Re: [h5md-user] Finalization of units module

[Pierre de Buyl]

On Thu, Jan 09, 2014 at 09:47:25AM +0100, Konrad Hinsen wrote:
> Felix Höfling writes:
> 
>  > Thinking about the "system" attribute again during the vacation I'm a bit
>  > unsure whether it is really needed. In particular given the fact that it
>  > can (and must) take a single value only.
>  > 
>  > Extending the set of units could be achieved by simply increasing the
>  > version number of the units module.
> 
> The underlying question is: do we want a single unit system, able to grow
> as new needs are identified, or do we envisage distinct unit systems
> that are completely independent?
> 
> I'd go for the second approach, and keep the "system" attribute, even
> if initially only "SI" is defined. The major alternative unit systems
> for which I see a need are domain-specific unit systems defined for
> theoretical models. A simple example is "reduced LJ units" but there
> are more elaborate ones. I see them arriving in coarse-grained models
> for macromolecules. These unit systems have no reference at all to the
> real world (and thus to SI), and may use arbitrary symbols.

I also support keeping "system".

> Finally, a minor point but I think it's worth considering. The SI
> system defines a fixed set of units and prefixes, plus some tolerated
> units that don't fit the system but are convenient, such as liters for
> volume.  A major goal of SI is simplification through standardization
> and thus the elimination of older units.  A unit system designed to
> include lots of other units as long as no name conflict arises should
> not be called "SI".

We can keep SI for the existing module and later have a "SI_strict" units system
if needed. It would make it clear what is intendent. Also, as we document nicely
the modules this should avoid confusion.

P