RFC: Units of measurement for D (Phobos?)

[Nicolas Sicard]

On Tuesday, 4 March 2014 at 13:20:02 UTC, Nordlöw wrote:
>> Thanks! I think David Nadlingers' is more elaborate (scaled and
>> affine units) and less prone to rounding errors. I haven't used
>> it yet. Mine is more basic but has a parsing system that I use 
>> to
>
> So you cannot defined things lika kPa from Pa in compile-time?

enum kPa = kilo(pascal);

which is equivalent to

enum kPa = 1000 * pascal;

Note that my module defines no real unit type, just quantities: a 
unit is only a special quantity with a value of 1 and a set of 
dimensions (a "base unit" in the sense of the SI). I think it 
better fits what physical units really are.  kPa would then be a 
quantity that stores 1000 and a set of dimensions corresponding 
to a pressure.

If you declare:

auto p  = 101.3 * kPa;

p stores a value that is 101.3 * 1000 internally, not 101.3. With 
David's library, I think you can declare kilopascal as a scaled 
unit derived from pascal, and then a quantity in kilopascal would 
actually store 101.3. The result is globally the same, but in 
fact the representations of the numeric values would be a bit 
different, and so would the rounding errors, when using floating 
point types.

>> define units quickly at compile-time
>
> Can't David's package also quickly define in compile-time?

I think it can.

>
> Have you thought about merging your effort vid Davids and 
> propose it to Phobos? This issue has stale for quite some time 
> know.

I just think that David's package could have been proposed for 
Phobos a long time ago. I don't know what has stopped it then. I 
developed mine afterwards, not being aware of David's one.

> I would like to have support add support for geometric units 
> like angles, coordinate systems, etc. Have you thought about 
> integrating these with your module?

My package is extendable. The SI module is just a predefined set 
of common quantities and prefixes. You can use your own. Some 
angle units are defined in quantites.si. I am not sure of what 
you call a coordinate systems in this respect.

--
Nicolas