On 2005-10-20, Matt Calabrese <rivorus@gmail.com> wrote:
What I mean be overridden is: Could I choose to use velocity, force, and pressure as the basis for my dimensional analysis (so rather than thinking of velocity as "length / time", length would be represented as "pressure^-1 force^1 velocity^-2") You can do that as well by just creating your own "velocity" base classification separate from the one I define. This would be rather silly
On 10/20/05, Phil Richards <news@derived-software.ltd.uk> wrote: though as well as pointless. Currently you wouldn't be able to convert your base classification to the more natural derived classification implicitly. This could easily be added, but I don't think it is necessary.
Actually, it would be ill-formed - you would have introduced a mechanism for allowing two things of the same dimensionality to be represented in two different ways. I'm really not convinced that this is good for dimensional analysis...
Well, implicit conversions is how it will be at least for this version since it's already all in place. I still don't see why you would want to force all explicit conversions. Quantities in meters, centimeters, feet, inches, miles, etc. all represent the same concept and should be able to be converted between behind the scenes. Nothing logically bad ever comes from it at that level of abstraction.
Repeated transformation of values is not something that I've ever found particularly good when doing numerical algorithms. It is generally better to encourage up-front conversion and do everything internally in a consistent set of units. And explicit conversion forces you to think about what operations are going on. If I saw an equation written using mixed units I'd run a mile - I'd wonder why the person hadn't normalised all the values first :-) In fact, if I was implementing it, I would normalise the values first - using explicit conversions to make it absolutely clear what was going...
At this moment, the only result I see from adding direct support for quantities all requiring explicit conversions to the library is uneccessary bloating since the restrictions you desire are better expressible with higher-level code than a quantities library should provide.
Erm, there is little difference in forcing explicit or implicit unit conversions in terms of coding. I would, off-the-top-of-my-head, expect implicit unit conversion to be ever so slightly messier to do right - as demonstrated by you expresssion rearrangement example. Anyway, I'll agree to disagree with you on this one. phil -- change name before "@" to "phil" for email