>
>>  I do have worries about the efficiency of this though:
>> when marshaling a closure across a socket to another process, it
>> must marshal all the necessary state as well.  I would imagine this
>> has the potential to get slow if there was much state to marshal
>> (i.e., a closure marshaled referencing a large data structure).
>
> This is a potential problem. This is like so many language features
> that if misused, can lead to huge performance degradations. Knowing
> about the problem beforehand allows programmers to avoid it.

Just throwing out an idea, so someone who is more familiar with the  
GC/runtime correct me if this can't be done, but what about a smaller  
separate heap in shared memory that is reserved for concurrent data?   
Maybe you could declare a type

stype 'a tree =
| Node of 'a * 'a tree * 'a tree
| Leaf of 'a

that is always allocated in the shared heap, and have a separate GC  
thread that manages that heap.  Could that GC be concurrent without  
affecting the performance of non-concurrent data?

--Jonathan