samdeepthink

Sep 6, 2026 · Backend Development

Functional Core, Imperative Shell: Refactoring Checkout Code for Testability and Reuse

The article demonstrates refactoring a tangled checkout method by separating pure calculation logic into a functional core using Java 17 records and sealed interfaces, leaving IO operations in an imperative shell, resulting in easily testable, reusable code without mocks or Spring containers.