Re: [Caml-list] Existential row types

[Alain Frisch <alain@frisch.fr>]

Hi Leo,

On 7/15/2014 6:11 PM, Leo White wrote:
 > There is a work-around, but it is quite convoluted:
 >
 >    https://sympa.inria.fr/sympa/arc/caml-list/2012-10/msg00131.html

Very clever trick. Thanks!

One can actually use the GADT only to carry the constraint and pass the 
value argument(s) separately.  This means the GADT definition remains 
very simple even if we need to add many fields:


=============================================================
class type c = object end

module type SIG =
sig
   type t = private #c
   val f: t -> unit
   val g: unit -> t
end

type 'a aux = Aux: (#c as 'a) aux

let create f g =
   let helper (type u) (Aux : u aux) f g =
     (module struct
        type t = u
        let f = f
        let g = g
      end : SIG)
   in
   helper Aux f g

(* or even: *)

let create f g =
   let module Aux = struct type 'a t = E: (#c as 'a) t end in
   begin fun (type u) (Aux.E : u Aux.t) f g ->
     (module struct
        type t = u
        let f = f
        let g = g
      end : SIG)
   end Aux.E f g
=============================================================



-- Alain