Re: [Caml-list] Structural subtyping problem

[Christophe TROESTLER <Christophe.Troestler+ocaml@umons.ac.be>]

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.