13, 2014, at 1:34 PM, Dirk Laurie <dirk.laurie@gmail.com> wrote:

> 2014-08-13 20:00 GMT+02:00 Tim Hill <drtimhill@gmail.com>:
>> On Aug 13, 2014, at 4:55 AM, Dirk Laurie <dirk.laurie@gmail.com> wrote:
>> [1] Binary chunks can have _ENV parameters, so (a) is possible.
>> 
>> 
>> Interesting, how do you do that?
> 
> $ lua
> Lua 5.2.3  Copyright (C) 1994-2013 Lua.org, PUC-Rio
>> f=function(_ENV) return _ENV,id end
>> cf=string.dump(f)
>> io.open("f.luac","wb"):write(cf):close()
>> 
> $ lua
> Lua 5.2.3  Copyright (C) 1994-2013 Lua.org, PUC-Rio
>> f=loadfile"f.luac"
>> print(_ENV)
> table: 0x1f205f0
>> return f{id="my_env"}
> table: 0x1f440d0    my_env
> 

Ah, interesting border-case. In fact, even if you DO specify an environment to loadfile() in this case it will be inaccessible to the function.

I’ve not really given it much thought, but a function that declares _ENV as one of its arguments in fact has NO access to the global environment at all so far as I can see (except indirectly via upvalues or other arguments, both of which the caller can control). Is this, then, a Lua idiom for writing (partially) pure functions? With the obvious caveat that such a function is free to call impure functions.

—Tim