At the start of looking at units, we looked at Unit<U : Unit<U>> and
ReifiedUnit<U: Unit<U>, RU: ReifiedUnit<U, RU>>.
Lets take a second look at why these are needed, and why they work.
Firstly, these generics allow for generic family lock-in, basically, code can be
written that is generic across all of RU: ReifiedUnit<U, RU>, and once you use
a specific unit there, nothing else can be used, by using RU rather than just
ReifiedUnit<U: Unit>, we ensure that only Distances can be used, as
opposed to any ReifiedUnit<DistanceUnit>, which means we can rely on the
properties and functions of Distance. The same applies for Unit<U: Unit<U>>.
We use this cycle to define a family, like DistanceUnit which extends
Unit<DistanceUnit>, this means that DistanceUnit needs to be able to handle
conversion between itself and any other DistanceUnit but not any Unit.
This is a fairly complex usage of generics and type systems, so don’t worry if
you don’t get it.
The basic idea, is that we can write code that works for any ‘RU’ or ‘U’ and
guarantee that it locks to that one, once its used.
