Why can't a coroutine yield inside an iterator?

[Matt Campbell <mattc@...>]

Hello:

I noticed that unless Lua is patched with something like Coco that uses 
machine-dependent tricks, a Lua coroutine can't yield inside an 
iterator.  Why is this?  I understand why a coroutine can't yield if 
there's a C function somewhere in the call stack; that would require 
switching C stacks.  However, it seems that it would be possible for the 
VM to save its state inside an iterator so a coroutine can yield.  Would 
this require a lot of work or add significant overhead per coroutine?  I 
think the ability to yield inside an iterator would be quite useful 
sometimes, particularly in some producer/consumer scenarios.  Thanks.

--
Matt Campbell
Lead Programmer
Serotek Corporation
www.freedombox.info
"The Accessibility Anywhere People"