Re: [Caml-list] Module/functor question

[ronniec95@lineone.net]

On Mon, Mar 15, 2004 at 04:22:18AM +0900, Yamagata Yoriyuki wrote:
> From: ronniec95@lineone.net
> Subject: [Caml-list] Module/functor question
> Date: Sun, 14 Mar 2004 19:05:28 +0000
> 
> > but I cannot figure out how to declare Foo having a signature of
> > Serialiser and then use it in main somehow. Assume also that I have
> > lots of other modules which have same basic interface but additional
> > methods specific to the modules.
> 
> You do not need to declare.  Foo.Make will accept any module whose
> signature is an *extension* of Serializer.
> 
> > (* was hoping for something like this *)
> > let _ = let m = module Sender(Foo.Make) in
> > 	m.serialisetoxml (Foo.Make (Xml.parse_from "..."));
> 
> By the way, a correct code would be
> 
>    let _ =
>       let module M = Sender(Foo.Make) in
>       M.serialisetoxml (Foo.Make (Xml.parse_from "..."));
> 
> --
> Yamagata Yoriyuki

Thanks that helped a lot - well it does work! but I'm a bit puzzled by
the extra declarations I had to do... 

I've copied the exact code below since it's not that long. I wonder
about header.mli which has 'type t=u' in it which was the only way of
getting it all to fit together. Why do I have to do that? Is there
another a more Ocaml way that I should be tackling this problem (I guess
a more design rather than implementation question...?)

Thanks for any ideas.

Ronnie


------- Code ----------
(*Header.mli*)
type u

module Make :
    sig
        type t = u
        val as_xml : t -> Xml.xml
        val create : Xml.xml -> t
    end

(* A function to construct Header.u objects *)
val make : ?version:int -> ?msgtypeid:int -> string -> string -> string -> u

(*Header.ml*)

(* some utility functions *)
let datetimefmt = "%Y%m%dT%H%M%S"
let find_node name root =
    let rec find (lst : Xml.xml list) res = match lst with
    | []    -> if List.length res = 0 
               then failwith ("Node not found:" ^ name)
               else res
    | Xml.Element(id,_,_) as hd::tl when id = name  -> find tl (hd::res)
    | _::tl                                         -> find tl res
    in
        find (Xml.children root) []



type u= { version : int; created : Calendar.t; environment : string; msgtypeid : int; msgtype : string; originatorid : string; machineid: string; }

(* OK I want everything to have this signature at least *)
module type Constructor =
    sig
        type t = u
        val as_xml : t -> Xml.xml
        val create : Xml.xml -> t
    end

module Make : Constructor =
struct
    type t = u (* why do I need this?*)
    let as_xml d = 
        Xml.Element("Header",[ 
                ("MsgVersion",(string_of_int d.version));
                ("Created",(fun x -> Printer.CalendarPrinter.sprint
                datetimefmt x)d.created);
                ("Environment",d.environment);
                ("MsgTypeId",(string_of_int d.msgtypeid));
                ("MsgType",d.msgtype)],
                [Xml.Element("Originator",[
                    ("OriginatorId",d.originatorid);
                    ("MachineId",d.machineid)],[])])

    let create (root:Xml.xml) : t =
        let originator = List.hd (find_node "Originator" root) in
        {   version = int_of_string(Xml.attrib root "MsgVersion");
            created = ((fun x -> Printer.CalendarPrinter.from_fstring datetimefmt x)
            (Xml.attrib root "Created"));
            environment = Xml.attrib root "Environment";
            msgtypeid = int_of_string(Xml.attrib root "MsgTypeId");
            msgtype = Xml.attrib root "MsgType";
            originatorid = Xml.attrib originator "OriginatorId";
            machineid = Xml.attrib originator "MachineId"; }

end

let make ?(version = 1) ?(msgtypeid=1) msgtype originator env =
        { version = version; created = Calendar.now(); environment = env; msgtypeid = msgtypeid; msgtype = msgtype; originatorid = originator; machineid = Unix.gethostname(); }

(*****************************************
 * main.ml - thanks to Yamagata Yoriyuki 
 *****************************************)

(* Don't like declaring this AGAIN! in main*)
module type Constructor =
    sig
        type t
        val as_xml : t -> Xml.xml
        val create : Xml.xml -> t
    end


module MessageSender (C : Constructor) =
    struct
        let send msg = print_string (Xml.to_string (C.as_xml msg))
    end

let _ = 
    let module M = MessageSender(Header.Make) in
    M.send (Header.make "Header" "TestMessage" "DEV") (* this works now!!*)

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