Re: SIunits and quantity library concerns

From: Eric Ford (eford_at_[hidden])
Date: 2001-10-05 12:19:40

--- In boost_at_y..., Jonathan H Lundquist <jhl_at_s...> wrote:
> I think they have to be more than pure numbers, they're units of
> measure in that bins can be converted to cartons, etc.  But the
> conversion factors do depend on the qualifiers.
> 
> We're converting to some fictional unit of volume, usually from
another
> unit of volume.  However, we also have to convert from units of
weight
> to units of volume.  A given number of lbs. of apples is a known
number
> of carton equivalents (approximated).

You can count cartons and call that the number of cartons. 
Alternatively you can measure a physical quantity in units of that
physical quantity of the carton.  Yes, a the unit
weight_of_bag_of_sugar may be different thatn weight_bag_of_rice.  If
you're converting a weight into a volume, you're probably using a
density and an acceleration, although some people "hide" those.

> At the time I wrote, I thought most of the discussion treated what
is 
> now being called tags as analagous to user defined units of measure.
> 
> I think the least which is needed are SI units, user defined units,
and
> tags.  I also think tags have to be able to express what is and is
not
> fungible.

In my qualified_unit the user specializes the operations they desire
to allow conversions between differently taged quantities.  While it
requires several lines of code, I think that your requests are
possible.  Once other things are working, we can try to reduce the
ammount of user code necessary to provide commonly requested
functionality.

E