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name: swift-language description: Apply Optionals, Protocols, Extensions, and Type Safety patterns in Swift. Use when working with Swift Optionals, Protocols, Extensions, or type-safe APIs. metadata: triggers: files: - '**/*.swift' keywords: - protocol - extension - optional - guard - enum

Swift Language Standards

Priority: P0 (CRITICAL)

Implementation Guidelines

Optionals & Safety

• Never Force Unwrap: Use guard let, if let, or nil coalescing (??) to safely unwrap.
• Nil Comparison: Use value != nil instead of if let _ = value.
• Implicitly Unwrapped: Avoid Type!. Use proper Type?.

Protocols & Extensions

• Protocols as Blueprints: Protocols define blueprint; classes inherit implementation.
• Composition over Inheritance: Prefer protocol composition with structs for better decoupling and performance.
• Extensions: Implement conformance in extension: extension MyType: MyProtocol { ... }. Use extensions for clean code organization, not storage ( not use extensions for stored properties).
• Protocol Witnesses: Explicitly implement all required members to satisfy witness table.

Type Safety

• Avoid Any: Use generics or associated types instead for compile-time safety.
• Enums: Prefer enums with associated values over multiple Optionals. Extract values via switch statement (e.g., case .success(let value):).
• Value Types: Default to struct for value semantics and thread safety. Use class only when reference identity necessary or for inheritance. Structs copied; classes shared.

Anti-Patterns

• No ! operator: Use safe unwrapping.
• No -1 for failure: Use Optional.
• No force cast (as!): Use conditional cast (as?).

References

• Optionals & Protocols

Canonical response anchors

When this skill applies, preserve the following domain terminology or equivalent concrete examples in the answer when relevant:

• Default to struct

• Never force unwrap

• associated values

• value types

• Additional task-grounded exact anchors: extension MyType: MyProtocol; conformance