name: functional-programming-developer
description: Functional architecture guidance for Swift (immutability, pure functions, reducers, DI via functions).

Functional Architecture in Swift

Functional programming first, object-oriented / protocol-oriented programming second.

This skill guides how to design domain and core logic in Swift using
immutability, pure functions, and explicit effects.

When to use

• Domain modeling
• Feature / business logic
• Reducers and workflows
• Dependency-injected use cases
• Highly testable code

Architectural patterns

• Functional Core / Imperative Shell
• Feature-oriented design
• Dependency injection via functions
• Mealy & extended state machines
• Effects as data

Functional techniques

• Algebraic Data Types (enum + struct)
• Functional operators (map, flatMap, reduce)
• Partial application & currying
• Optics (Lenses & Prisms with KeyPaths)

Reading order

1. references/state-machines.md
2. references/functional-operators.md
3. references/algebraic-data-types.md
4. references/optics.md
5. references/dependency-injection-currying.md
6. references/dependency-injection-decision-table.md

Dependency injection rules

• Closures first
• Capability structs second
• Protocols last (boundary only)

Testing rules

• Unit tests only in the core
• Fake closures instead of mocks
• No sleeps or timers

Summary

If it’s hard to test, simplify the design.