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Date: Sun, 28 Mar 2010 19:32:37 +0200
From: =?UTF-8?B?U3TDqXBoYW5lIEdsb25kdQ==?= <steph@glondu.net>
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To: Dario Teixeira <darioteixeira@yahoo.com>
Cc: caml-list@yquem.inria.fr
Subject: Re: [Caml-list] Structural subtyping problem
References: <61102.44294.qm@web111503.mail.gq1.yahoo.com>
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X-Spam: no; 0.00; subtyping:01 recursion:01 bool:01 bool:01 ocaml:01 cheers:01 phane:98 steph:98 phane:98 polymorphic:01 rec:01 caml-list:01 int:01 int:01 parameter:02 

Dario Teixeira a =C3=A9crit :
> A simple nonsensical example that illustrates the problem is listed bel=
ow; the
> type-checking error occurs in function "step1", where the optional para=
meter "story"
> is used as an object of type "< title:string; .. >".  In function "step=
3", this
> parameter "story" is actually instantiated with objects of type "< titl=
e:string >"
> and "< title:string; count:int >".

As said elsewhere, this is because you are trying to do polymorphic
recursion. Standard tricks work here:

------------------------------------------------------------------------
type steps =3D {
  step1 : 'a. ?story:(<title : string; ..> as 'a) -> unit -> bool;
  step2 : string -> bool;
  step3 : 'a. story:(<title : string; count : int; ..> as 'a) -> bool
}

let rec steps =3D {

  step1 =3D
    begin fun ?story () ->
      match story with
        | Some s -> steps.step2 s#title
        | None -> steps.step2 "title1"
    end;

  step2 =3D
    begin fun title ->
      let story =3D object
        method title =3D title
        method count =3D 0
      end in
      steps.step3 story
    end;

  step3 =3D
    begin fun ~story ->
      match story#count with
        | 0 ->
	    steps.step1 ~story ()
        | 1 ->
	    let story =3D object
              method title =3D "title2"
            end in
            steps.step1 ~story ()
        | _ ->
	    true
    end
}

let step1 =3D steps.step1
let step2 =3D steps.step2
let step3 =3D steps.step3
------------------------------------------------------------------------

With OCaml 3.12, I guess this should be feasible without using the
intermediate "steps" record.


Cheers,

--=20
St=C3=A9phane