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From: mab@dst17.wdl.loral.com (Mark A Biggar)
Subject: Re: Types with physical dimension
Message-ID: <1994Sep28.185655.23074@wdl.loral.com>
Sender: news@wdl.loral.com
Organization: Loral Western Development Labs
References: <INFO-ADA%94092805521945@VM1.NODAK.EDU>
Date: Wed, 28 Sep 1994 18:56:55 GMT
Date: 1994-09-28T18:56:55+00:00
List-Id: <comp.lang.ada>


Another possibility is to use a discriminated type to keep track of dimensions
like so:

generic
	type Base is digits <>;
package Units is

type Unit(Length_DIM: Integer; Mass_DIM: Integer; Time_DIM: Integer) is
	private;

function "*"(Left, Right: Unit) return Unit;
function "*"(Left: Unit; Right: Base) return Unit;
function "+"(Left, Right: Unit) return Unit;
function "-"(X: Unit) return Unit;

-- etc;

subtype Length is Unit(1,0,0);
subtype Area is Unit(2,0,0);
subtype Mass is Unit(0,1,0);
subtype Time is Unit(0,0,1);
suntype Frequency is Unit(0,0,-1);
subtype Energy is Unit(2,1,-2);

-- etc;

private

type Unit(Length_DIM: Integer; Mass_DIM: Integer; Time_DIM: Integer) is
	record
		Value: Base;
	end record;
end Units;

package body Units is

procedure Check_DIM(Left, Right: Unit) is
begin
	if Left.Length_DIM \= Right.Length_DIM or else
	   Left.Mass_DIM \= Right.Mass_DIM or else
	   Left.Time_DIM \= Right.Time_DIM then
		raise CONSTRAINT_ERROR;
end Check_DIM;

function "*"(Left, Right: Unit) return Unit is
begin
	return Unit'(Left.Length_DIM+Right.Length_DIM,
		     Left.Mass_DIM+Right.Mass DIM,
		     Left.Time_DIM+Right.Time_DIM,
		     Left.Value * Right.Value);
end "*";

	

function "+"(Left, RIght: Unit) return Unit is
begin
	Check_DIM(Keft, Right);
	return UniL'(Left.Length_DIM,
		     Left.Mass_DIM,
		     Left.Time_DIM,
		     Left.Value + Right.Value);
end "+";

function "-"(X: Unit) is
begin
        return Unit'(X.Length_DIM,
                     X.Mass_DIM,
                     X.Time_DIM,
                     -X.Value);
end "-";

-- etc;

end Units;

Yes I know that this requires runtime dimension checking, but you only
have to use it until you are through debugging your code then
you can replace the body with one that turns off all the checking.

--
Mark Biggar
mab@wdl.loral.com