On Thu, Mar 14, 2024 at 1:48 PM Tiago Freire via SG14 <sg14_at_[hidden]>
wrote:

> > The `Money` class seems isomorphic to Mateusz Pusz (cc'ed)'s work with
> mp_units. You should look into the state of WG21 proposals around "units
> library."
>
> It would be interesting to hear from Mateusz (or, I bet, he's written
> something somewhere already) on the question of whether there are
> important/insurmountable differences between physical units like "meter"
> and monetary units like "euro," or whether the same C++ approach can work
> for both just fine. I can't think of any differences — except that monetary
> units should never be interconverted without some kind of
> GetCurrentExchangeRate sigilling — but I also wouldn't be terribly
> surprised to hear that there were some subtle-but-insurmountable
> differences in practice.
>
>
>
> I can answer that question. Money conversions are not a compile time
> constant, which is a requirement for mp-units
>

For GetCurrentExchangeRate, sure, I covered that in the part you quoted.
But there's at least three more parts to mp_units:
- Exact representations of fractional parts, and conversions among those
parts. It feels to me as if the relationship between pounds, shillings, and
pence (or yuan, jiao, and fen) is exactly isomorphic to the relationship
between miles, yards, and inches; but I wonder if I'm missing something
subtle.
- Type-safe manipulations; e.g. it should be physically impossible to store
a `yards` value into a `miles` variable, let alone into a `liters`
variable. It feels as if `pounds` and `yuan` should work the same in this
respect, but I wonder if I'm missing something subtle.
- Automatic handling of products and powers; e.g. 1 newton times 1 meter
should type-safely produce one newton-meter; 1 newton divided by 1 kilogram
should produce 1 m/s^2. It feels as if e.g. "dollars per day" should work
the same in this respect, but I wonder if I'm missing something subtle.
- And then there's the unknown unknowns!

HTH,
Arthur