Gunther, (01)
Your description looks reasonable if you restrict the range of values that Unit
can take to only the set of coherent derived units. (02)
jbc (03)
Gunther Schadow wrote: (04)
>
> Actually I believe the answer should be "don't count on it!" -- meaning,
> you asked the wrong question. May be the best way to describe what I mean
> is this. We have (essentially) in HL7 the following design:
>
> Quantity {
> specificKindOfQuantity : KindOfQuantity;
> value : NumberAndUnit;
> }
>
> NumberAndUnit {
> number : REAL;
> unit : Unit;
> canonicalForm : NumberAndUnit;
> }
>
> Unit extends DimensionedQuantity {
> /*and adds*/ symbol : String;
> }
>
> DimensionedQuantity {
> number : REAL;
> dimension : VectorOfExponentsOverBaseUnits;
> }
>
> As you can see in this, there is a difference between Quantity and
> NumberAndUnit. NumberAndUnit does not preserve the specific kind of
> quantity, only the dimension. But Quantity does preserve this.
>
> Now, you can do or not do things with Quantity that you can or
> can not do with simple NumberAndUnit.
>
> For instance, for NumberAndUnit, 1 N.m = 1 N.m = 1 J always and without
> any qualification. But if you have an ontology of mechanics, you can
> tie that into the Quantity.specificKindOfQuantity, and it might
> tell you that you can't compare a N.m of torque with a N.m of energy.
>
> On the opposite end, you may have an ontology of chemistry in there
> which will tell you that you can equate 180 g/dL of Glucose mass
> concentration with 1 mol/dL of Glucose substance concentration something
> you would not have thought from just looking at the dimension.
>
> There is significant potential to create very powerful but also very
> complex ontologies of kinds of quantity which would be used to
> compute freely with Quantities. But this is not a matter of Units,
> and the NumberAndUnit (or Quantity.value) computations and comparisons
> would be much less powerful, just as DimensionedQuantities.
> (05)
--
_______________________________
Joseph B. Collins, Ph.D.
Code 5583, Adv. Info. Tech.
Naval Research Laboratory
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(202) 767-1122 (fax)
B34, R221C
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