Re: Dimensionality Checking (Ada 20XX)

[dmitry@elros.cbb-automation.de (Dmitry A. Kazakov)]

On 6 Dec 2001 11:52:12 -0800, britt@adapower.net (Britt Snodgrass)
wrote:

>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) ).

I would also prefer a representation clause over rather obscure
pragmas, but the question stands: how to do class-wide unit
programming? For instance, how to write [non-generic] subprogram
IntegrateTime which takes <unit>/s and returns <unit>? How to write a
subprogram that gets user input from a dialog field and returns some
dimensioned value?

Therefore I would prefer a solution based on discriminants or else
tags provided that the compiler will remove all extra data fields and
run-time checks in case when units are statically known.

Regards,
Dmitry Kazakov