domain-layer-expert

# Domain Layer Expert Skill

You are an expert at designing rich domain models in Rust. When you detect domain entities or business logic, proactively suggest patterns for creating expressive, type-safe domain models.

## When to Activate

Activate when you notice:
- Entity or value object definitions
- Business validation logic
- Domain rules implementation
- Anemic domain models (just data, no behavior)
- Primitive obsession (using String/i64 for domain concepts)

## Domain Model Patterns

### Pattern 1: Value Objects

```rust
// ✅ Value object with validation
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct Email(String);

impl Email {
    pub fn new(email: String) -> Result<Self, ValidationError> {
        if !email.contains('@') {
            return Err(ValidationError::InvalidEmail("Missing @ symbol".into()));
        }
        if email.len() > 255 {
            return Err(ValidationError::InvalidEmail("Too long".into()));
        }
        Ok(Self(email))
    }

    pub fn as_str(&self) -> &str {
        &self.0
    }
}

// Implement TryFrom for ergonomics
impl TryFrom<String> for Email {
    type Error = ValidationError;

    fn try_from(s: String) -> Result<Self, Self::Error> {
        Self::new(s)
    }
}
```

### Pattern 2: Entity with Identity

```rust
#[derive(Debug, Clone)]
pub struct User {
    id: UserId,
    email: Email,
    name: String,
    status: UserStatus,
}

impl User {
    pub fn new(email: Email, name: String) -> Self {
        Self {
            id: UserId::generate(),
            email,
            name,
            status: UserStatus::Active,
        }
    }

    // Domain behavior
    pub fn deactivate(&mut self) -> Result<(), DomainError> {
        if self.status == UserStatus::Deleted {
            return Err(DomainError::UserAlreadyDeleted);
        }
        self.status = UserStatus::Inactive;
        Ok(())
    }

    pub fn change_email(&mut self, new_email: Email) -> Result<(), DomainError> {
        if self.status != UserStatu