Re: Dimensionality Checking (Ada 20XX)

[britt@adapower.net (Britt Snodgrass)]

dmitry@elros.cbb-automation.de (Dmitry A. Kazakov) wrote in message news:<3c1dd328.10829203@News.CIS.DFN.DE>...
> On Sat, 15 Dec 2001 02:07:41 -0500, "Steven Deller"
> <deller@smsail.com> wrote:
> 
> >What is wrong with using Pat Roger's dimensioning system.  It has units
> >and scaling.  It allows making metric, English, or other dimensional
> >units, with all "interconversions" straightforward and easy.  For
> >example, the following user code works with dimension checking and
> >appropriate scaling conversions:

I agree with Steve.  Pat Roger's approach seems pretty elegant to me.

> 
> I used a similar approach. The difference was that I used only one
> discriminant - a modular number holding all 7 powers. With
> Interfaces.Unsigned_32 it holds powers -8..7. Then I used an
> additional field for the offset [to cope with damned Celsius degree
> (:-))]

Can you provide an example of your modular type definition?

> 
> >The only difficulties I see are getting the RIGHT system of dimensions
> >for such a package (but then again, not being "built in", it can be
> >changed is so needed).
> 
> The approach is thinkable and it has a great advantage - an ability to
> write "class-wide" subroutines for dimensioned types. But it still
> faces several problems.
> 
> 1. Optimization issues. I saw no compiler able to remove run-time
> checks and storage for the dicriminants for constrained subtypes. [I
> wrote a small test program which calculated performance penalty. It is
> 6..20 times when compared with regular float]. I agree with you that
> such kind of optimizations could be even more useful than dimensioned
> values itself: some sort of user-defined compile-time expressions,
> removal of static discriminants as well as tags (the later one might
> require revison of tagged types, because of redispatch).
>

Could a large part of the performance penalty be attributed to your
use of a single modular number discriminant for the dimensional
powers?  I think this would require a lot of bit unpacking/repacking. 
Separate discriminants might be faster.

I would probably remove the unit checks in final production code by
redefining the unit types to be reqular floating types, recompiling,
and retesting.

Britt Snodgrass

 <<snip>>
 
> Regards,
> Dmitry Kazakov