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costello@mitre.org@INTERNET@wtgw'(Roger L. Costello)
If we grant that kilometer is a property/relation/function between
a distance dimension object and a number, then the choice of
representations would be:

 <River id="Yangtze">
 <length kilometers="6300"/>
 </River>

or alternatively:

 <River id="Yangtze">
 <length>
 <kilometer>6300</kilometer>
 </length>
 </River>

This is quite a radical approach. I am surprised that there
aren't more comments, since the "conventional wisdom" is
not to treat kilometer as a property/relation/function, but rather
to treat it as the value of a property, e.g.,

 <River id="Yangtze">
 <length unit="kilometer">6300</length>
 </River>

Thoughts? /Roger
In one of the systems I've designed which allow for units to be specified on
properties, the 'units' attribute gave the name which was mapped to a object
comprising integers representing the dimensionality of the numerator and
denominator (bases being prime, eg length > 2, time>5 so 2/25 is acceleration)
and a real factor to convert to unit SI. As such this was treated as an additional
property of the length property, and a seperate functions used it to do the
conversion to and from other compatible units. No checks were provided against
converting torque and energy in this system (both force times distance). Names were
parsed for combinations of the predefined units in the implementation (eg "m/s2" is
the combination of metres and 1/(seconds squared)). Names such as "D3/T:4.54" were
also allowed (distance cubed per time, conversion factor 4.54) for units not in the
implementation's map.
In a functional language, the name 'kilometer' could be mapped to a function to
convert to the basis units, but this may also require a second parameter for
dimensionality to be passed around, depending on whether the language's type system
could catch these. There would also be a need for a system to composite these
functions for combinations of units.
In terms of the use cases I've met, you have one property specified as having one
value and one units label. The user wants the representation of the value and the
units to be preserved in the persistent data; the underlying maths wants the value
in SI (this is particularly true with degrees/radians, where floating point errors
make 60 degrees>pi/3 radians>59.9999* degrees). The units attribute of the
property carries the dimensionality and conversion factor for the value,
comparisons are made first to check that the units are equal, if so the value are
compared, if not then the values are converted to a common units then compared.
Doing such a comparison where the value in different units is in differently named
attributes or elements would be much more complicated, and doesn't gain anything in
the use cases I've met.
Pete
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