axiom-codable

# Swift Codable Patterns

Comprehensive guide to Codable protocol conformance for JSON and PropertyList encoding/decoding in Swift 6.x.

## Quick Reference

### Decision Tree: When to Use Each Approach

```
Has your type...
├─ All properties Codable? → Automatic synthesis (just add `: Codable`)
├─ Property names differ from JSON keys? → CodingKeys customization
├─ Needs to exclude properties? → CodingKeys customization
├─ Enum with associated values? → Check enum synthesis patterns
├─ Needs structural transformation? → Manual implementation + bridge types
├─ Needs data not in JSON? → DecodableWithConfiguration (iOS 15+)
└─ Complex nested JSON? → Manual implementation + nested containers
```

### Common Triggers

| Error | Solution |
|-------|----------|
| "Type 'X' does not conform to protocol 'Decodable'" | Ensure all stored properties are Codable |
| "No value associated with key X" | Check CodingKeys match JSON keys |
| "Expected to decode X but found Y instead" | Type mismatch; check JSON structure or use bridge type |
| "keyNotFound" | JSON missing expected key; make property optional or provide default |
| "Date parsing failed" | Configure dateDecodingStrategy on decoder |

---

## Part 1: Automatic Synthesis

Swift automatically synthesizes Codable conformance when all stored properties are Codable.

### Struct Synthesis

```swift
// ✅ Automatic synthesis
struct User: Codable {
    let id: UUID              // Codable
    var name: String          // Codable
    var membershipPoints: Int // Codable
}

// JSON: {"id":"...", "name":"Alice", "membershipPoints":100}
```

**Requirements**:
- All stored properties must conform to Codable
- Properties use standard Swift types or other Codable types
- No custom initialization logic needed

### Enum Synthesis Patterns

#### Pattern 1: Raw Value Enums

```swift
enum Direction: String, Codable {
    case north, south, east, west
}

// Encodes as: "north"
```

The raw value itself becomes the JSON representation.

#### P