std.concurrency and efficient returns

[dsimcha]

== Quote from Jonathan M Davis (jmdavisprog at gmail.com)'s article
> Okay. From what I can tell, it seems to be a recurring pattern with threads that
> it's useful to spawn a thread, have it do some work, and then have it return the
> result and terminate. The appropriate way to do that seems to spawn the thread
> with the data that needs to be passed and then using send to send what would
> normally be the return value before the function (and therefore the spawned
> thread) terminates. I see 2 problems with this, both stemming from immutability.
> 1. _All_ of the arguments passed to spawn must be immutable. It's not that hard
> to be in a situation where you need to pass it arguments that the parent thread
> will never use, and it's highly probable that that data will have to be copied
> to make it immutable so that it can be passed. The result is that you're forced
> to make pointless copies. If you're passing a lot of data, that could be
> expensive.
> 2. _All_ of the arguments returned via send must be immutable. In the scenario
> that I'm describing here, the thread is going away after sending the message, so
> there's no way that it's going to do anything with the data, and having to copy
> it to make it immutable (as will likely have to be done) can be highly
> inefficient.
> Is there a better way to do this? Or if not, can one be created? It seems to me
> that it would be highly desirable to be able to pass mutable reference types
> between threads where the thread doing the receiving takes control of the
> object/array being passed. Due to D's threading model, a copy may still have to
> be done behind the scenes, but if you could pass mutable data across while
> passing ownership, you could have at most 1 copy rather than the 2 - 3 copies
> that would have to be taking place when you have a mutable obect that you're
> trying to send across threads (so, one copy to make it immutable, possibly a
> copy from one thread local storage to another of the immutable data (though I'd
> hope that that wouldn't require a copy), and one copy on the other end to get
> mutable data from the immutable data). As it stands, it seems painfully
> inefficient to me when you're passing anything other than small amounts of data
> across.
> Also, this recurring pattern that I'm seeing makes me wonder if it would be
> advantageous to have an addititon to std.concurrency where you spawned a thread
> which returned a value when it was done (rather than having to use a send with a
> void function), and the parent thread used a receive call of some kind to get
> the return value. Ideally, you could spawn a series of threads which were paired
> with the variables that their return values would be assigned to, and you could
> do it all as one function call.
> Overall, I really like D's threading model, but it seems to me that it could be
> streamlined a bit.
> - Jonathan M Davis

BTW, the other thing we need to make message passing viable even when small
efficiencies don't count is an easy way of sending a deep clone of an arbitrarily
complex object graph (clone would, I guess, return a Unique type) as a message to
another thread.  Of course there are some obvious limitations on what can be
cloned.  For example, raw pointers to data structures containing more pointer
indirection couldn't be automagically deep cloned because you wouldn't know where
the boundary between valid data and invalid data is.  However, if I could just do
something like:

// myObj is unshared and is an arbitrarily complex object graph
// that consists entirely of SafeD-style objects, not raw pointers.
auto myObj = buildReallyComplicatedObject();
auto tid = spawn(&myFun, myObj.clone());

then I'd consider D's implementation of message passing viable for non-trivial use
cases, as long as simplicity and safety matter more than performance and flexibility.