Re: Why no pairs() for userdata, __len for tables?

["Jose Luis Hidalgo" <joseluis.hidalgo@...>]

Hi Thomas,

2007/4/19, Thomas Lauer <thomas.lauer@virgin.net>:
> I have a userdatum that looks (almost) like a Lua table: accesses are
> caught through __index/__newindex/__len. (Nifty, this metastuff.)
>
> What I can't (but would like to) do, is iterating over this table with
> the standard functions. Writing my own iterator is easy enough but it
> feels inconsistent: with a "normal" table I'd use pairs() (or ipairs()).
>
> There may be very good reasons why there's no metamethod support for
> this. If so, can anyone light a candle?

I'm not sure if is there any reason, but maybe this can help you, it
is a replacement of pairs written in pure lua.

do
	local _pairs = pairs
	function pairs (value)
		if type(value) == "userdata" then
			local g = getmetatable(value)
			return g.__pairs(value)
		else
			return _pairs(value) -- original
		end
	end
end

makes a closure with the original pairs function, and in case the
given value is an userdata then tries to access its metatable and ask
for a "__pairs" function.... otherwise uses the original pairs.

To make this work, you should return three values, a function, an
state, and an initial value... as is explained in the section 2.4.5 of
the lua manual.


Cheers,
   Jose L.

--
 Jose L. Hidalgo Valiño (PpluX)
 ---- http://www.pplux.com ----