functional-core-imperative-shell

Enforce Functional Core and Imperative Shell boundaries with file classification comments.

13|Updated Jan 18, 2026

Search Tags:#testing #refactoring #design-patterns #functional-core #software-architecture #fcis

Ready to run

npx skills add https://github.com/pbdeuchler/llm-plugins --skill functional-core-imperative-shell-pbdeuchler

💬 Prompt for ChatGPT / Claude Chat▼

Please help me install this Agent Skill.
Skill: functional-core-imperative-shell
Source: https://github.com/pbdeuchler/llm-plugins/tree/main/plugins/house-style/skills/howto-functional-vs-imperative
Command: npx skills add https://github.com/pbdeuchler/llm-plugins --skill functional-core-imperative-shell-pbdeuchler

SYSTEM DOCUMENTATION & REQUIREMENTS

What problem does it solve?

FCIS helps separate functional core logic from side effects, improving testability and maintainability by enforcing a clear split between pure computations and I/O orchestration.

Core Features & Use Cases

Identify and extract the Functional Core: move pure calculations into pure functions.
Isolate I/O in Imperative Shell: orchestrate file, network, and environment interactions in thin shell wrappers.
Enforce consistent classification: require pattern comments on modified/created files to document responsibilities.
Use cases: new code, refactoring, and architecture decisions across projects.

Quick Start

Identify pure logic, move it to a Functional Core, and wrap all I/O in an Imperative Shell.

Frequently Asked Questions about functional-core-imperative-shell

High-intent search queries and answers about installing and using this skill.

FAQPage Schema

How do I separate business logic from side effects to improve testability?▼

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.

What is the functional core imperative shell pattern in software architecture?▼

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.

Can I use FCIS refactoring on large existing projects?▼

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.

How do I enforce consistent boundaries when refactoring to functional core and imperative shell?▼

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.

When should I not use functional core imperative shell separation?▼

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.