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Date: Tue, 3 Aug 2004 16:50:27 +0100
To: John Prevost <j.prevost@gmail.com>
Cc: Ocaml Mailing List <caml-list@inria.fr>
Subject: Re: [Caml-list] Functors and classes
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From: Richard Jones <rich@annexia.org>
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On Tue, Aug 03, 2004 at 11:06:33AM -0400, John Prevost wrote:

I've found a problem with this approach.  If I replace the connect
function here:

> module ThingPool (Thing : ThingIntf) :
>     (ThingPoolT with type connection = Thing.connection) =
>   struct
>     type connection = Thing.connection
>     let connect () = Thing.connect ()
>   end

with:

  let connect () =
    let conn = Thing.connect () in
    ignore (conn#a);
    conn

then I get the following error:

  File "test.ml", line 30, characters 14-18:
  This expression has type Thing.connection
  It has no method a

So I extended the type of connection as follows to make it work:

  module type ThingIntf =
    sig
      type connection = <a : int; b : int; c : int>
      val connect : unit -> connection
    end

That's all well and good.  BUT I need to define something more like
this:

  module type ThingIntf =
    sig
      type connection = <a : int; ..>
      val connect : unit -> connection
    end

which gives me:

  File "test.ml", line 14, characters 8-35:
  A type variable is unbound in this type declaration

(I'm only uses a small subset of the much larger interface in my
pooling code).

Full code which fails is attached.

Thanks,

Rich.

----------------------------------------------------------------------
module TestThing =
  struct
    class connection =
    object
      method a = 1
      method b = 1
      method c = 1
    end
    let connect () = new connection
  end

module type ThingIntf =
  sig
    type connection = <a : int; ..>
    val connect : unit -> connection
  end

module type ThingPoolT =
  sig
    type connection
    val connect : unit -> connection
  end

module ThingPool (Thing : ThingIntf) :
    (ThingPoolT with type connection = Thing.connection) =
  struct
    type connection = Thing.connection
    let connect () =
      let conn = Thing.connect () in
      ignore (conn#a);
      conn
  end

module X = ThingPool(TestThing)

let x = X.connect ()

let a = x #a
let b = x #b
let c = x #c
----------------------------------------------------------------------

-- 
Richard Jones. http://www.annexia.org/ http://www.j-london.com/
Merjis Ltd. http://www.merjis.com/ - improving website return on investment
If I have not seen as far as others, it is because I have been
standing in the footprints of giants.  -- from Usenet

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