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Date: Tue, 15 Apr 2003 12:12:41 +0100
From: Axel Simon <A.Simon@ukc.ac.uk>
To: caml-list@inria.fr
Subject: [Caml-list] polymorphic modules in classes
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Precedence: bulk

Hi,

I would like to use a Hashtbl which is polymorphic in its elements. The 
elements should by compared by physical equality so I cannot use the 
"generic interface". My goal is to create a hash table over the type 'a 
which maps to int. I wrote:

module H = Hashtbl;;

module PhysEq : H.HashedType = struct
  type t 
  let equal = (==)
  let hash  = H.hash
end;;

module PhysHashtbl : H.S = H.Make(PhysEq);;

class ['a] bulkWriter ((fname, wr) : string * (out_channel -> 'a -> unit)) 
=
  let outCh = open_out_bin fname
  and stored   : (a' PhysHashtbl.t) = (PhysHashtbl.create 100)
  in object
  (* something here *)   
end

When I compile, I get "Unbound type constructor a'" for the line which 
introduces stored, although it works with int. I guess there are many 
other problems with my solution: PhysEq.t is abstract and PhysHashtbl.key 
is not bound to any type. How do I solve all this?

I am really new to OCaml and I couldn't make sense of the tutorials and 
other messages posted on this list.

Any help appreciated,
Axel.

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