From mboxrd@z Thu Jan  1 00:00:00 1970
Received: (from weis@localhost) by pauillac.inria.fr (8.7.6/8.7.3) id DAA07102 for caml-redistribution; Thu, 8 Jul 1999 03:50:18 +0200 (MET DST)
Received: from nez-perce.inria.fr (nez-perce.inria.fr [192.93.2.78]) by pauillac.inria.fr (8.7.6/8.7.3) with ESMTP id MAA28948 for <caml-list@pauillac.inria.fr>; Mon, 5 Jul 1999 12:57:15 +0200 (MET DST)
Received: from nenuphar.saclay.cea.fr (nenuphar.saclay.cea.fr [132.166.192.7])
	by nez-perce.inria.fr (8.8.7/8.8.7) with ESMTP id MAA04062
	for <caml-list@inria.fr>; Mon, 5 Jul 1999 12:57:14 +0200 (MET DST)
Received: from muguet.saclay.cea.fr (muguet.saclay.cea.fr [132.166.192.6])
	by nenuphar.saclay.cea.fr (8.9.1a/8.9.1/CEAnet-relay-5.0.D20) with ESMTP id MAA27577
	for <caml-list@inria.fr>; Mon, 5 Jul 1999 12:57:13 +0200 (MET DST)
Received: from harpie.saclay.cea.fr (harpie.saclay.cea.fr [132.166.133.152])
	by muguet.saclay.cea.fr (8.9.1a/8.9.1/CEAnet-relay-5.0.D20) with ESMTP id MAA17466;
	Mon, 5 Jul 1999 12:57:12 +0200 (MET DST)
Received: by harpie.saclay.cea.fr (8.8.8+Sun/CEANET.2.0.1)
	id MAA04036; Mon, 5 Jul 1999 12:57:08 +0200 (MET DST)
From: STARYNKEVITCH Basile <Basile.Starynkevitch@cea.fr>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Transfer-Encoding: 8bit
Message-ID: <14208.36740.406400.710367@gargle.gargle.HOWL>
Date: Mon, 5 Jul 1999 12:57:08 +0200 (MET DST)
To: caml-list@inria.fr
Subject: Q: parametric types in simple functors?
X-Mailer: VM 6.72 under Emacs 20.3.1
Content-Transfer-Encoding: 8bit
Content-Transfer-Encoding: 8bit
Sender: weis


The subject title is poor. I have a concrete simple question.

(All CAML code is indented to the right here)
Consider

   type 'a symbol = { sy_name: string; sy_hash: int; mutable sy_prop: 'a option };;

This is essentially a symbol, memoizing its hash code for fast
access. Some structures are naturally hashed map, whose key are
symbols (eg bindings). We make symbols with

  let get_symbol n =
    try Hashtbl.find symdict n 
    with Not_found ->
     let s = { sy_name=n; sy_hash=Hashtbl.hash n; sy_prop=None } in
      Hashtbl.add symdict n s;
      s
  ;;


The property field of symbols could be used the way you like it.

I want to define a binding as an hashtable mapping symbols to (say)
integers - or even any type 'b. But I can't code:

  module SymbolHash = Hashtbl.Make(
     struct 
       type t = symbol
       let equal s1 s2 = s1 == s2
       let hash {sy_hash=h} = h
     end)
;;

Because the type constructor t has no argument in the
Hashtbl.HashedType signature.

Any comments or suggestions (could define a class abstract_symbol
with a subclass symbol holding the properties. But I don't want
classes because I don't need inheritance).


The intent is that the symbol properties would be defined in another
module. I don't want to fix the 'a in symbol_t here, yet I want to
make an hashtable whose keys are symbols.

In other words, 

  why the stdlib doesn't give parametrized types for many modules
  (Hashtbl, Set...)

  when should I use a type constructor with an argument, or a functor?


Any comments?


N.B. Any opinions expressed here are only mine, and not of my organization.
N.B. Les opinions exprimees ici me sont personnelles et n engagent pas le CEA.

---------------------------------------------------------------------
Basile STARYNKEVITCH   ----  Commissariat à l Energie Atomique 
DTA/LETI/DEIN/SLA * CEA/Saclay b.528 (p111f) * 91191 GIF/YVETTE CEDEX * France
phone: 1,69.08.60.55; fax: 1.69.08.83.95 home: 1,46.65.45.53
email: Basile point Starynkevitch at cea point fr