Re: Function environment table interface

[Doug Rogers <doug.rogers@...>]

Peter Cawley wrote:
> On Mon, Jul 6, 2009 at 9:20 PM, Rob Kendrick<lua-l@nun.org.uk> wrote:
> > I just had a thought; why are function environments not implemented via
> > a field in a metatable?  For example;
> Firstly, environment tables are per-function, whereas all functions
> share a single metatable, so function environments could not be set
> via metatable. Secondly, separating the environment from the metatable
> allows for one to be per-instance and one to be per-class.

It may be that OP meant also to ask "Why not a per-instance metatable 
for functions?" If so, I'd recommend a fun journey through these threads:

  http://lua-users.org/lists/lua-l/2009-05/msg00298.html
  http://lua-users.org/lists/lua-l/2009-03/msg00479.html

IMHO, the short reply to the original question is "performance". 
Requiring the attachment of a metatable for __env would levy the 
overhead of a table lookup, possibly a chain of them due to __index, for 
each function call.

I like the general direction you're going - unifying and generalizing - 
but it seems that separate environments for functions are too useful for 
such a price to be paid per call.

Doug


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