[boost] Re: Unit library

From: Dan W. (danw_at_[hidden])
Date: 2004-01-11 16:35:34

Deane Yang wrote:
> (By now, I think they should be dimensions with respect to explicitly
> stated units, which is, in fact, what I think some of the proposals
> actually do. You want to allow the same dimension but with different 
> units, AND you want to allow different dimensions that use the same
> units. A given type should specify BOTH the dimension and the unit.)

I think you got the solution!

Dimension IS needed, but not in the semantic context that it is used in 
the current proposals --namely, as an identifier for a set of 
dimensionally equivalent units. It is rather needed to disambiguate 
dimensionally equivalent units that should non-the-less be treated 
differently:

Sets of dimensionally equivalent units may need to be split into 
different dimensions, to denote != meanings. Cases found so far:
     torque vs. work
     hubble constant vs. frequency
     radians vs. inflation adjustment
as well as, perhaps, cases of absolute versus relative or differential 
quantities, such as:
     date vs. days
     position vs. distance
     temperature vs. temperature-diff

Perhaps another semantic use of 'dimension' might be to separate sets of 
units that are rationally related, from those that are irrationally 
related. Thus, inches, feet and yards would be in one set;  while mm, 
cm, dm, m, decam, hm, km would be in another set. Automatic conversions 
might be allowed within a set, but made explicit between sets.

I think there's something here... Opinions?