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From: eachus@spectre.mitre.org (Robert I. Eachus)
Subject: Re: Help on deciding a data type.
Date: 1996/09/05
Message-ID: <EACHUS.96Sep5193850@spectre.mitre.org>#1/1
X-Deja-AN: 178758841
references: <322C50E0.167E@sdrc.com> <ytc20gicxob.fsf@erlang.praxis.co.uk>
organization: The Mitre Corp., Bedford, MA.
newsgroups: comp.lang.ada
Date: 1996-09-05T00:00:00+00:00
List-Id: <comp.lang.ada>


In article <ytc20gicxob.fsf@erlang.praxis.co.uk> sgb@erlang.praxis.co.uk (Stephen Bull) writes:

  > I would suggest defining a record type with two fields: the first is a flag
  > indicating whether the second is a valid value:

  > type T is record
  >   Valid : Boolean;
  >   Value : Symbol;
  > end record;

   There are two choices here.  The one I would recommend if
efficiency is an issue is to go through the pain of redefining all of
the operations in a generic package, then instantiating that package
for all moduli of interest.

   If however you want a simple, useable method that will never steer
you wrong try:

   type Real_Mod7 is mod 7;
   type Mod7 is access Mod7;

   Indeterminate: Mod7 := null;

   ...

   function "+" (L,R: Mod7) return Mod7 is
   begin
     return new Real_Mod7'(L.all + R.all);
   exception
     when others return Indeterminate;
   end "+";

   ...and so on.

   Notice that even on a garbage collecting Ada implementation you are
going to spend a lot of extra CPU cycles over a more sophisitcated
solution.  But you will get the correct answers.

--

					Robert I. Eachus

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