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"Ref" and copy of functions
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Date: 2007-12-13 (17:27)
From: Pierre-Evariste Dagand <pedagand@g...>
Subject: "Ref" and copy of functions
Hi Caml-list,

I'm looking for advices about a "clean way" of doing something which,
by design, isn't. So, let states the problem.

I'm doing a combinator library based on Arrows for a Yampa-like system :

type ('a,'b) arrow = Arrow of ( 'a -> 'b )

Thus, I can instantiate an arrow with :

let arr f = Arrow ( f )
val arr : ('a -> 'b) -> ('a, 'b) arrow

Then, I have, for example, the composition of arrows :

let (>>>) (Arrow f) (Arrow g) =Arrow ( fun  c -> g  ( f  c ) )
val ( >>> ) : ('a, 'b) arrow -> ('b, 'c) arrow -> ('a, 'c) arrow

Right here, everything plays well (excepted some weak type variables
issues but that's not a big deal).

But (and that's here that the trouble is) I need a "loop" combinator,
that I wrote this way :

let loop init (Arrow f) =
  let state = ref init in
    Arrow (
      fun c ->
        let new_state , output = f ( !state , c ) in
          state := new_state;
val loop : 'a -> ('a * 'b, 'a * 'c) arrow -> ('b, 'c) arrow

Finally, I can write a small arrow  :

let arr_counter = loop 0 ( arr ( fun ( counter , x ) -> counter+1 ,
counter+x ) )
let arr_double = arr ( fun x -> 2*x )
let arr_my_small_arrow = arr_counter >>> arr_double

And I execute it with :

let run (Arrow f) input =
  f  input
val run : ('a, 'b) arrow -> 'a -> 'b

And it works. Right.

But now, I would like to be able to "copy" a built arrow and to be
able to execute the copy without side-effecting on the first one.
Obviously, I cannot do that in this implementation.

My current solution is really *ugly* :

let arrow_string = Marshal.to_string arrow [ Marshal.No_sharing ;
Marshal.Closures ]

Then I "Marshal.from_string arrow_string". I'm not even sure if it
will not blow out of my hands with "strange" things in the Ref cell.

I'm aware of implementations in CPS but I use to think that it has a
performance cost that I'm not ready to pay (I'm fighting against a C++
code so...).

So if someone knows an elegant solution to my problem (if someone has
read this mail until here), please let me know :-)

Best regards,

P.S.: For those who are just curious, these combinators are used in a
functional-reactive library for Peer-to-Peer overlays programming.

Pierre-Evariste DAGAND