Re: [h5md-user] units module

[Felix Höfling]

Am 16.10.2013, 15:11 Uhr, schrieb Pierre de Buyl  
<address@hidden>:

> Hi Konrad,
>
> Thanks for your replies.
>
> On Wed, Oct 16, 2013 at 08:35:54AM +0200, Konrad Hinsen wrote:
> > Pierre de Buyl writes:
> >
> >  > I just toyed a bit with udunits2 and it seems very nice. The fact  
> > that we can
> >  > rely on this software if need is good.
> >
> > Indeed. On the other hand, if we allow units to be as general as
> > allowed by udunits, this means that every program wanting to read H5MD
> > must either use udunits or re-implement a large subset of it.
>
> Indeed, your restriction is fine.

I find udunits' grouping into SI-base units, SI-derived units etc. very  
reasonable. Let's keep it for H5MD rather than introducing a different  
subset. Then a simple reader may want to implement the base units only,  
and an advanced one relies on udunits. The modules/units section would  
make a promise which unit set is used, and the reader can immediately tell  
whether it can cope with it or not.

Actually, whether a reader can "understand" a small or large set of units  
is mainly a matter of the database defining the units. Do I overlook  
something here? Why not copying the full list from udunits?

BTW, a more advanced functionality that discriminates between "simple" and  
"advanced" readers is automatic conversion between units ...

> >  > Is there some kind of "ultimate ASCII SI units definition"  
> > available? The xml
> >  > files from udunits2 seem close to that :-)
> >
> > It's the best collection I know of.
>
> So, to get back to Peter's message:
>
> I propose that we follow udunits grammar by restricting it similarly to  
> Mosaic.
> For reference, Mosaic's definition is
> """
> The value of the units field is a text string in ASCII encoding. It  
> contains a
> sequence of unit factors separated by a space. A unit factor is a unit  
> symbol
> optionally followed by a non-zero integer which indicates the power to  
> which
> this factor is taken.
> """
>
> I would remove the constants defined ("c" and "Nav"), however. We may  
> want to
> add "a unit string must be parseable by udunits"?

This should follow automatically if the grammar is specified correctly as  
a subset of udunits grammar. I would not like to make the spec depend on  
an external piece of software (with the exception of HDF5 of course). The  
reference to udunits should go to the implementation section.

Felix