# Swift Best Practices

> Apply Guard, Value Types, Immutability, and Naming conventions in Swift. Use when writing idiomatic Swift with guard, value types, immutability, or naming conventions—not Swift concurrency/actor-isolation, persistence, linting, or release tooling.

- Skill: `hoangnguyen0403/swift-best-practices` (Agent Skill, multi-file: 3 files)
- Install (CLI): `npx skillmds@latest add hoangnguyen0403/swift-best-practices`
- Raw SKILL.md: https://api.skillmd.com/api/skills/hoangnguyen0403/swift-best-practices/raw
- Safety review: pending (external: skill-scanner PASS, skillspector PASS)
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: HoangNguyen0403 (https://skillmd.com/u/hoangnguyen0403)
- Updated: 2026-09-09
- Page: https://skillmd.com/skills/hoangnguyen0403/swift-best-practices

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# Swift Best Practices

## **Priority: P0 (CRITICAL)**

## Implementation Guidelines

### Control Flow (Guard over If)

- **Guard for Early Exit**: Use **`guard let`** over **nested if** statements for better readability and to unwrap optionals early.
- **Nested Checks**: Use **`guard`** for **precondition** checks at top of function to reduce nested depth.
- **Switch Exhaustiveness**: Always handle all cases; use **`@unknown default`** for freezing enums (enums from frameworks).
- **if-case**: Use **`if case .success(let value) = result`** for simple enum pattern matching.

### Value Types & Immutability

- **Prefer Structs**: **Default to struct** for **value semantics** and thread safety. Use `class` only when reference identity or **inheritance** required.
- **Immutability**: Always **default to let** for all properties and constants. Use `var` only when change required.
- **Modifiers**: Use **`final`** for all classes that not intended to subclassed to improve performance (static dispatch).
- **Static Dispatch**: Favor methods in structs and `final` classes.

### Naming & Style

- **Clear Intent**: Prefix booleans with **`is, has, or can`**. Example: **`isValid`**, `hasErrors`, **`canEdit`**. Makes boolean state clear.
- **API Guidelines**: Follow official **Swift API Design Guidelines**. Use **`camelCase`** for **clear names** and `PascalCase` for types.
- **Protocols**: Name protocols with **`-able`**, `-ible`, or `-ing` suffixes (e.g., `Codable`, `Identifiable`).
- **Opaque Types**: Use **`some View`** or `some Collection` for return types where underlying type internal.

### Collection Performance

- **Sequence API**: Use **`compactMap`**, **`filter`**, and **`reduce`** instead of explicit **for-where** loops for data transformations.
- **Lazy Collections**: Use **`.lazy`** for large sequences when result consumed partially.
- **Dictionaries**: Use **`default`** values in dictionary access to avoid double optional unwrapping.

## Anti-Patterns

- **No Pyramid of Doom**: Use **`guard`** for early exits.
- **No force unwrap**: Never use **`!`** on optionals. Use **`??`** (nil-coalescing) or **`if let`**.
- **No global var**: Avoid mutable global state. Use **Singletons** (accessed via `static let shared`) or DI.

## References

- [Guard Patterns & Immutability](references/implementation.md)