Generally, the idea is that there is no incompatibility between units. Money is a bit exotic given its abstract nature wrt units / exchange rates.

For instance, physical dimensions store values as SI units, regardless of supplied unit, but retain the supplied unit type as a display hint.

Here, the Length dimension illustrates working with amounts of any unit.

Length orbit = 250 mi;

Length moon = 238000 km;

Length landing = moon + orbit;

display(landing); // prints 148,136.344 mi

display(landing.to(km)); // prints 238,402.336 km

// where

display(landing.to(mi) == landing.to(km)); // prints true

// perhaps a better example

Rate rate = 1.41 km/s;

Time duration = 76 hours;

Length distance = rate * duration;

// make your own unit constants

RateUnit kps = km/s;

rate = 1.41 kps;

You can play with the manifold-science[1] project, it implements the complete SI physical dimensions and many derived dimensions along with comprehensive operator overloading[2] support etc. to make working with and reading code using quantities more productive in Java.

1. https://github.com/manifold-systems/manifold/tree/master/man...

2. https://github.com/manifold-systems/manifold/tree/master/man...