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From: Deane Yang (deane_yang_at_[hidden])
Date: 2005-10-21 09:59:56

Andy Little wrote:
I believe that pqs is a great solution
> for SI quantities. The primary purpose of a physical quantity library is to
> cater for common practise, rather than being some sort of tool kit for ad-hoc
> unit systems to be created. Defining a unit system is a non-trivial task. The
> dimensional analysis part is trivial really. The difficult part is output and to
> be honest I havent seen a library apart form pqs that tackles this well. Output
> is messy even in the SI. I believe that users ( including myself) want
> simplicity. I dont want to have to spend time creating output facilities or
> making complex typedefs for commonly used SI quantities. The design of an SI
> quantity library is possible because they have published detailed definitions of
> their base units and the output format.

Again, it's probably just me, but this all sounds backwards to me. I
certainly agree that adding a "generalized dimensions" library as a
layer over an SI quantities library is not a good idea.

But wouldn't it make sense to develop a core dimensions/units library
without any predefined dimensions in it and develop the SI quantities
library as a layer above that?

I am not proposing that the dimensions library should be distributed
without predefined SI dimensions and units; that would of course be
silly. I am just proposing that the underlying infrastructure of the
library (I am assuming that the physical dimensions and units are not
hardwired in the library from top to bottom) be exposed, so that someone
like me can build our own customized dimension/units system (analogous
to but different from the SI units).

But it appears that the only people willing to devote effort to
developing such a library are people who always use SI quantities and
are not familiar with applications that don't use them. So I will now
bow out of this discussion (until that indefinite time in the future
when I can submit actual code for the kind of library I want), but I
would encourage all of you to move ahead with the SI quantities library.
It definitely sounds like a very useful library to me.

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