Lightweight FsCheck generators

🏷️ #ActivePattern #FsCheck #Test

Writing custom FsCheck generators can be verbose — especially for domain types with many constraints. A lighter alternative is to use active patterns that transform types FsCheck already knows how to generate:

let (|PositiveEuros|) (PositiveInt amount) : Money = Money.Euros(decimal amount)

PositiveInt is a built-in FsCheck wrapper. The PositiveEuros active pattern converts it into a valid Money value — no custom Arb or Gen required. It can be used directly in the test parameter:

member _.``return the average price of purchases within 1Y``
    (PositiveInt q1, PositiveEuros p1, PositiveInt q2, PositiveEuros p2) = ...

The rule of thumb: use active patterns when you can derive a valid domain value from a single primitive that FsCheck already generates; use custom generators when the domain type has structural invariants spanning multiple fields.