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Date: Sun, 28 Mar 2010 21:20:35 +0200 (CEST)
Message-Id: <20100328.212035.735321043546824606.Christophe.Troestler+ocaml@umons.ac.be>
To: steph@glondu.net
Cc: darioteixeira@yahoo.com, caml-list@yquem.inria.fr
Subject: Re: [Caml-list] Structural subtyping problem
From: Christophe TROESTLER <Christophe.Troestler+ocaml@umons.ac.be>
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X-Spam: no; 0.00; subtyping:01 christophe:01 troestler:01 christophe:01 troestler:01 ocaml:01 0200,:01 bool:01 bool:01 recursive:01 sig:01 val:01 val:01 struct:01 imho:01 

Hi,

On Sun, 28 Mar 2010 19:32:37 +0200, Stéphane Glondu wrote:
> 
> type steps = {
>   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 = {  [...]

IMHO, recursive modules are also a nice way to achieve the same
effect :

module rec X :
sig
  val step1 : ?story:<title : string; ..> -> unit -> bool
  val step2 : string -> bool
  val step3 : story:<title : string; count : int; ..> -> bool
end = struct
  let rec step1 ?story () = match story with
    | Some s -> X.step2 s#title
    | None -> X.step2 "title1"

  and step2 title =
    let story = object
      method title = title
      method count = 0
    end in
    X.step3 story

  and step3 ~story = match story#count with
    | 0 -> X.step1 ~story ()
    | 1 ->
        let story = object
          method title = "title2"
        end in
        X.step1 ~story ()
    | _ -> true
end

My 0.02¤,
C.