Domain Modeling Made Functional

Units of Measure

For numeric values, another way of documenting the requirements while ensuring type-safety is to use units of measure. With a units of measure approach, numeric values are annotated with a custom “measure.” For example, we might define some units of measure for kg (kilogram) and m (meter) like this:

[<Measure>]
type kg

[<Measure>]
type m

And then we annotate some values with those units of measure like this:

let fiveKilos = 5.0<kg>
let fiveMeters = 5.0<m>

You don’t need to define measure types for all the SI units. They are available in the Microsoft.FSharp.Data.UnitSystems.SI namespace.

Once this is done, ...