Re: [boost] [review] pqs

From: David Greene (greened_at_[hidden])
Date: 2006-06-08 13:48:35

Leland Brown wrote:

>>I think restricting the library to "physical quantities" is a big
>>mistake.  You've put a _lot_ of effort into getting the metaprogramming
>>right for his and we ought to try to re-use it wherever possible.  I
>>don't relish the idea of creating a parallel "electrotechnology system"
>>framework that looks exactly like pqs but with powers of 2 and 10 (to
>>do the conversions).  Let's get it right in a single library.
> 
> 
> Several reviewers have made comments to this effect.  It seems there is a need 
> in a variety of non-physics contexts as well for a general units conversion 
> library.
> 
> However, IMHO, PQS is *not* a units library.  Its purpose is to do dimensional 
> analysis.  For this reason (as well as a weaker one I mentioned in another 
> followup post), I oppose the name "units" for the library.  That would only add 
> to the confusion.  Units conversion is a necessary part of the library (or 
> adjuct to it) in order to create the dimensional quantities, but the core 
> purpose and utility of the library is its compile-time checking of dimensional 
> formulas and enforced documentation of dimensions (as well as units).

The two go hand-in-hand.  What good is a "units" library without
dimensional analysis?  All I'm saying is that we should not need
to create multiple versions of pqs to handle different sets of units.

Where does it stop?  I want one for "electrotechnology."  Someone
else wants one for non-SI units.  Someone gave an example of
dimensioning fruits and vegetables.  Do we really want separate
frameworks for each of these uses?

It should all be in one framework.

I'm not saying everything needs to be implemented right now.  Just
that the flexibility needs to be there for _someone_ to do it.
The assumption of powers-of-10 prefixes precludes that.

> Perhaps, as Gerhard Wesp suggested in one of his posts (sorry, I don't know how 
> to include a message reference):
> 
>>I think that handling of dimensional quantities and conversion factors
>>are orthogonal concepts and should be separated.
> 
> 
> If there's a way to do this, perhaps the units conversion can be made more 
> general and put into a separate library, and the dimensional analysis library 
> can make use of it.  But please, let's not lose sight of the great importance 
> of a dimensional analysis library.  Many users (though perhaps a minority) find 
> this *very* useful.

As Andy has stated, the two are orthogonal in pqs, it's just that
metaprogramming is used to encode the conversions in the type system,
thereby saving runtime calculation by doing it at compile time.  This
is the correct approach, because for t1_quantity, all of the necessary
information is known by the compiler.

                      -Dave