Re: [Caml-list] Utopian idea ?

["Hezekiah M. Carty" <hez@0ok.org>]

On Fri, Sep 16, 2011 at 5:33 PM, Pierre-Alexandre Voye
<ontologiae@gmail.com> wrote:
>
> I implemented the directory, and as i expected i had a problem : the
> compiler doesn't accept it because it doesn't know the type of object.
>
> So here the code :
>
> class _DIRECTORY = object  (self :'self)
>     val mutable indexDico = Hashtbl.create 128
>     val mutable dictionnaires =  Hashtbl.create 128
>     val mutable maxIndex = 0
>     method getDict = dictionnaires
>     method getIndex = indexDico
>     method getMaxIndex = maxIndex
>     method incMaxIndex = maxIndex <- (self#getMaxIndex +1)
>     method addDict (name:string) = (self#incMaxIndex) ;
>                            let elem = Hashtbl.create 128 in Hashtbl.add
> (self#getIndex) name elem
>   end;;
>
> Error: Some type variables are unbound in this type:
>          class _DIRECTORY :
>            object
>              val mutable dictionnaires : ('a, 'b) Hashtbl.t
>              val mutable indexDico : (string, ('c, 'd) Hashtbl.t) Hashtbl.t
>              val mutable maxIndex : int
>              method addDict : string -> unit
>              method getDict : ('a, 'b) Hashtbl.t
>              method getIndex : (string, ('c, 'd) Hashtbl.t) Hashtbl.t
>              method getMaxIndex : int
>              method incMaxIndex : unit
>            end
>        The method getDict has type ('a, 'b) Hashtbl.t where 'b is unbound
>
>
> It doen't surprised me anymore, but i there a strategy to allow that and to
> get round this difficulties ?
>

You need to provide some explicit types:

class ['a, 'b] _DIRECTORY = object  (self :'self)
  val mutable indexDico : ('a, 'b) Hashtbl.t = Hashtbl.create 128
  val mutable dictionnaires : ('a, 'b) Hashtbl.t =  Hashtbl.create 128
  val mutable maxIndex = 0
  method getDict = dictionnaires
  method getIndex = indexDico
  method getMaxIndex = maxIndex
  method incMaxIndex = maxIndex <- (self#getMaxIndex +1)
  method addDict (name:string) =
    (self#incMaxIndex) ;
    let elem = Hashtbl.create 128 in Hashtbl.add (self#getIndex) name
elem
end

See the class section of the manual for more information:

http://caml.inria.fr/pub/docs/manual-ocaml/manual005.html#toc27

I hope this helps.

Hez