Domain Modeling Made Functional

Modeling Optional Values, Errors, and Collections

While we are discussing domain modeling, let’s talk about some common situations and how to represent them with the F# type system, namely:

• Optional or missing values
• Errors
• Functions that return no value
• Collections

Modeling Optional Values

The types that we have used so far—records and choice types—are not allowed to be null in F#. That means that every time we reference a type in a domain model, it’s a required value.

So how can we model missing or optional data?

The answer is to think about what missing data means: it’s either present or absent. There’s something there, or nothing there. We can model this with a choice type called Option, defined like this:

type Option<'a> =
  | Some of ...