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Date: Tue, 9 Jul 2013 22:43:12 +0200
From: Goswin von Brederlow <goswin-v-b@web.de>
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Subject: [Caml-list] GADTs and associative container

Hi,

I'm wondering if one can have an ascociative container, like a Hashtbl.t
with dependent types (GADTs as the key, value depending on the key).
Something like this:

module H = struct
  type ('a, 'b) t = ('a, 'b) Hashtbl.t
  let create : type a b . int -> (a b, a) t =
		 fun x -> Hashtbl.create x
  let add : type a b . (a b, a) t -> a b -> a -> unit =
	      fun h k v -> Hashtbl.add h k v
  let find : type a b . (a b, a) t -> a b -> a =
	      fun h k -> Hashtbl.find h k
end
    
type one = ONE
type two = TWO

type _ g =
| ONEt : one g
| TWOt : two g

let () =
  let h = H.create 0
  in
  H.add h ONEt ONE;
  H.add h TWOt TWO;
  assert (H.find h ONEt = ONE);
  assert (H.find h TWOt = TWO);
  ()


BUT:

    let create : type a b . int -> (a b, a) t =
                                    ^^^
Error: Unbound type constructor b


Is there some special syntax I'm missing or is it simply impossible to
declare such a container in the abstract?

MfG
	Goswin