I am thinking about how Currencies would fit with software for handling SI
and other physical dimensions and the usual units of measure.
The following approach seems workable and (mostly) not disruptive to the
working and evolving code Andrew Keller offers the community.
A currency behaves as a representation of, or a stand-in or proxy for all
the banknotes, sovereign wealth and other property interests of a nation.
Two or more currencies for which there is a market that allows one to be
converted into another at an agreed upon, current rate of exchange are not
that different from two or more units of length or units of time that are
allowed to be converted into one another using a predefined multiplicative
ratio.
(now, 1.0 US Dollar = 8.45536 Swedish Krona, refreshing the page, now
1.0 US Dollar = 8.45518 Swedish Krona)
The most impactful distinction is that rates of exchange are dynamic while
the physical conversion factors (BIPM standards, CODATA values) persist.
However currencies may be accommodated, it must protect our ability work
with units with ease and with invisibly amortized processing overhead.
So, the request that gets the appropriate exchange rate to use when
converting from one currency to another must come from the Units code in a
way that does not require other physical unit conversions to operate any
differently.
One of the things on Andrew's future to do list is allow measures and their
uncertainties to be used and to interact respecting the uncertainty math.
In any currency exchange trade, there is a buyer and a seller. Usually,
when they (or their requests) meet, they disagree. The buyer wants to pay
as little as possible and the seller wants to receive as much as possible.
That separation is called the bid-ask spread (the buyer bids, the seller
asks); almost always it is very, very small relative to the total value
being exchanged. The uncertainties associated with physical measures and
conversions usually are very, very small relative to the total quantity
being determined or the value being mapped into different units. When
uncertainty management is introduced, it is conceivable that currency
support could take advantage of that analogous role.
