# Default

The `Default` macro generates a static `defaultValue()` factory method that creates
instances with default values. This is analogous to Rust's `Default` trait, providing
a standard way to create "zero" or "empty" instances of types.

## Generated Output

| Type | Generated Code | Description |
|------|----------------|-------------|
| Class | `static defaultValue()` + `classNameDefaultValue()` | Static factory method + standalone function |
| Enum | `enumNameDefaultValue(): EnumName` | Standalone function returning `@default` variant |
| Interface | `ifaceNameDefaultValue(): InterfaceName` | Standalone function returning object literal |
| Type Alias | `typeNameDefaultValue(): TypeName` | Standalone function with type-appropriate default |

Names use **camelCase** conversion (e.g., `UserSettings` -> `userSettingsDefaultValue`).


## Default Values by Type

The macro uses Rust-like default semantics:

| Type | Default Value |
|------|---------------|
| `string` | `""` (empty string) |
| `number` | `0` |
| `boolean` | `false` |
| `bigint` | `0n` |
| `T[]` | `[]` (empty array) |
| `Array<T>` | `[]` (empty array) |
| `Map<K,V>` | `new Map()` |
| `Set<T>` | `new Set()` |
| `Date` | `new Date()` (current time) |
| `T \| null` | `null` |
| `CustomType` | `CustomType.defaultValue()` (recursive) |

## Field-Level Options

The `@default` decorator allows specifying explicit default values:

- `@default(42)` - Use 42 as the default
- `@default("hello")` - Use "hello" as the default
- `@default([])` - Use empty array as the default
- `@default({ value: "test" })` - Named form for complex values

## Example

```typescript before
/** @derive(Default) */
class UserSettings {
    /** @default("light") */
    theme: string;

    /** @default(10) */
    pageSize: number;

    notifications: boolean;  // Uses type default: false
}
```

```typescript after
class UserSettings {
    theme: string;

    pageSize: number;

    notifications: boolean; // Uses type default: false

    static defaultValue(): UserSettings {
        const instance = new UserSettings();
        instance.theme = 'light';
        instance.pageSize = 10;
        instance.notifications = false;
        return instance;
    }
}

export function userSettingsDefaultValue(): UserSettings {
    return UserSettings.defaultValue();
}
```

## Enum Defaults

For enums, mark one variant with `@default`:

```typescript before
/** @derive(Default) */
enum Status {
    /** @default */
    Pending,
    Active,
    Completed
}
```

```typescript after
enum Status {
    /** @default */
    Pending,
    Active,
    Completed
}

export function statusDefaultValue(): Status {
    return Status.Pending;
}
```

## Error Handling

The macro will return an error if:

- A non-primitive field lacks `@default` and has no known default
- An enum has no variant marked with `@default`
- A union type has no `@default` on a variant