# Re: [std-proposals] On the standardization of mp-units P3045R1

From: Charles R Hogg <charles.r.hogg_at_[hidden]>
Date: Tue, 18 Jun 2024 21:30:22 -0400
On Tue, Jun 18, 2024 at 9:03 PM Thiago Macieira via Std-Proposals <
std-proposals_at_[hidden]> wrote:

> On Tuesday 18 June 2024 10:06:09 GMT-7 Charles R Hogg via Std-Proposals
> wrote:
> > A basic design principle for a units library is that the result of a
> > computation shouldn't conjure up any units not mentioned in the inputs.
>
> Why not? If I multiply an acceleration by mass, why should I not get
> newtons?
> If I divide a charge by time, should I not get amperes?
>

Thanks for asking! Here's what I meant, in more detail.

If you multiply an acceleration (m / s^2) by a mass (kg), the units of
this *intermediate
computation result* should be (kg * m / s^2). If you print it out,
the unit label should be this (or something equivalent). Perfectly
correct, perfectly consistent with the basic rules of quantity calculus.
No surprises.

If you then try to assign this result to a quantity of newtons (N), it
should be implicitly convertible. It has the same dimension, and same
magnitude, so the library should permit this as a direct assignment, with
no runtime conversion factor. So, for intermediate computations, just use
whatever unit falls out naturally, and for assignment, check the dimension
and magnitude to see whether it's permitted.

(Aside: mp-units also supports more sophisticated reasoning, whereby we can
distinguish between different kinds-of-quantity that have the same
dimension and magnitude, but I wanted to keep it simple for this example.
That, and if I don't mention this, then Mateusz will jump in and correct
me. :)

Now, if you wanted the result of an acceleration times a mass to *simply
already be* in Newtons, you *could* make that work, but at a cost. You
would have to introduce the notion of a globally "preferred" unit for each
(dimension, magnitude) combination. Adding global registries to the
library would increase complexity and fragility, and make the library
harder to reason about. mp-units abandoned this approach pre-2.0 and has
never looked back.

PS: would this proposed API support a unit like ns/√km ?
>

Yep!

Cheers,
Chip

>
> --
> Thiago Macieira - thiago (AT) macieira.info - thiago (AT) kde.org
> Principal Engineer - Intel DCAI Fleet Systems Engineering
>
>
>
> --
> Std-Proposals mailing list
> Std-Proposals_at_[hidden]

>