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On 4/06/2009, at 12:25 PM, James Snyder wrote:
Would lazy evaluation work? You could not do anything until the object is used by a __call, __tostring, __add or some other metamethod. There would be the usual problem with comparison operators, but it would work some of the time I think.
That works for a fair number of things so long as there's a metamethod that can be hooked prior to an operation. 

It does get weird with something like:

value_from_server = handle.some_value
value_from_server + local_value

or even things like:
value_from_server + handle.some_other_value
These would both work wouldn't they? Your helper's __add would get its (and the other operand's, if it was also a helper) value from the server and do the job locally. (Which is what I think you wrote next). I don't see a problem with that - am I missing something? You *could* build a syntax tree and serialise that and send it to the server, but that'd be probably more complication than is necessary.
To me it would make the most sense to, when possible, pull both to the local environment, and then perform the local operation as it would with that type. I wonder if that could be cleanly generalized somehow? It would be nice to be able to do things like call math.cos(handle.some_remote_value) without having to explicitly copy it from the remote to the local state. For things like this,
That should be possible, right? You're serialising the call anyway, so if you can just serialise the helper too, it should be possible to handle it on the remote machine?
It doesn't have to perfectly handle every situation. My main goals are to make it easy to execute functions on a remote session, and to make it pretty easy to get data from the remote environment for further local processing.
I think a combination of laziness and offering a "get" as you mention below would work fine. There'd be some odd corners when you weren't sure if you needed a get or forgot one, but that wouldn't be too bad would it?
Perhaps the easiest approach is to do lazy evaluation, where the helper can handle __call (to call a remote function), __index (to index deeper into multiple layers of tables), and a method like "get" or "copy" that would take anything it knows how to serialize and copy it to the local stack. 

Thanks for your comments and thoughts :-)
Any other suggestions? (doing it all from a module without modifying Lua itself would be best)
Although the goal is completely different, this has a lot in common with rima's late bound symbolic math stuff - which is far from elegant or finished at the moment - so I'll look forward to seeing what you come up with. Rima does have an eval, and it includes the "connection" (called a scope), so, in your context you'd have something like: 

local a, b = rpc.helper"a, b"
local e = a + b.some_value

local res1 = rpc.eval(e, connection1)

and then you could ask another server/scope the same question

local res2 = rpc.eval(e, connection2)
I doubt that there's any use for that kind of functionality in what you're doing, but I'm pleasantly surprised that some of the insides are so similar. 

Cheers,
Geoff