Can I use logging in a Functional Core file?

Yes, logging is an explicitly permitted exception in this skill's implementation of FCIS, though it is recommended to pass no-op loggers during unit testing to maintain purity.

How does this help with software testing?

By isolating business logic in pure functions within the Functional Core, you can write unit tests with simple assertions and zero mocks, making your test suite faster and more resilient to change.

Why does this skill require file classification comments?

These comments act as a structural safeguard, forcing both the AI and the developer to consciously declare the purpose of a file, which prevents accidental mixing of logic and side-effects.

When should I use the 'Mixed' pattern classification?

The 'Mixed' tag should only be used for existing code awaiting refactoring or rare performance-critical paths where separating I/O would cause unacceptable overhead, requiring specific technical justification.

What is the Functional Core, Imperative Shell pattern?

It is an architectural pattern that separates pure, deterministic business logic (the Core) from impure operations like I/O, database queries, and API calls (the Shell) to improve maintainability.

FCIS Architect Claude Code Skill - Pure Logic Separation

概要

This skill implements the 'Functional Core, Imperative Shell' (FCIS) pattern to improve code testability and reduce architectural debt. It provides Claude with a strict framework for classifying files as either pure logic containers or I/O orchestrators, mandating pattern comments on all modified files. By separating computational complexity from environment-dependent side-effects, it ensures that business logic remains deterministic and easy to unit test without complex mocks, while isolating external dependencies in thin imperative shells. It is particularly useful when refactoring legacy code or architecting new features that require high reliability.

主な機能

Mandatory file classification using pattern comments for every code file
Isolated I/O and orchestration handling via the Imperative Shell pattern
Comprehensive decision framework for determining function placement
45 GitHub stars
Strict enforcement of pure, side-effect-free Functional Core logic
Standardized refactoring patterns for converting mixed logic into clean architectures

ユースケース

Refactoring monolithic functions that mix database calls with complex business rules
Decoupling application logic from external dependencies like APIs, file systems, and environment variables
Establishing high-testability architectural standards for new project modules

概要

主な機能

Mandatory file classification using pattern comments for every code file
Isolated I/O and orchestration handling via the Imperative Shell pattern
Comprehensive decision framework for determining function placement
45 GitHub stars
Strict enforcement of pure, side-effect-free Functional Core logic
Standardized refactoring patterns for converting mixed logic into clean architectures

ユースケース

Refactoring monolithic functions that mix database calls with complex business rules
Decoupling application logic from external dependencies like APIs, file systems, and environment variables
Establishing high-testability architectural standards for new project modules

FCIS Architect & Refactoring Guide

FCIS Architect & Refactoring Guide

概要

主な機能

ユースケース

概要

主な機能

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