Re: stack level parameter

[Mark Hamburg <mark@...>]

On Jan 21, 2010, at 4:21 AM, David Given wrote:

> On 21/01/2010 11:29, M Joonas Pihlaja wrote:
> [...]
>>> 	in _G:pushenv( env ) do ... end
> [...]
>> You left out the stack unwinding with popenv().  Of course you need to 
>> pop it on errors as well, so every pushenv() ought to be called inside 
>> a pcall().  The required error handling makes your approach less 
>> appealing than what your example might suggest.
> 
> Ah, but you don't! pushenv() would be implemented like this:
> 
> function pushenv(env)
>  local newenv = {}
>  setmetatable(newenv, { __index = env })
>  return newenv
> end
> 
> Note that the new environment contains a pointer to the old one, but not
> vice versa.
> 
> On exit, nothing needs popping, because the old environment has not been
> modified. in..do..end ensures that the new environment is only used
> inside the lexical block. When the block is exited, through whatever
> mechanism, the new environment becomes unused and will be safely garbage
> collected.

Yes though that code isn't quite right since it needs to actually build something that looks up in env followed by the current environment.

Mark