Re: Announce: Grace project site operational

[Dmitry A. Kazakov <mailbox@dmitry-kazakov.de>]

On 17 May 2002 17:03:39 +0200, Fraser Wilson
<newsfraser@blancolioni.org> wrote:

>Unfortunately, there's a wee problem with magnitude changes; say I add
>the following:
>
>   function "*" (Left : Float; Right : Metres) return Metres;
>
>Then for M : Metres, M := 2.0 * M is ambiguous, because of Standard."*"
>
>What's more, Standard."*" has no meaning in this domain, but there's
>no way (as far as I know) of getting rid of it.

Override it with abstract. It is possible for non-tagged types. Here
is how it is made in my implementation of units:

with Interfaces;

package Units is
   pragma Pure (Units);
   subtype UnitPower is Interfaces.Unsigned_32;
   type Unit is new UnitPower;

   function "**" (Left : Unit; Right : Integer) return Unit;
   function "*"  (Left, Right : Unit) return Unit;
   function "/"  (Left, Right : Unit) return Unit;
   function Sqrt (X : Unit) return Unit;
   pragma Inline ("**", "*", "/", Sqrt);
   function Image
            (  Value  : Unit;
               Latin1 : Boolean := True
            )  return String;
   --
   -- The following operations are disallowed.
   --
   function "abs" (      Right : Unit) return Unit is abstract;
   function "and" (Left, Right : Unit) return Unit is abstract;
   function "mod" (Left, Right : Unit) return Unit is abstract;
   function "not" (      Right : Unit) return Unit is abstract;
   function "or"  (Left, Right : Unit) return Unit is abstract;
   function "rem" (Left, Right : Unit) return Unit is abstract;
   function "xor" (Left, Right : Unit) return Unit is abstract;

   function "+" (Right : Unit) return Unit is abstract;
   function "-" (Right : Unit) return Unit is abstract;

   function "+" (Left, Right : Unit) return Unit is abstract;
   function "-" (Left, Right : Unit) return Unit is abstract;

private
   PowerBits    : constant := (UnitPower'Size - 1) / 7;
   Current      : constant := 2**(PowerBits*0);
   Luminescence : constant := 2**(PowerBits*1);
   Temperature  : constant := 2**(PowerBits*2);
   Mass         : constant := 2**(PowerBits*3);
   Length       : constant := 2**(PowerBits*4);
   Quantity     : constant := 2**(PowerBits*5);
   Time         : constant := 2**(PowerBits*6);
   Unitless     : constant := 0;
end Units;

---
Regards,
Dmitry Kazakov
www.dmitry-kazakov.de