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Message-ID: <46CC8D72.50205@ed.ac.uk>
Date: Wed, 22 Aug 2007 20:24:34 +0100
From: Jeremy Yallop <jeremy.yallop@ed.ac.uk>
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To: Philippe Wang <lists@philippewang.info>
Cc: Yitzhak Mandelbaum <yitzhak@research.att.com>,
	caml-list@yquem.inria.fr
Subject: Re: [Caml-list] "opening" record types
References: <67ECB685-E69A-43CA-867D-30B879DB0510@research.att.com> <46CC6502.5090003@philippewang.info>
In-Reply-To: <46CC6502.5090003@philippewang.info>
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X-Spam: no; 0.00; struct:01 sig:01 struct:01 wrote:01 caml-list:01 int:01 int:01 expression:02 module:03 module:03 types:05 definition:07 quite:08 useful:09 philippe:11 

Philippe Wang wrote:
> Instead of :
> #  module M = struct type t = { a : int ; b : int } end ;;
> module M : sig type t = { a : int; b : int; } end
> 
> Write :
> #  module M = struct type t = { a : int ; b : int } module N = struct 
> type t = M.t = { a : int ; b : int } end end ;;

I don't think this code is quite what you intended.

   # ({a = 0 ; b = 0 } : M.t );;
   Characters 1-17:
      ({a = 0 ; b = 0 } : M.t );;
      ^^^^^^^^^^^^^^^^
   This expression has type t = M.t but is here used with type M.t

I like the idea, though.  You can avoid duplicating the type definition 
by writing the inner module first.

   module M =
   struct
     module N = struct type t = { a : int; b : int } end
     include N
   end

Then, as before:

    open M.N

    { a = 0; b = 0 }

> That way, when you change or add definitions, it's easy to spread 
> changes, and you export only what you want to export...

Yes.  Unforunately, the technique is only useful if you have control of 
the module that you want to import.

Jeremy.