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From: britt@adapower.net (Britt Snodgrass)
Newsgroups: comp.lang.ada
Subject: Re: Dimensionality Checking (Ada 20XX)
Date: 6 Dec 2001 11:52:12 -0800
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Date: 2001-12-06T19:52:12+00:00
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Perhaps this is not a new proposal but it seems to me that units
(dimensionality) in Ada code would be most naturally implemented as
attributes of objects or types.  The units of an object or type would
be specified with a representation clause.  If unspecified, the unit
attribute would default to "null" (no units).  The compiler could then
enforce unit consistency during compilation or generate runtime unit
checking code.

For example:

with Ada.Text_IO;

with Ada.Dimensionality.ISO_Metric_Units; 
-- predefined for kilograms, meters, etc.
 
with Ada.Dimensionality.My_User_Defined_Units
-- parsecs_per_picosecond, etc.

use  Ada.Dimensionality;

package body Proposed_Unit_Syntax_Example is

   type Magnitude is digits 15;

   Delta_Position : Magnitude;
   for Delta_Position'Unit use ISO_Metric_Units.Meter;

   Delta_Time : Magnitude;
   for Delta_Time'Unit use ISO_Metric_Units.Second;

   Current_Speed : Magnitude;
   for Current_Speed'Unit use 
     ISO_Metric_Units.Meter / ISO_Metric_Units.Second;

   Accel : Magnitude;
   for Accel'Unit use
     ISO_Metric_Units.Meter / ISO_Metric_Units.Second**2;
     -- note use of operators in attribute definition.

begin

   Delta_Position := 500.0;  -- meters implied by unit attribute

   Delta_Interval := 10.0;  -- seconds implied by unit attribute

   Current_Speed := Delta_Position / Delta_Interval;
   -- OK, units match

   Current_Speed := Delta_Position / Delta_Interval**2;
   -- won't compile or raises a predefined Unit_Error exception;

exception

   when Unit_Error =>
      Ada.Text_IO.Put_Line
        ("Oops! I just passed Mars without stopping to orbit!");

end Proposed_Unit_Syntax_Example;


Since all of the variables in this example are of the same type, no
new arithmetic operators need to be defined.  However the compiler
would use the unit attributes to ensure that any resulting right-side
composite unit matches the left-side unit before allowing an
assignment to proceed.  Most unit inconsistencies would be caught
during compilation. If a unit mismatch can't be caught until runtime,
then a predefined Unit_Error exception would be raised (unless the
code had been compiled with Pragma Suppress (Unit_Checks) ).

Britt Snodgrass