Re: destructive iteration patterns

[Josh Haberman <jhaberman@...>]

Sam Roberts <vieuxtech <at> gmail.com> writes:
> > It's sad to have to give up the very nice iterator syntax, but this
> > seems like the cleanest way to provide these semantics which
> > can be implemented much more efficiently!
> >
> > Thoughts?
> 
> You don't want accesses to an old entry to "crash", but are you ok
> with it returning nils, or erroring?

I would be ok with throwing an error, but what worries me is the
case where I can't detect incorrect usage:

last_entry = nil
for entry in ParseFast(io.stdin) do
  if last_entry then
    -- whoops, will actually return this entry's IP address!
    print(last_entry.ip_address)  
  end
  -- Doesn't do what you intend!
  last_entry = entry
end

Maybe the thing to do is to expose the more efficient version, but
also expose an iterator that is by nature less efficient (by doing a
clone of the entry every time).  A lot of people probably won't be
as concerned about efficiency as I am, so they can use the nicer
syntax.

> Why not use userdata, so if they keep an entry in a global, they can
> keep it, and if they don't assign it to a global it will get garbage
> collected?

I ultimately want to avoid having to do a per-entry allocation.
malloc() is *expensive* when performed this often.

Thanks,
Josh