Re: Suggestion: "usertype()" function

[Tomas Guisasola Gorham <tomas@...>]

	Hi Tim

On Wed, 30 May 2012, Tim Hill wrote:
> However, it seems to me that there is one hole in this clean
> model: the inability of a metatable to project a "type" into the
> Lua code space. Specifically, regardless of metatable, the value
> of type(someuserdata) is always "userdata".
	This is consistent with type(sometable).  The inability also
affects tables with metatables.

> function usertype(v)
> 	local ty = getmetatable(v).__type
> 	return ty and "_" .. ty or type(v)
> end
>
> Of course, it's not necessary to add this to the language; after all I just wrote the code in the above example. However, I feel that this is a generic feature that needs to always be available for people who are writing addon libraries or packages for Lua.
>
> Thoughts anyone?
	I would suggest to return a second value:

-- Untested!
function usertype(v)
	local tv = type(v)
	if tv == "table" or tv == "userdata" then
		local mt = getmetatable(v)
		return tv, mt and mt.__type or mt
	else
		return tv
	end
end

	It can be used in place of type(), but it has extra cost...
	In fact, I discarded type-checking in my Lua code (I experimented
a library to check types and a tool to remove it from production code,
but gave up: the time spent on it were bigger than the bugs it warned),
which is one use of this kind of function.  In other situations, when I
need to check the type to avoid problems, a generic test like that is an
overkill (for me, at least) then I use a direct solution (like checking
an especific field of the value or something like that).

	Regards,
		Tomás