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[Caml-list] Is arrow programming impossible in ocaml?
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Date: -- (:)
From: Nick Name <nick.name@i...>
Subject: Re: [Caml-list] Is arrow programming impossible in ocaml?
Alle 02:22, mercoledì 15 ottobre 2003, Jacques Garrigue ha scritto:
> I'm afraid you will have to state in more detail what you want to do.
> Or to show some Haskell code.

Here it is:

module type ARROW =
sig
  type ('a,'b) t

  val arr : ('a -> 'b) -> ('a,'b) t
  val (>>>) : ('a,'b) t -> ('b,'c) t -> ('a,'c) t
  val (&&&) : ('a,'b) t -> ('a,'c) t -> ('a,('b * 'c)) t
end

module Time : sig

  include ARROW

  val time : ('a,float) t
  val run : (unit,'a) t -> float -> 'a option
  val const : 'a -> ('b,'a) t
end =
struct
  type state = (float * float)

  (* A signal transformer is a function wich takes the current time (and  
     dt) and maybe returns a value, or stops with None *)

  type ('a,'b) t = T of ((state * 'a) -> (('b * ('a,'b) t) option))

  let rec arr f = T (fun (_,x) -> Some (f x,arr f))

  let rec (>>>) (T a1) (T a2) =
    T (fun (s,x) ->
	 match a1 (s,x) with
	     None -> None
	   | Some (r1,n1) ->
	       match a2 (s,r1) with
		   None -> None
		 | Some (r2,n2) ->
		     Some (r2,n1 >>> n2))

  let rec (&&&) (T a1) (T a2) =
    T (fun arg ->
	 match (a1 arg,a2 arg) with
	     ((None,_)|(_,None)) -> None
	   | (Some (r1,n1),Some (r2,n2)) -> Some ((r1,r2),n1 &&& n2))

  let rec time = T (fun ((t,_),_) -> Some (t,time))

  let run (T f) t = 
    match f ((t,0.0),()) with
	None -> None
      | Some (r,_) -> Some r

  let const x = arr (fun _ -> x) 
end

(* now in the toplevel

  # let t = time >>> time;;
  val t : ('_a, float) Var.Time.t = <abstr> *)
 
Well, that '_a is exactly what I wish to avoid; I had this suggestion 
privately (with simplified type declarations):

# type ('a,'b) sf = Arr of ('a * float -> 'b);;
type ('a, 'b) sf = Arr of ('a * float -> 'b)
# let (>>>) f1 f2  = match f1(),f2() with (Arr f1),(Arr f2) -> Arr (fun 
(a,s) -> f2 (f1(a,s),s));;
val ( >>> ) : (unit -> ('a, 'b) sf) -> (unit -> ('b, 'c) sf) -> ('a, 'c) 
sf =
  <fun>
# let time () = Arr (fun (_,t) -> t);;
val time : unit -> ('a, float) sf = <fun>
# let t () = time >>> time;;
val t : unit -> ('a, float) sf = <fun>


and yet I don't know if it's satisfactory, however I realize that this 
is a general problem with caml view of polymorphism, probably driven by 
type-inference decidability issues; in fact I can't write a polymorphic 
compose operator in general, because I will get exactly the same 
problems.

Vincenzo

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