Re: [boost] quantity libraries

From: Dean Foster (foster_at_[hidden])
Date: 2001-09-09 15:54:41

> > I just joined the list a day or so ago. 
> Hi Dean, welcome to boost.  
> - Michael Kenniston

Thanks for the kind introduction to boost.  I posted my library on
yahoo with the name of units.zip.  As an academic, I'm used to
receiving nasty referee's reports, so hopefully I'll be thick skinned
about comments.  I thought I'd say a few things that I learned about a
quantities library from using one for the past year.

In reading over the postings over the summer I think there are two
different goals a quantities library could aspire to.  The first is
the SI goal: avoiding crashing into Mars and all that.  The second is
catching programming errors by making signatures of functions more
useful.  I built my library assuming that the primary goal was the
first one.  So I built in the ability to have inches, yards, rods,
miles, meters, etc.  But when it came time to actually use it, I
almost never gave explicit values.  So the first goal seemed to be
less important than I originally thought.  

What turned out to be very important to me was the ability to have
signatures of functions that were meaningful.  To achieve this goal it
is very important for the quantities to be user defined.  Let me
explain via example.

A physicist has nice concepts like length, weight, etc.  Obviously
they need funding (lots of it in fact) so they also would want to have
a unit to represent money.  For them, they would be happy with a
single type called "dollars."  But now consider a trader on wall
street.  They need not only dollar, but also yen, pounds, etc.  From
their perspective all the fancy units of physics could be lumped
together as a single unit called "physical unit".  So they would
measure corn in dollars/physical unit, where physical unit would be a
bushel, and gold in yen/physical unit, where physical unit in this
case would be a Troy ounce.  This mixing up bushels (a measure of
volume) and ounces (a measure of mass/weight) would case them almost
no confusion.

But units can get even more divided than that.  A trader might want to
have different sorts of dollars.  Not only is it important to
differentiate current_dollars, from future_dollars, but bids might
want to be separated from asks.  So there could be many different
sorts of dollars.

In fact, the system that I use my units classes in has two sorts of
time.  Both CPU time and wall clock time are different units.  Both of
them are naturally measured in something called seconds, BUT these two
different times measure different things.  

My point is that as far as making programs easy to read, the crucial
goal of a quantities class is to make the signature of a function say
what the function expects when it is called.  Thus you want as few
different types as possible subject to the constraint that there
sufficient types so that functions rarely have a useless signature.
So my goal is to switch useless signatures like:

        f(double a, double b, double c);

to:

        f(Dollars a, Bytes b, Time c);

later,

dean

p.s. If it seems of enough interest I can move over my cvs directory
and do a proper submission.  The Makefile probably only works under
gnu/linux/g++.  Hopefully the code itself is more portable.  :-)

=============================================================================
Dean Foster                                                   dean_at_[hidden]
Statistics, Wharton, U. Penn                                     215 898 8233
Philadelphia PA 19104-6302                 http://diskworld.wharton.upenn.edu