Functional Core Architecture

About

This skill guides developers in adopting the Functional Core, Imperative Shell architectural pattern, a strategy designed to decouple pure business logic from impure I/O operations. By modeling business rules as deterministic functions and delegating side effects to a thin imperative layer, it enables high-speed unit testing, reduces framework dependency churn, and creates more maintainable codebases. The skill provides structured guidance for inventorying side effects, designing command schemas, and refactoring legacy systems into a more resilient, testable structure, complete with automated boundary validation.

Key Features

• Incremental refactoring workflows for legacy imperative code
• Automated architecture boundary validation and test generation
• Side-effect inventory mapping for clear system boundary definition
• 10 GitHub stars
• Command schema design for core-to-shell communication
• Pure function modeling for deterministic business logic

Use Cases

• Building highly testable backend services with 90%+ deterministic unit test coverage
• Refactoring legacy systems where logic is tightly coupled with database calls
• Drafting Architecture Decision Records (ADRs) for complex system migrations