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

• references/state-machines.md

• references/functional-operators.md

• references/algebraic-data-types.md

• references/optics.md

• references/dependency-injection-currying.md

• 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.