Can I use logging inside the Functional Core?

Yes, this skill includes a specific exception for loggers, allowing them in the Functional Core as long as they don't change the function's output.

What should I do if a file cannot be separated?

In rare cases where separation causes performance issues, the skill allows a 'Mixed (unavoidable)' classification provided you document the specific technical justification.

What is the Functional Core, Imperative Shell (FCIS) pattern?

FCIS is an architectural pattern where business logic is kept in 'pure' functions (Functional Core) and I/O or side effects are isolated in a thin wrapper (Imperative Shell).

Why does this skill require mandatory file comments?

The '// pattern:' comments serve as explicit architectural metadata that ensures both the AI and human developers maintain the intended boundary between logic and I/O.

How does this skill improve code testability?

By moving logic into pure functions, you can write unit tests with simple assertions that require no mocks for databases, files, or network calls.

Functional Core Imperative Shell (FCIS) Architect

Name: Functional Core Imperative Shell (FCIS) Architect
Author: pbdeuchler

bypbdeuchler

0•

API Development

Enforces architectural separation of pure business logic from side effects using the Functional Core, Imperative Shell pattern.

This skill implements the Functional Core, Imperative Shell (FCIS) pattern by guiding Claude to strictly separate deterministic, testable business logic from non-deterministic I/O operations. It mandates file-level classification to ensure architectural integrity, drastically reducing the need for complex mocking in unit tests and making codebases more maintainable. By isolating side effects like database calls, file system access, and HTTP requests into a thin shell, this skill ensures that your core logic remains pure, predictable, and easy to verify.

Key Features

01Decision framework for determining function placement based on side effects

02Pre-defined implementation patterns for Python, JavaScript, and other languages

03Strict prohibitions on non-deterministic functions within the Functional Core

040 GitHub stars

05Mandatory file-level pattern classification comments

06Automated refactoring of mixed-concern functions into Core and Shell components

Use Cases

01Refactoring 'spaghetti' code where business logic and database calls are tightly coupled

02Designing new systems from scratch with a focus on high testability and clean architecture

03Enforcing architectural standards during code reviews and AI-driven code generation

What are Skills?·How to Install

Install with 🐟 Skill.Fish

npx skillfish add pbdeuchler/llm-plugins howto-functional-vs-imperative

For use in Claude.ai and ChatGPT

Download Skill