]> 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 ------------------- To unsubscribe, mail caml-list-request@inria.fr Archives: http://caml.inria.fr Bug reports: http://caml.inria.fr/bin/caml-bugs FAQ: http://caml.inria.fr/FAQ/ Beginner's list: http://groups.yahoo.com/group/ocaml_beginners