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From: "Nick Roberts" <nickroberts@adaos.worldonline.co.uk>
Newsgroups: comp.lang.ada
Subject: Re: Dimensionality Checking (Ada 20XX)
Date: Mon, 10 Dec 2001 03:33:18 -0000
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Date: 2001-12-10T03:33:18+00:00
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"James Rogers" <jimmaureenrogers@worldnet.att.net> wrote in message
news:3C141540.96E5EB0D@worldnet.att.net...
> Mark Lundquist wrote:
> >
> > Would you not prefer a solution in which no particular set of units was
> > built in to the core language?  That is, I think it would be fine if the
> > standard library had a package Ada.SI_Units or whatever, written in
ordinary
> > Ada 20xx (a unit-aware language), just to save everyone the trouble of
> > writing it themselves, but I don't think that the core language itself
> > should have any intrinsic units.  Please comment!
>
> I agree. The basic language capability of units should not declare any
> particular set of units.
>
> The SI_Units package proposed can be useful, but also terribly limiting.
> The chosen units would be inappropriate for calculations dealing with
> submolecular physics, or astronomy. Neither of these fields should be
> systematically ignored by Ada.


Perhaps I did not make it clear enough, but the user can declare her own
units using the predefined units as a base, e.g.:


   Parsec:   constant Unit_Type := Ada.Units.Meter * 3.01e16;
   Angstrom: constant Unit_Type := Ada.Units.Meter * 10.0e-10;


User-defined dimensions could be accommodated by making the Ada.Units
package generic, and maybe renaming it, e.g.:


   generic
      type Dimension is (<>);

   package Ada.Dimension_Checking is

      type Dimensionality is array (Dimension) of Integer;

      type Scaling_Factor is digits ...; -- highly accurate

      type Unit_Type is
         record
            Is_Unitless: Boolean := True;
            Dim: Dimensionality := (others => 0);
            Scaling: Scaling_Factor := 1.0;
         end record;

      Unitless: constant Unit_Type := (Is_Unitless => True, Scaling => 1.0);

      -- functions "*", "/", and "**"

   end Ada.Dimension_Checking;


There might then be:


   package Ada.ISO_Units is

      type ISO_Dimension is (Mass, Length, Time, Temperature, Matter,
Current);

      package ISO_Dimension_Checking is new
Ada.Dimension_Checking(ISO_Dimension);

      subtype Unit_Type is ISO_Dimension_Checking.Unit_Type;

      Meter: constant Unit_Type := (Is_Unitless => False,
                                    Dim         => (Length => 1, others =>
0),
                                    Scaling     => 1.0);

      -- etc.

   end Ada.ISO_Units;


This complicates things for the compiler, so it would be nice if it were not
necessary.

--
Best wishes,
Nick Roberts