Functional Core, Imperative Shell (FCIS)

Overview

Core principle: Separate pure business logic (Functional Core) from side effects (Imperative Shell). Pure functions go in one file, I/O operations in another.

Why this matters: Pure functions are trivial to test (no mocks needed). I/O code is isolated to thin shells. Bugs become structurally impossible when business logic has no side effects.

When to Use

Use FCIS when:

• Writing any new code file
• Refactoring existing code
• Reviewing code for architectural decisions
• Deciding where logic belongs