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On Jun 4, 2009, at 8:05 PM, Geoff Leyland wrote:


On 5/06/2009, at 2:39 AM, James Snyder wrote:

math.cos(handle.some_remote_value)


Hmm..  So, executing the line above gives you the following bytecode:
main <test.lua:0,0> (6 instructions, 24 bytes at 0x1dd5530)
0+ params, 2 slots, 0 upvalues, 0 locals, 4 constants, 0 functions
	1	[1]	GETGLOBAL	0 -1	; math
	2	[1]	GETTABLE 	0 0 -2	; "cos"
	3	[1]	GETGLOBAL	1 -3	; handle
	4	[1]	GETTABLE 	1 1 -4	; "some_remote_value"
	5	[1]	CALL     	0 2 1
	6	[1]	RETURN   	0 1
This is tougher than the other metatable stuff because after getting the cos function onto the stack, locally, I'm putting a some_remote_value userdata for it to operate on locally. Unfortunately, it's not a lua number however, it's a userdata. Looking at how this is implemented in lua, I don't see how I can hook this in such a way that I can have that userdata behave like a number.
Sorry, I got that one wrong - I can see how handle.math.cos(handle.some_remote_value) could be done in one round trip to the server, but I agree that math.cos(handle.some_remote_value) is not going to work and I can't see a way around math.cos(rpc.get(handle.some_remote_value)). You'd need some kind of c++ cast operator - maybe you could campaign for a __tonumber metamethod in Lua 5.2?
That, I think, would be quite nice to have for this situation. I did find this earlier discussion that suggests __tonumber (for different reasons), and includes a patch that modifies luaB_tonumber to try __tonmber: 
http://lua-users.org/lists/lua-l/2009-02/msg00071.html

There are some upsides and downsides discussed there as well.
One idea that I have no idea whether it might work would be some way of __index to be able to tell who is asking who is about to consume the value. Such a mechanism could have interesting implications, certainly including possibility for good and evil. I haven't really investigated to see what one has available through debug hooks when index is called, so this could be really messy or impossible :-)
Another way to do things would be to have index2adr optionally call a metamethod. I don't know if this ever gets called early such that you might accidentally do pro-active rather than lazy evaluation. Also, I'm sure such a change would potentially slow down everything else in Lua.
There's some interesting stuff here too, although I don't think it really solves the problem: 
http://lua-users.org/wiki/TableScope
This, I suppose, is the crux of this issue: how do I know whether to push userdata onto the local stack, or to serialize the remote value and put that actual value on the stack instead?
handle.math.cos(handle.some_remote_value) wouldn't be as much of a problem, since I could push over the index events to get some_remote_value onto the remote stack for the remote cos.
I guess one way to deal with this would be to handle things this way for remote indexes: 

if type is number, string, boolean, or nil:
	serialize it and put it on the local stack,
otherwise:
	provide lazy evaluation helper
There are probably a bunch of disadvantages I'm not thinking of here, but I think, for sure, I have to point the index to some sort of proxying function that actually gets the remote value, otherwise I'll end up with a local cached value on the local table (which would be undesirable if the value changed on the remote side and I wanted to poll it).
Does this mean you'd have to go and ask the server at each __index? But then, "for k, v in pairs(handle.some_remote_table) do..." wouldn't work either. (Campaign for __pairs in Lua 5.2?)
Hmm.. that would be true as well. I suppose another alternative would be to serialize only tables that contain all serializable entries, but I think that starts getting too weird and inconsistent.
Interesting, is Rima available somewhere? I might be interested in taking a look just to see if I might learn a bit from the implementation. 

http://www.incremental.co.nz/download/rima-0.01.tar.gz
Just ignore all the solver stuff and have a look at the symbolic math stuff. rima/ref.lua is probably where the interesting stuff happens. Rima does not do this nicely yet - it's more that I aim to have it looking like what you're talking about one day.
Thanks. I will take a look. I do have some interest in symbolic math stuff as well, though I don't end up using it a great deal in day-to- day.
I could see some advantage to doing something like what you describe above where you might have a client talking to multiple lua instances to either poll them for state information, or to do work on them. If you wrap things with some sort of map function and use it to work on large chunks of data over a series of connections registered on a table or something.
I'm mainly working on this for use with eLua (hence the serial link protocol being supported (which is giving me some headaches) ), so that I can have a link to an embedded device and do development and testing while having the benefit of being able to get large quantities of data back to a desktop machine. There might be benefit to being able to use this sort of thing to poll a bunch of embedded devices in succession over a network or wireless link. 

Distributing workloads seems like an obvious use?
Perhaps someone will take a look at it for uses like that at some point. There are certainly already some models out there for concurrency in Lua. I know the serialization and protocol here aren't necessarily new, but being able to manipulate other Lua environments as if they were a table seems like a powerful metaphor.
At least the path for doing this with functions seems fairly straightforward :-) 

--
James Snyder
Biomedical Engineering
Northwestern University
jbsnyder@fanplastic.org
http://fanplastic.org/key.txt
ph: (847) 448-0386

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