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From: Ivan Gotovchits <ivg@ieee.org>
To: Martin DeMello <martindemello@gmail.com>
Cc: "caml-list\@inria.fr" <caml-list@inria.fr>
References: <CAFrFfuHwyDw1u41bnWnZNzUATRn+5inB1h8udW09sDVH134Wjg@mail.gmail.com>
Date: Mon, 16 Sep 2013 08:20:57 +0400
In-Reply-To: <CAFrFfuHwyDw1u41bnWnZNzUATRn+5inB1h8udW09sDVH134Wjg@mail.gmail.com>
	(Martin DeMello's message of "Sun, 15 Sep 2013 15:29:25 -0700")
Message-ID: <87txhlbdna.fsf@golf.niidar.ru>
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Subject: Re: [Caml-list] "subclassing" a char map

Martin DeMello <martindemello@gmail.com> writes:

> I have a char multiset defined via
>
> module MultiSet =3D Map.Make(struct type t =3D char let compare =3D compa=
re end)
>
> Now I would like to split off a distinct type that can only contain
> A-Z as keys. What's the best way to do this?
>
> martin

Not sure, that I correctly understood your needs... but you can
implement =C2=ABA-Z keys=C2=BB as an abstract type, contained in a module w=
ith the
following signature:

module type Caps =3D  sig
  type t
  val create: char -> t option
  val compare: t -> t -> int
  val project: t -> char
end

Next, you can instatiate a Map from abstract type Cap.t to 'a:

module CapMap =3D Map.Make(Caps)

where Caps is an implementation, conforming to the signature Caps. For
example like this:

module Caps : Caps =3D struct
    type t =3D char
    let create =3D function
      | 'A'..'Z' as ch -> Some ch
      | otherwise      -> None
    let compare =3D compare
    let project ch =3D ch
end

--=20
         (__)=20
         (oo)=20
   /------\/=20
  / |    ||=20=20=20
 *  /\---/\=20
    ~~   ~~=20=20=20
...."Have you mooed today?"...