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From: Leland Brown (lelandbrown_at_[hidden])
Date: 2006-06-06 17:44:02

Hello! I'm a newbie to the Boost mailing list, but I want to add my comments
because I believe Andy's PQS library is very useful for scientific
programming. I hope that it gets accepted into Boost, either in its present
form, or possibly with some improvements as I'll discuss below.

Unfortunately I haven't had time to examine the code in depth, but I feel very
qualified to comment because I have implemented a similar template library as
part of my job. (I got the inspiration from Barton & Nackman's book,
_Scientific and Engineering C++_.) I can't share the code because it's
proprietary, but we have been using it for the last six years in an application
that's heavy on physical and geometric calculation and numerical analysis. It
has provided all the benefits Andy promises - self-documenting code, compiler
detection of errors in physical formulas, elimination of units confusion, etc.
The benefits are excellent, and it's easy to use for the most part. It has
saved us a lot of headaches. Andy's library looks to be similar in all those
respects, and I expect it will provide the same benefits. Many of our design
choices are the same. Actually, the PQS library is in many ways much more
extensive than my work and should be useful in a broader range of applications.

One important point is that we found that our library was extremely efficient,
with all of the work being done at compile-time. A comparison of run-times
using the quantities library versus built-in floating-point types showed a
speed degradation of only 1% on highly math-intensive code.

What I implemented was more similar to Andy's "t2_quantity" and "t3_quantity,"
and I find these (his future planned parts of the library) to be the most
useful in what I do. I look forward especially to their completion, and I can
testify to their utility. If the value of the library is not evident to some
people as it stands now, I think a vision of the completed library may make it
more clear. For that reason (and because it's what I'm most familiar with),
I'll make a comment about where I think the library is going - realizing that
I'm departing briefly from the topic of reviewing the library as it is today.
Then I'll come back to specific comments on the current code.

In my work, the advantage of the "t2_quantity" is that it provides an
abstraction like a "length" that's independent of the units used to represent
it - i.e., 1000 m is the *same* as 1 km, and a variable of type "t2_quantity"
could contain that length as a value. All my scientific formulas then work on
values like length and time rather than meters and seconds. A specific choice
of units to represent the values is then only needed for input (e.g., from
numeric values in a text file or hard-coded constants) or output (e.g., for
human-readable results). All the computations become independent of units, and
all numeric values input or output are enforced by the compiler to have
documented units. Code is clear and simple, like this:

   length x = 1000.0 * meter;
   velocty v = 2.0 * nautical_mile / hour;
   time t = x / v; // typically LOTS of computations here
   cout << "Time in seconds = " << t / second;

I think the currently implemented PQS library is a good start and provides most
of the same benefits to the user. The unit-specific types of the "t1_quantity"
probably in some cases can also avoid some of the multiplication/division that
would occur with my example above.

Returning to the current library per se, I want to add some specific comments
to some of those that have already been made in the reviews:

There's been quite a bit of discussion about names. First, the name of the
library. I wouldn't mind a different name than PQS, but I'd prefer to stay
away from calling it "units" - precisely because of what I said above
about "t2_quantity" - the advantage is actually in the ability to make your
physical formulas units-INDEPENDENT. Obviously, units are still needed and
handling them is a big part of the library, but the part of value is in making
the units more transparent to the users. So to focus on the units (meters,
seconds) versus the dimensions/quantities (length, time) would seem misleading
in the library name. Then again, it's just a name.

Secondly, the term "historical units" has been suggested to replace "incoherent
units." To me, "historical units" suggests they are archaic and no longer in
use, and as an American I unfortunately know better. Even in certain
scientific applications, units like feet and nautical miles, for instance, are
still in common use. "Traditional units" was also suggested - I like that one

I have to agree with the other reviewers that the
names "t1_quantity," "t2_quantity," and "t3_quantity" are not very
descriptive. Let me explain the naming scheme I used - maybe it will help get
us thinking about better names. In my case, I tried to stick to one class
template (the equivalent of t2_quantity), but I found a need for a type that
did its dimension-checking at runtime so that the dimensions were not part of
the type itself. This came up whenever I had an aggregate class (such as a
covariance matrix, or a set of polynomial coefficients) that needed to contain
a mix of values with different physical dimensions. Even though (in my
application) the dimensions of each element of the aggregate are known a
priori, I couldn't find another way to implement those aggregates that was time-
efficient without also being impossibly cumbersome. So, because of this
property of being able to have different kinds of quantities "mixed" within an
aggregate, I called this type "mix_measure" (i.e., t3_quantity), while the
other type was simply "measure" (for t2_quantity). I'm not necessarily
proposing you use my names, but I'm saying I think it's possible to find names
that do reflect something of their differences in functionality or purpose.

Lastly, it does seem from some of the other reviews that the documentation
could be more clear. I'm disappointed that the purpose and the value and ease
of use of the library is not more obvious to people from reading the
documentation. It's clear to me because I'm very familiar with the concept and
I've used it successfully.

One example is the questions about adding monetary conversions or pixel units
to the library. I think PQS is definitely the wrong place to put these
things. It's more than just a units conversion tool. The most obvious
difference is that I've never heard anyone talk in units like dollars squared.
The PQS library provides functionality meant for scientific comoutations
involving things like cubic meters per second squared. Dollars (and to a
lesser extent, pixels) are just a linear thing that does not make use of the
machinery afforded by this library.

One final note: I do think it's important that this library eventually be
extended and completed, and also that the various parts (t1_quantity,
t2_quantity, etc.) work well together and have similar interfaces. It's
possible that in implementing the future parts of the library, Andy will find
he needs to change some things in the current code to make this happen. I'm
not too familiar with the Boost process for changing existing libraries. So I
don't know if there would be any value in putting the acceptance on hold until
the other parts of the library are complete. But I would hate to see this
important work get dropped, and I recommend its acceptance, with this one
caveat about possibly accepting a more extended version instead at a later
date. However, I think the library is quite useful as it stands now, and it
provides a good foundation for the future work.


-- Leland Brown

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