Domain-Driven-Design

A lot of code is hard to test not because the business logic is complex, but because decisions and side effects are woven together. Functional Core, Imperative Shell is a simple rule for separating the two — and I pointed Claude Code at a tangled handler to do the split.

An order model with a status enum and a handful of nullable fields can represent far more states than are legal. I pointed Claude Code at one, had it count the illegal combinations, and let it redesign the type so they can’t be constructed at all.