functional-core-imperative-shell

Apply functional core imperative shell separation by moving pure calculations into a functional core and wrapping file, network, and environment I/O in thin imperative shell wrappers. This architectural pattern enables reliable testing by isolating side effects from computations.

The functional core imperative shell pattern is a software architecture design that splits pure computations from I/O orchestration. It enforces a clear boundary where pure functions handle calculations and thin shells manage file, network, and environment interactions.

Yes, FCIS refactoring applies to both small and large existing projects. It guides where to place logic versus I/O across new code, refactors, and architecture decisions, requiring explicit file classification comments to document responsibilities during the transition.

Enforce consistent FCIS boundaries by requiring explicit pattern comments on modified or created files to document their responsibilities. This maintains a clear core of pure functions and thin shells for I/O orchestration across the codebase.

FCIS separation is less suited for trivial scripts or prototypes where long-term maintainability and reliable testing are not priorities. It provides the most value during new code creation, refactoring, and architecture decisions across small to large projects.