Re: Function environment table interface
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- Subject: Re: Function environment table interface
- From: Doug Rogers <doug.rogers@...>
- Date: 2009年7月06日 17:44:07 -0400
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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