Swift Best Practices

---
name: swift-best-practices
description: 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.
metadata:
  triggers:
    files:
    - '**/*.swift'
    keywords:
    - guard
    - let
    - struct
    - final
    - swift idiomatic
    - swift naming
    - swift best practice
    - swift conventions
    - value type
    - immutability swift
    - guard let
---
# 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)