Functional Core Imperative Shell

This skill helps you structure code by separating pure business logic from side effects. The core idea is simple: put your pure functions in one file where they're easy to test without mocks, and isolate all I/O operations in a thin shell around them. It's based on the functional core, imperative shell pattern that makes entire classes of bugs structurally impossible. The author suggests using it whenever you're writing new code, refactoring, or deciding where logic belongs. It's a disciplined approach that pays off in testability, though it requires thinking carefully about boundaries from the start.