[Camllist] Is arrow programming impossible in ocaml?
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Date:  20031015 (01:48) 
From:  Nick Name <nick.name@i...> 
Subject:  Re: [Camllist] 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 typeinference 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 camllistrequest@inria.fr Archives: http://caml.inria.fr Bug reports: http://caml.inria.fr/bin/camlbugs FAQ: http://caml.inria.fr/FAQ/ Beginner's list: http://groups.yahoo.com/group/ocaml_beginners