Swift allows us to add both computed instance properties and computed type properties to an existing type. These are the only types of properties that we can add to an existing type, so we cannot add simpler stored properties using extensions.
When you need to perform calculations with values that have an associated unit of measurement, it is very common to make mistakes by mixing its different units. It is also common to perform incorrect conversions between the different units that generate wrong results. Swift doesn't allow us to associate a specific numerical value with a unit of measurement. However, we can add computed properties to provide some information about the units of measurement for a specific domain.
Note
We worked with units when we analyzed the object-oriented approach of the HealthKit framework in Chapter 1, Objects from the Real-Word to the Playground. However, in this case, we just want to simplify a sum operation...