On Mon, 13 Feb 2006, Basile STARYNKEVITCH wrote:

>
> Hello All,
>
> I'm a bit ashamed to ask such a quasi FAQ, but I don't understand why
> the following does not compile
>
>
>  class virtual ['VarT,'NodeT] myclass = object(self)
>    method virtual v : 'VarT
>    method virtual n : 'NodeT
>  end;;
>
>  type
>    'a vart = [> `NothingV | `IntegerV of int | `NodeV of 'a nodet ]
>  and
>    'a nodet = Empty | LeafI of int | ObLeaf of 'a instancet | Node of nodet list
>  and
>    'a instancet = ('a,nodet) myclass constraint 'a = 'a vart

Maybe something like that:

class virtual ['VarT,'NodeT] myclass = object(self)
   method virtual v : 'VarT
   method virtual n : 'NodeT
end;;

(* 'a = 'VarT
    'b = 'NodeT *)
type ('a, 'b) nodet_poly =
     Empty
   | LeafI of int
   | ObLeaf of ('a, 'b) instancet_poly
   | Node of ('a, 'b) nodet_poly
and
   ('a, 'b) instancet_poly = ('b, ('a, 'b) nodet_poly) myclass

(* 'b = 'NodeT is constrained in vart *)
type ('a, 'b) vart = 'b
constraint 'b =
     [> `NothingV | `IntegerV of int | `NodeV of ('a, 'b) nodet_poly ]

type ('a, 'b) instancet = (('a, 'b) vart, ('a, 'b) nodet_poly) myclass


Martin

> I'm getting the following error
>
> File "ess.ml", line 8, characters 12-66:
> Unbound type parameter [..]
>
> which is positionned inside the bracket [] of vart line.
>
> Sorry for asking such a nearly FAQ; I thought that this would be a
> nearly canical example of the use of constraint.
>
> The real context of all this is some scriptable Wiki-like application
> which contains a (dynamically typed) interpreter (quasi scheme
> semantics with some ml-like or ada-like syntax). The actual need is to
> box in an open variant type (like vart above) all the "special" values
> of the interpreter.
>
>
> Again, my apologies for asking a nearly FAQ... (I googled a bit, but
> did not understood the relation between the answers & my question).
>
> Regards
>
> -- 
> Basile STARYNKEVITCH         http://starynkevitch.net/Basile/
> email: basile<at>starynkevitch<dot>net
> aliases: basile<at>tunes<dot>org = bstarynk<at>nerim<dot>net
> 8, rue de la Faïencerie, 92340 Bourg La Reine, France
>
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--
Martin Jambon, PhD
http://martin.jambon.free.fr

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