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Zhongwei Li
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skills/domain-layer-expert/SKILL.md
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279
skills/domain-layer-expert/SKILL.md
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---
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name: domain-layer-expert
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description: Guides users in creating rich domain models with behavior, value objects, and domain logic. Activates when users define domain entities, business rules, or validation logic.
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allowed-tools: Read, Grep
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version: 1.0.0
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---
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# Domain Layer Expert Skill
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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.
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## When to Activate
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Activate when you notice:
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- Entity or value object definitions
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- Business validation logic
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- Domain rules implementation
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- Anemic domain models (just data, no behavior)
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- Primitive obsession (using String/i64 for domain concepts)
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## Domain Model Patterns
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### Pattern 1: Value Objects
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```rust
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// ✅ Value object with validation
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#[derive(Debug, Clone, PartialEq, Eq)]
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pub struct Email(String);
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impl Email {
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pub fn new(email: String) -> Result<Self, ValidationError> {
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if !email.contains('@') {
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return Err(ValidationError::InvalidEmail("Missing @ symbol".into()));
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}
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if email.len() > 255 {
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return Err(ValidationError::InvalidEmail("Too long".into()));
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}
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Ok(Self(email))
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}
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pub fn as_str(&self) -> &str {
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&self.0
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}
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}
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// Implement TryFrom for ergonomics
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impl TryFrom<String> for Email {
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type Error = ValidationError;
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fn try_from(s: String) -> Result<Self, Self::Error> {
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Self::new(s)
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}
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}
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```
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### Pattern 2: Entity with Identity
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```rust
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#[derive(Debug, Clone)]
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pub struct User {
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id: UserId,
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email: Email,
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name: String,
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status: UserStatus,
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}
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impl User {
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pub fn new(email: Email, name: String) -> Self {
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Self {
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id: UserId::generate(),
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email,
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name,
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status: UserStatus::Active,
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}
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}
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// Domain behavior
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pub fn deactivate(&mut self) -> Result<(), DomainError> {
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if self.status == UserStatus::Deleted {
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return Err(DomainError::UserAlreadyDeleted);
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}
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self.status = UserStatus::Inactive;
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Ok(())
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}
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pub fn change_email(&mut self, new_email: Email) -> Result<(), DomainError> {
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if self.status != UserStatus::Active {
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return Err(DomainError::UserNotActive);
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}
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self.email = new_email;
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Ok(())
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}
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// Getters
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pub fn id(&self) -> &UserId { &self.id }
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pub fn email(&self) -> &Email { &self.email }
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}
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```
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### Pattern 3: Domain Events
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```rust
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#[derive(Debug, Clone)]
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pub enum UserEvent {
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UserCreated { id: UserId, email: Email },
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UserDeactivated { id: UserId },
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EmailChanged { id: UserId, old_email: Email, new_email: Email },
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}
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pub struct User {
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id: UserId,
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email: Email,
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events: Vec<UserEvent>,
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}
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impl User {
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pub fn new(email: Email) -> Self {
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let id = UserId::generate();
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let mut user = Self {
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id: id.clone(),
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email: email.clone(),
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events: vec![],
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};
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user.record_event(UserEvent::UserCreated { id, email });
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user
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}
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pub fn change_email(&mut self, new_email: Email) -> Result<(), DomainError> {
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let old_email = self.email.clone();
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self.email = new_email.clone();
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self.record_event(UserEvent::EmailChanged {
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id: self.id.clone(),
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old_email,
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new_email,
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});
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Ok(())
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}
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pub fn take_events(&mut self) -> Vec<UserEvent> {
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std::mem::take(&mut self.events)
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}
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fn record_event(&mut self, event: UserEvent) {
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self.events.push(event);
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}
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}
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```
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### Pattern 4: Business Rules
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```rust
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pub struct Order {
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id: OrderId,
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items: Vec<OrderItem>,
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status: OrderStatus,
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total: Money,
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}
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impl Order {
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pub fn new(items: Vec<OrderItem>) -> Result<Self, DomainError> {
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if items.is_empty() {
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return Err(DomainError::EmptyOrder);
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}
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let total = items.iter().map(|item| item.total()).sum();
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Ok(Self {
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id: OrderId::generate(),
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items,
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status: OrderStatus::Pending,
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total,
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})
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}
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pub fn add_item(&mut self, item: OrderItem) -> Result<(), DomainError> {
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if self.status != OrderStatus::Pending {
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return Err(DomainError::OrderNotEditable);
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}
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self.items.push(item.clone());
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self.total = self.total + item.total();
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Ok(())
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}
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pub fn confirm(&mut self) -> Result<(), DomainError> {
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if self.status != OrderStatus::Pending {
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return Err(DomainError::OrderAlreadyConfirmed);
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}
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if self.total < Money::dollars(10) {
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return Err(DomainError::MinimumOrderNotMet);
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}
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self.status = OrderStatus::Confirmed;
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Ok(())
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}
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}
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```
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## Anti-Patterns to Avoid
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### ❌ Primitive Obsession
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```rust
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// BAD: Using primitives everywhere
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pub struct User {
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pub id: String,
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pub email: String,
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pub age: i32,
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}
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fn create_user(email: String, age: i32) -> User {
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// No validation, easy to pass wrong data
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}
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// GOOD: Domain types
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pub struct User {
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id: UserId,
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email: Email,
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age: Age,
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}
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impl User {
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pub fn new(email: Email, age: Age) -> Result<Self, DomainError> {
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// Validation already done in Email and Age types
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Ok(Self {
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id: UserId::generate(),
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email,
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age,
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})
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}
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}
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```
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### ❌ Anemic Domain Model
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```rust
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// BAD: Domain is just data
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pub struct User {
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pub id: String,
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pub email: String,
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pub status: String,
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}
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// Business logic in service layer
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impl UserService {
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pub fn deactivate_user(&self, user: &mut User) {
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user.status = "inactive".to_string();
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}
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}
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// GOOD: Domain has behavior
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pub struct User {
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id: UserId,
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email: Email,
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status: UserStatus,
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}
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impl User {
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pub fn deactivate(&mut self) -> Result<(), DomainError> {
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if self.status == UserStatus::Deleted {
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return Err(DomainError::UserAlreadyDeleted);
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}
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self.status = UserStatus::Inactive;
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Ok(())
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}
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}
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```
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## Your Approach
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When you see domain models:
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1. Check for primitive obsession
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2. Suggest value objects for domain concepts
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3. Move validation into domain types
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4. Add behavior methods to entities
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5. Ensure immutability where appropriate
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Proactively suggest rich domain patterns when you detect anemic models or primitive obsession.
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