architecture-paradigm-functional-core

Employ the "Functional Core, Imperative Shell" pattern to isolate deterministicbusiness logic from side-effecting code for superior testability.Triggers: functional core, imperative shell, pure functions, immutable data,side effect isolation, testability improvement, business logic isolation,deterministic code, FP architecture, onion architectureUse when: business logic is entangled with I/O, tests are brittle or slow,significant time spent on adapter rewrites, need fast deterministic unit testsDO NOT use when: selecting from multiple paradigms - use architecture-paradigms first.DO NOT use when: simple scripting or automation without complex logic.Consult this skill when implementing functional core patterns or refactoring for testability.