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A Few Questions
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Date: 2006-12-18 (01:52)
From: Jonathan T Bryant <jtbryant@v...>
Subject: A Few Questions
I've been reasearching into a parallel extension to OCaml (based on 
Reppy's CML and the OCaml Event
module).  Mostly it's extending the semantics of channels and using 
CamlP4 to add syntactic constructs
for concurrency, but there are a few extensions I'm having trouble with 
because of OCaml language
"problems", and so I have a few questions.

1) Why can't a functorized module be used as a functor to another 
module?  I don't know if this is
semantically not possible or if I am just doing it wrong.  I've played 
around with the syntax and
reread the manuals, but I can't seem to find a solution.  For example:

module type AModule =
    type t
module type BModule =
  functor (A : AModule) ->
    type t
    type s = A.t
module S : sig type t end 
module Make = functor (B : BModule) -> S with type t = B.t  (* This is 
what fails *)
module AImpl =
    type t = int
module BImpl =
  functor (A : AModule) ->
    type t = int
    type s = A.t
module X = Make (BImpl (AImpl))

2) Why, in general, are there not first class modules?  I've looked at 
Russo's paper on this and it
shouldn't conflict with static typing or type inference.  Again, for 
example, why isn't this allowed:

val make : ('a -> 'b) -> ('a -> 'c) -> sig type t val x : 'a -> 'b val 
y : 'a -> 'c end
let make a b =
    type t = int
    let x = a
    let y = b
module X = make (fun x -> x) (fun y -> y)

It doesn't seem like it would be very different from the already 
allowed immediate objects and local
module bindings.

3) Since CML's threads are implemented via continuations, speculative 
computation is allowed because
threads are simply GCed once they are not referenced any more.  Since 
OCaml's threads are implemeted
via system calls, is this still the case or do threads need to be 
manually joined?  I've run into
some instances where I can't create any more threads because the 
"Thread limit" of 1024 has been
reached, but in code where nowhere near that many threads should be 
left active.  Is this limit an
OCaml limit or a system limit?  Does it make a difference whether using 
native code and system
threads vs. bytecode and vmthreads?  Also, what about threads that are 
not referenced anymore but
should still be running (i.e., "background services" and the like)?  Is 
there any way to keep the GC
from collecting them?

4) I've found that in sending functions across sockets, I can only send 
them between copies of the
exact same binary image.  Is it possible to marshal functions to 
different binaries of the same code,
i.e., different platforms?  Again, does native vs. bytecode make a 

5) One possible extension is a vector type.  Is it possible as is to 
make the type inference
engine "as is" include the size of the underlying array as part of the 
type information or does that
require modifications to the type system?  Adding to the type 
information allows runtime size checks
to be avoided and allows code generation to take advantage of external 
vector processors and/or GPUs.
The ideal setup is vectors that are unboxed arrays of fixed length, 
similar to tuples.

--Jonathan Bryant