Domain Modeling

Replacing Primitive Data with Domain Types

Many bugs begin with a value that is technically valid for its programming-language type but invalid for the job it represents. A quantity is negative. A percentage is passed as 20 where another function expects 0.20. Two string arguments are swapped because both look identical to the type system. The problem is not that strings and numbers are poor types. They are useful building blocks. The problem appears when a primitive value has accumulated rules or meaning that the primitive cannot express.