Re: __index and function args

[Andreas Stenius <kaos@...>]

Wesley Smith skrev:
> Let's say I have
>
> Obj = {}
> Obj_mt = { __index = function(t, k) -- do something
>                     end
>         }
>
> function Obj.new(o)
>    setmetatable(o, Obj_mt)
>    return o
> end
>
> obj1 = Obj.new{}
> obj1:draw_circle(0, 0, 0.5)
>
>
> When draw_circle is called, it looks up __index and falls the function
> defined in the metatable above where t = obj1 and k = "draw_circle".
>
> My question is, how do I get at the arguments?  What happens to 0, 0,
> 0.5 and how can I access them?
>
> thanks,
> wes

What you could do is to return a anonymous (lambda) function that calls
the requested function. That might not be what you need in this case, 
but was usefull for me when implementing XML-RPC.

function Obj_mt.__index(t, k)
	return function(...)
		-- the function name to call is in `k'
		-- `...' is the arguments passed to this function

		-- dummy example
		if k == "draw_circle" then
			return my_draw_circle_impl(...)
		else
			return nil, "unknown function"
		end

		-- or, another dummy example
		return my_func_impl[k](...)
		-- where my_func_impl probably should have a metatable
		-- returning a default function for undefined k's giving
		-- a descriptive error message
	end
end

Another meta method to look at is __call, which can be quite nice :)

Cheers,
Andreas